IB DP Chemistry 11.2 Graphical techniques SL Paper 3

 

https://play.google.com/store/apps/details?id=com.iitianDownload IITian Academy  App for accessing Online Mock Tests and More..

Question 

Alloys containing at least $60 \%$ copper reduce the presence of bacteria on their surface. The percentage of copper in brass, an alloy of copper and zinc, can be determined by UV-vis spectrometry.
A sample of brass is dissolved in concentrated nitric acid and then made up to $250.0 \mathrm{~cm}^3$ with water before analysis.
$$
\begin{gathered}
\mathrm{Cu}(\mathrm{s})+4 \mathrm{HNO}_3(\mathrm{aq}) \rightarrow \mathrm{Cu}\left(\mathrm{NO}_3\right)_2(\mathrm{aq})+2 \mathrm{NO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \\
3 \mathrm{Zn}(\mathrm{s})+8 \mathrm{HNO}_3(\mathrm{aq}) \rightarrow 3 \mathrm{Zn}\left(\mathrm{NO}_3\right)_2(\mathrm{aq})+2 \mathrm{NO}(\mathrm{g})+4 \mathrm{H}_2 \mathrm{O}(\mathrm{l})
\end{gathered}
$$
The concentration of copper(II) ions in the resulting solution is then determined from a calibration curve, which is plotted by measuring the light absorbance of standard solutions.

Titration is another method for analysing the solution obtained from adding brass to nitric acid.
a. Outline why the initial reaction should be carried out under a fume hood.
b. Deduce the equation for the relationship between absorbance and concentration.

c. Outline how a solution of $0.0100 \mathrm{~mol} \mathrm{dm}^{-3}$ is obtained from a standard $1.000 \mathrm{~mol} \mathrm{dm}^{-3}$ copper(II) sulfate solution, including two essential pieces of glassware you would need.
d.i. The original piece of brass weighed $0.200 \mathrm{~g}$. The absorbance was 0.32 .
Calculate, showing your working, the percentage of copper by mass in the brass.
d.ii.Deduce the appropriate number of significant figures for your answer in (d)(i).
e.i. Comment on the suitability of using brass of this composition for door handles in hospitals.
If you did not obtain an answer to (d)(i), use $70 \%$ but this is not the correct answer.
e.ii.Suggest another property of brass that makes it suitable for door handles.
f.i. Copper(II) ions are reduced to copper(I) iodide by the addition of potassium iodide solution, releasing iodine that can be titrated with sodium thiosulfate solution, $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3(\mathrm{aq})$. Copper(I) iodide is a white solid.
$$
\begin{gathered}
4 \mathrm{I}^{-}(\mathrm{aq})+2 \mathrm{Cu}^{2+}(\mathrm{aq}) \rightarrow 2 \mathrm{Cul}(\mathrm{s})+\mathrm{I}_2(\mathrm{aq}) \\
\mathrm{I}_2(\mathrm{aq})+2 \mathrm{~S}_2 \mathrm{O}_3{ }^{2-}(\mathrm{aq}) \rightarrow 2 \mathrm{I}^{-}(\mathrm{aq})+\mathrm{S}_4 \mathrm{O}_6{ }^{2-}(\mathrm{aq})
\end{gathered}
$$
Deduce the overall equation for the two reactions by combining the two equations.
f.ii. Suggest why the end point of the titration is difficult to determine, even with the addition of starch to turn the remaining free iodine black.

▶️Answer/Explanation

Markscheme

a. $\mathrm{NO}_2 / \mathrm{NO} / \mathrm{NO}_{\mathrm{x}} / \mathrm{HNO}_3 /$ gas is poisonous/toxic/irritant
Accept formula or name.
Accept ” $\mathrm{HNO}_3$ is corrosive” OR “poisonous/toxic gases produced”.
Accept “reaction is harmful/hazardous”.
b. Slope (gradient):
40
Equation:
absorbance $=40 \times$ concentration
OR
$$
y=40 x
$$
Accept any correct relationship for slope such as $\frac{1.00}{0.025}$.
Award [2] if equation in M2 is correct.
c. dilute $1.00 \mathrm{~cm}^3$ «of the standard solution with water» to $100 \mathrm{~cm}^3$
OR
dilute sample of standard solution «with water» 100 times
«graduated/volumetric» pipette/pipet

volumetric flask
Accept any $1: 100$ ratio for $M 1$.
Accept “mix $1 \mathrm{~cm}^3$ of the standard solution with $99 \mathrm{~cm}^3$ of water” for M1.
Do not accept “add $100 \mathrm{~cm}^3$ of water to $1.00 \mathrm{~cm}^3$ of standard solution” for M1.
Accept “burette/buret” for M2.
Accept “graduated/measuring flask” for M3 but not “graduated/measuring cylinder” or “conical/Erlenmeyer flask”.
d.i. concentration of copper $=0.0080 \ll \mathrm{mol} \mathrm{dm}^{-3} »$
mass of copper in $250.0 \mathrm{~cm}^3=« 0.0080 \mathrm{~mol} \mathrm{dm}^{-3} \times 0.2500 \mathrm{dm}^3 \times 63.55 \mathrm{~g} \mathrm{~mol}^{-1}=» 0.127$ «g
OR
mass of brass in $1 \mathrm{dm}^3=« 4 \times 0.200 \mathrm{~g}=» 0.800 \mathrm{~g} \mathrm{AND}$ [Cu2+] $=« 0.0080 \mathrm{~mol} \mathrm{dm}^{-3} \times 63.55 \mathrm{~g} \mathrm{~mol}^{-1}=» 0.5084 \mathrm{~g} \mathrm{dm}^{-3}$
« $\%$ copper in this sample of brass $=\frac{0.127}{0.200} \times 100=» 64$ «\%»
OR
$« \%$ copper in this sample of brass $=\frac{0.5084}{0.800} \times 100=» 64 « \% »$
Accept any value in range $0.0075-0.0085$ «mol dm${ }^{-3}$ ) for M1.
Accept annotation on graph for $M 1$.
Award [3] for correct final answer.
Accept “65 «\%»”.

d.ii.wo
Do not apply ECF from 1(d)(i).
e.i. «since it is greater than $60 \%$ » it will reduce the presence of bacteria «on door handles»
e.ii resistant to corrosion/oxidation/rusting
OR
low friction surface «so ideal for connected moving components»
Accept “hard/durable”, “«high tensile» strength”, “unreactive”, “malleable” or any reference to the appearance/colour of brass (eg “gold-like”, “looks nice” etc.).

Do not accept irrelevant properties, such as “high melting/boiling point”, “non-magnetic”, “good heat/electrical conductor”, “low volatility”, etc.
Do not accept “ductile”.
f.i. $2 \mathrm{I}^{-}(\mathrm{aq})+2 \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{~S}_2 \mathrm{O}_3{ }^{2-}(\mathrm{aq}) \rightarrow 2 \mathrm{Cul}(\mathrm{s})+\mathrm{S}_4 \mathrm{O}_6{ }^{2-}(\mathrm{aq})$
correct reactants and products
balanced equation
M2 can only be awarded if M1 is correct.

f.ii. precipitate/copper(I) iodide/Cul makes colour change difficult to see
OR
release of $\mathrm{I}_2 /$ iodine from starch- $\mathrm{I}_2$ complex is slow so titration must be done slowly

 
 

Question 

Physical properties of elements vary according to atomic number. Sections 6 to 9 of the data booklet list some of these properties.

a. Deduce, giving a reason, the group of elements in the periodic table most likely to undergo sublimation.
$b$ (i)Describe the density trend across periods 4 and 5 of the periodic table.

b(ii) Suggest, with a reason, whether the lanthanoids or actinoids of the f-block would have the higher density.
b(iiifompare the ease of oxidation of s-block and d-block metals to their melting points and densities. Use section 25 of the data booklet.
b(iVsketch how the first ionization energies of elements vary with their atomic radius.

▶️Answer/Explanation

Markscheme
a. group $18 /$ noble gases [ $[$ ]
smallest difference between melting and boiling points
OR
weakest intermolecular forces «in that period» $[\boldsymbol{V}]$
Note: Accept “group 17/halogens”.
b(i)density increases «to a maximum in the transition elements» AND then decreases $[\boldsymbol{V}]$
b(iiłctinoids $A N D$ density increases down all groups «due to large increase in atomic mass for small increase in atomic volume»
OR
actinoids AND «much» greater atomic mass with similar type of bonding
OR
actinoids $A N D$ density «of actinoids» atomic number 90 to 95 is greater than corresponding lanthanoids [ $\mathbf{V}]$
Note: Accept “actinoids AND on graph actinoids have “much» greater density than lanthanoids”.

b(iii) Alternative 1:
«metals with» low densities oxidize easier [ $\boldsymbol{C}$
«metals with» low melting points oxidize easier [ $\boldsymbol{c}$ ]
Alternative 2:
in s-block «metals with» high densities oxidize easier
OR
in s-block «metals with» low melting points oxidize easier [ $\boldsymbol{\swarrow}$ ]
in d-block «metals with» low densities oxidize easier
OR
in d-block «metals with» low melting points oxidize easier $[\boldsymbol{\sim}$ ]
Note: Award [1 max] for “s-block metals more easily oxidized” OR “s-block metals have lower melting points” OR “s-block metals have lower densities”.
Accept “have greater activity” for “oxidize easier”.

b(iv).

Note: Accept any negative sloping line.
Do not award mark if line touches either axis.

 
 

Question 

A student investigated how the type of acid in acid deposition affects limestone, a building material mainly composed of calcium carbonate.

The student monitored the mass of six similarly sized pieces of limestone. Three were placed in beakers containing $200.0 \mathrm{~cm}^3$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ nitric acid, $\mathrm{HNO}_3(\mathrm{aq})$, and the other three in $200.0 \mathrm{~cm}^3$ of $0.100 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ sulfuric acid, $\mathrm{H}_2 \mathrm{SO}_4$ (aq).

The limestone was removed from the acid, washed, dried with a paper towel and weighed every day at the same time and then replaced in the beakers. The student plotted the mass of one of the pieces of limestone placed in nitric acid against time.

The student hypothesized that sulfuric acid would cause a larger mass loss than nitric acid.
a. Draw a best-fit line on the graph.
b(i)Determine the initial rate of reaction of limestone with nitric acid from the graph.
Show your working on the graph and include the units of the initial rate.
$\mathrm{b}$ (ii) Explain why the rate of reaction of limestone with nitric acid decreases and reaches zero over the period of five days.
b(iii\$uggest a source of error in the procedure, assuming no human errors occurred and the balance was accurate.
c(i)Justify this hypothesis.
c(ii)The student obtained the following total mass losses.

She concluded that nitric acid caused more mass loss than sulfuric acid, which did not support her hypothesis.
Suggest an explanation for the data, assuming that no errors were made by the student.

▶️Answer/Explanation

Markscheme
a. best-fit smooth curve
NOTE: Do not accept a series of connected lines that pass through all points OR any straight line representation.
b(i)tangent drawn at time zero
$\mathrm{g} \mathrm{day}^{-1}$
0.16
$M 3$ can only be awarded if the value corresponds to the correct unit given in $M 2$.
Accept values for the initial rate for M3 in the range: $0.13-0.20 \mathrm{~g} \mathrm{day}^{-1}$ OR $1.5 \times 10^{-6} \mathrm{~g} \mathrm{~s}^{-1}-2.3 \times 10^{-6} \mathrm{~g} \mathrm{~s}^{-1} \mathrm{OR}^{-5} .5 \times 10^{-8}-1.2 \times 10^{-7} \mathrm{~mol} \mathrm{dm}^{-3}$ $\mathrm{s}^{-1}$ OR $4.5 \times 10^{-6}-6.9 \times 10^{-6} \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{~min}^{-1}$ OR $9.0 \times 10^{-5}-1.4 \times 10^{-4} \mathrm{~g} \mathrm{~min}^{-1}$ OR a range based on any other reasonable unit for rate.
Ignore any negative rate value.
Award [ 2 max] for answers such as $0.12 / 0.11 \mathrm{~g} \mathrm{day}^{-1}$, incorrectly obtained by using the first two points on the graph (the average rate between $t=0$ and 1 day).
Award [1 max] for correctly calculating any other average rate.

,b(iigcid used up
OR
acid is the limiting reactant
concentration of acid decreases
OR
less frequent collisions
NOTE: Award [1 max] for “surface area decreases” if the idea that $\mathrm{CaCO}_3$ is used up/acts as the limiting reactant” is conveyed for $\mathrm{M1}$.
Do not accept “reaction reaches equilibrium” for $M 2$.
b(iis)urface area not uniform
NOTE: Accept “acids impure.
OR
limestone pieces do not have same composition/source
NOTE: Accept “«limestone” contains impurities”.
OR
limestone absorbed water «which increased mass»
OR
acid removed from solution when limestone removed
NOTE: Accept “loss of limestone when dried” OR “loss of limestone due to crumbling when removed from beaker”.
OR
«some» calcium sulfate deposited on limestone lost
OR
pieces of paper towel may have stuck to limestone
OR
beakers not covered/evaporation

c(i)sulfuric acid is diprotic/contains two $\mathrm{H}^{+}$«while nitric acid contains one $\mathrm{H}^{+}$»/releases more $\mathrm{H}^{+}$«so reacts with more limestone»
OR
higher concentration of protons $/ \mathrm{H}^{+}$
NOTE: Ignore any reference to the relative strengths of sulfuric acid and nitric acid.
Accept “sulfuric acid has two hydrogens “whereas nitric has one»”.
Accept “dibasic” for “diprotic”.
c(ii)calcium sulfate remained/deposited on limestone «in sulfuric acid»
OR
reaction prevented/stopped by slightly soluble/deposited/layer of calcium sulfate
NOTE: Answer must refer to calcium sulfate.

 
 

Question

The absorption of infrared (IR) radiation by molecules in the atmosphere affects global temperatures.

Using the graph, state, giving your reasons, whether or not oxygen and ozone are greenhouse gases.

▶️Answer/Explanation

Markscheme

Ozone: yes because it absorbs IR/is IR active
Oxygen: no because it does not absorb IR/is IR inactive

Award [1 max] for stating “ozone/O3 is a greenhouse gas but oxygen/O2 is not”.

Award [1 max] for stating “ozone/O3 absorbs IR/is IR active but oxygen/O2 does not/is IR inactive”.

Accept “oxygen/O2 is not a greenhouse gas because it absorbs UV”.

Examiners report

[N/A]

Question

In order to provide safe drinking water, a water supply is often treated with disinfectants, which aim to inactivate disease-causing bacteria in the water.

To compare the effectiveness of different disinfectants, a CT value is used as a measure of the dosage of disinfectant needed to achieve a certain level of inactivation of specific bacteria.

CT value (mg min dm−3) = C (mg dm−3) concentration of disinfectant × T (min) contact time with water

The table below compares the CT values of different disinfectants necessary to achieve 99% inactivation of two types of bacteria, listed as A and B.

(i) Deduce the oxidation state of chlorine in the following disinfectants.

(ii) From the data on CT values, justify the statement that bacterium B is generally more resistant to disinfection than bacterium A.

(iii) CT values can be used to determine whether a particular treatment process is adequate. Calculate the CT value, in mg min dm−3, when 1.50 × 10−5 g dm−3 of chlorine dioxide is added to a water supply with a contact time of 9.82 minutes.

(iv) From your answer to (a) (iii) and the data in the table, comment on whether this treatment will be sufficient to inactivate 99% of bacterium A.

[4]
a.

CT values are influenced by temperature and by pH. The table below shows the CT values for chlorine needed to achieve 99% inactivation of a specific bacterium at stated values of pH and temperature.

(i) With reference to the temperature data in the table, suggest why it may be more difficult to treat water effectively with chlorine in cold climates.

(ii) Sketch a graph on the axes below to show how the CT value (at any temperature) varies with pH.

(iii) Comment on the relative CT values at pH 6.0 and pH 9.0 at each temperature.

(iv) Chlorine reacts with water as follows:

Cl2 (g) + H2O (l) \( \rightleftharpoons \) HOCl (aq) + HCl (aq)

HOCl (aq) \( \rightleftharpoons \) OCl (aq) + H+ (aq)

Predict how the concentrations of each of the species HOCl (aq) and OCl (aq) will change if the pH of the disinfected water increases.

[4]
b.

Despite widespread improvements in the provision of safe drinking water, the sale of bottled water has increased dramatically in recent years. State one problem caused by this trend.

[1]
c.
▶️Answer/Explanation

Markscheme

i

HOCl: +1
AND
ClO2:
+4

Accept “I” and “IV” but not “1+/1” and “4+/4” notations.

ii

«most» CT values are higher for «bacterium» B
OR
«generally» higher dosage needed for «bacterium» B 

Accept converse arguments. Accept “concentration” for “dosage”

iii

«CT = 1.50 × 10–5 × 103 mg dm–3 × 9.82 min =» 1.47 × 10–1 «mg min dm–3»

iv

lower than CT value/minimum dosage/1.8 × 10–1 «mg min dm–3»
AND
no/insufficient

Accept “concentration” for “dosage”.

a.

i

higher CT value at lower temperature
OR
higher dosage «of chlorine» needed at low temperature

Accept “effectiveness decreases at lower temperature”.
Accept “concentration” for “dosage”.
Accept converse arguments.

ii

labeled axes ( y: CT and x: pH)
AND
curve with increasing gradient 

Do not accept axes the wrong way round.
Accept a linear sketch.

iii

values at pH 9.0 approximately 3 times values at pH 6.0
OR
increase in CT values in same ratio 

The exact ratio is 2.9 times
Do not accept just “increase in value”.

iv

[HOCl] decreases AND [OCl] increases

b.

plastic disposal/pollution
OR
plastic bottles use up petroleum/non-renewable raw material

OR
chemicals in plastic bottle can contaminate water

OR
«prolonged» storage in plastic can cause contamination of water

OR
plastic water bottles sometimes reused without proper hygiene considerations

Accept other valid answers.
Accept economic considerations such as “greater production costs”, “greater transport costs” or “bottled water more expensive than tap water”

c.

Question

A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.

Volume of aqueous NaOH = 25.0 ± 0.5 cm3

Concentration of HCl = 1.00 ± 0.01 mol dm−3

Calculate the percentage uncertainty of the volume of the aqueous sodium hydroxide.

[1]
a.

Suggest how the precision of this measurement could be improved.

[1]
b.
▶️Answer/Explanation

Markscheme

«\(\frac{{0.5}}{{25.0}} \times 100\)» = 2 «%»

[1 mark]

a.

pipette/pipet «rather than a measuring cylinder»

Accept “using a burette/buret”.

Accept “using a volumetric/measuring flask”.

Do not accept “use of a more precise measuring cylinder”.

[1 mark]

b.

Question

A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.

Volume of aqueous NaOH = 25.0 ± 0.5 cm3

Concentration of HCl = 1.00 ± 0.01 mol dm−3

Suggest how the end point of the titration might be estimated from the graph.

▶️Answer/Explanation

Markscheme

volume «found by extrapolation of the two best fit lines» required to give the highest temperature
OR
extrapolate «two best fit» lines to the point where they meet

Accept “where lines through the points meet”.

Accept “at maximum temperature”.

Accept “at 35 cm3 of HCl”.

[1 mark]

Examiners report

[N/A]

Question

A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.

Volume of aqueous NaOH = 25.0 ± 0.5 cm3

Concentration of HCl = 1.00 ± 0.01 mol dm−3

State and explain how the graph would differ if 1 mol\(\,\)dm−3 sulfuric acid had been used instead of 1 mol\(\,\)dm−3 hydrochloric acid.

Answer/Explanation

Markscheme

graph would peak/maximum at 17.5 cm3
OR
smaller volume of acid «needed to reach equivalence» 

sulfuric acid is dibasic/diprotic

higher temperature would be reached

Accept “gradient/slope «of graph» is greater/steeper” for M1.

Accept “one mole of sulfuric acid neutralizes two moles of NaOH” for M2.

[2 marks]

Examiners report

[N/A]

Question

Medicines have a variety of different effects on the body and act at the molecular level.

Morphine and codeine are strong analgesics. Their structures are given in section 37 of the data booklet.

Dose response curves are determined for each drug.

M17/4/CHEMI/SP3/ENG/TZ1/XX

Outline the significance of range “a”.

[1]
a.

Suggest the type of reaction used to convert morphine to codeine.

[1]
b.i.

State and explain the action of opiates as painkillers.

[2]
b.ii.
▶️Answer/Explanation

Markscheme

«measures» therapeutic window/margin «of a drug»
OR
range of doses that produce a therapeutic effect without causing toxic effects

Accept “difference between ED50/minimum effective/therapeutic dose «for 50% of population» AND TD50 /toxic dose «for 50% of population»”.

Do not accept “therapeutic index”.

Do not accept “lethal/fatal dose” as this is not LD50.

[1 mark]

a.

«nucleophilic» substitution/SN

Accept “methylation”.

[1 mark]

b.i.

work directly on opioid/pain receptors «in brain»

suppress pain impulses in brain/CNS

resemble endorphins/enkephalins/natural chemical painkillers «produced in the brain and spinal cord»

Do not award mark for “resemble hormones”.

[2 marks]

b.ii.

Question

There is a link between world energy consumption and carbon dioxide production.

Climate induced changes in the ocean can be studied using measurements such as the Atmospheric Potential Oxygen (APO). Trends in APO concentration from two stations, one in each hemisphere, are shown below.

Trends in atmospheric potential oxygen (APO) based on monthly averages between 1990 and 2010.

[Source: www.ioos.noaa.gov]

The following graph represents world energy consumption by type for the years 1988–2013.

Estimate the percentage of energy consumption which did not directly produce CO2 in 2013.

[1]
a.

O2 is consumed in producing CO2 for electricity generation. The graph shows the relationship between the world’s electricity generation and CO2 production between 1994 and 2013.

Calculate the mass, in million tonnes, of oxygen gas ultimately found in CO2 which is consumed in generating 18\(\,\)000 terawatts of electricity using the equation given for the best fit line. Give your answer to 2 significant figures.

Assume coal is the only energy source.

[2]
b.

The equilibrium expression for O2 exchange between the atmosphere and ocean is O2(g) \( \rightleftharpoons \) O2(aq). Identify one factor which shifts the equilibrium to the right.

[1]
c.i.

Factors such as photosynthesis and respiration are excluded so that APO is influenced by oceanic changes only. Suggest why the seasonal cycles from Alert station and Cape Grim observatory are different.

[2]
c.ii.

The change in APO O2/N2 ratio, per meg, is measured relative to an O2/N2 reference.

\[\Delta {\text{(}}{{\text{O}}_{\text{2}}}{\text{/}}{{\text{N}}_{\text{2}}}{\text{)}} = \left( {\frac{{{{({{\text{O}}_2}/{{\text{N}}_2})}_{{\text{sample}}}}}}{{{{({{\text{O}}_2}/{{\text{N}}_2})}_{{\text{reference}}}}}} – 1} \right) \times {10^6}\]

Calculate the APO Δ(O2/N2) value for an oxygen concentration of 209\(\,\)400 ppm assuming that any change in N2 concentration is negligible. Reference values for O2 and N2 are 209 460 and 790 190 ppm respectively.

[1]
c.iii.

Suggest a reason for the general negative gradient of the APO curve given in (c).

[1]
c.iv.
▶️Answer/Explanation

Markscheme

«\(\frac{{\sum {({\text{renewables}} + {\text{hydroelectricity}} + {\text{nuclear)}}} }}{{{\text{total}}}}\)»

«\(\left( {\frac{{8800 – 7200}}{{12600}}} \right) \times 100 = \)» 13 «%»

Accept range of “11–16%”.

[1 mark]

a.

«18000 = 0.54x – 2000»
x = 37037 «million tonnes of CO2»
«\(\frac{{32.00}}{{44.01}}\) x 37037 = 26930»
27000/2.7 x 104 «million tonnes of O2»

Accept “37000 «million tonnes of CO2»” for M1.

Award [2] for correct final answer with two significant figures.

Award [1] for non rounded answers in range 26903–26936 «million tonnes of O2».

[2 marks]

b.

increase in «atmospheric» pressure
OR
increase in [O2(g)]/concentration of O2(g)
OR
decrease in [O2(aq)]/concentration of O2(aq)
OR
decrease in temperature

Accept “increase in volume of oceans «due to polar ice cap melting»” OR “consumption of O2 in oceans/O2(aq)
«by living organisms»”.

State symbols required for oxygen concentration.

[1 mark]

c.i.

summer in one station while winter in other
OR
stations are at different latitudes

oxygen dissolves better in colder water

Accept “opposite seasons «in each hemisphere»”.

Do not accept “different locations with different temperatures” OR “stations are in different hemispheres”.

[2 marks]

c.ii.

«\({\text{(}}\frac{{209400}}{{209460}} – 1{\text{)}} \times {10^6}\) =» − 286.5 «per meg»

The nitrogen cancels so is not needed in the calculation.

Negative sign required for mark.

[1 mark]

c.iii.

decrease in [O2]/concentration of O2
OR
increasing combustion of fossil fuels «consumes more O2 so [O2]/concentration of O2 decreases»
OR
warmer oceans/seas/water «as oxygen dissolves better in colder water»
OR
deforestation

Accept “decrease in level of O2”.

Accept “increasing CO2 production «consumes more O2 so [O2]/concentration of O2 decreases»”.

Do not accept “decrease in amount of O2OR “increase in greenhouse gases”.

[1 mark]

c.iv.

Question

A student set up a simple voltaic cell consisting of a copper electrode and a zinc electrode dipped in sodium chloride solution.

The student gradually increased the distance, d, between the electrodes to study the effect on the initial current, I, passing through the light bulb.

The student hypothesized that the initial current would be inversely proportional to the distance between the electrodes.

The following data was collected over five trials.

The data did not support the student’s hypothesis. He investigated other possible relationships by plotting a graph of the average current against the distance between the electrodes. He obtained the following best-fit line with a correlation coefficient (r) of −0.9999.

Sketch a graph that would support the student’s hypothesis.

[1]
a.

Suggest what the correlation coefficient of −0.9999 indicates.

[1]
b.i.

State the equation of the straight line obtained using the data.

[1]
b.ii.

Outline how current flows in the sodium chloride solution.

[1]
b.iii.
▶️Answer/Explanation

Markscheme

OR

OR

Correct labels of axes required for mark.

Accept d–1 instead of \(\frac{1}{d}\).

Accept I–1 instead of \(\frac{1}{I}\).

Plot of I vs d should not be linear.

a.

negative correlation

OR

model/prediction matches results

OR

99% of variance accounted for

b.i.

I = – 0.001631 d + 0.09939

OR

y = – 0.001631 x + 0.09939

Accept correctly rounded values for m and b in equation.

Do not accept “y = mx + b”.

b.ii.

ions move «across electrolyte»

b.iii.

Question

Sodium chloride, NaCl, can be spread on icy roads to lower the freezing point of water.

The diagram shows the effects of temperature and percentage by mass of NaCl on the composition of a mixture of NaCl and H2O.

Estimate the lowest freezing point of water that can be reached by adding sodium chloride.

[1]
a.

Estimate the percentage by mass of NaCl dissolved in a saturated sodium chloride solution at +10 ºC.

[1]
b.

Calculate the percentage of water by mass in the NaCl•2H2O crystals. Use the data from section 6 of the data booklet and give your answer to two decimal places.

[2]
c.

Suggest a concern about spreading sodium chloride on roads.

[1]
d.
▶️Answer/Explanation

Markscheme

–21 «ºC»

a.

28 «%»

Accept any specific answer in the range 27 to 29 «%».

b.

Mr = 94.48

«\(2\frac{{\left( {1.01 \times 2 + 16.00} \right)}}{{94.48}} \times 100 = \)» 38.15 «%»

Award M2 only if answer is to 2 decimal places.

Award [2] for correct final answer.

Award [1 max] for 38.10 %.

c.

rust/corrosion «of cars and bridges»
OR
waste of important raw material
OR
soil/water salination/pollution «from run off»
OR
erosion of/damage to the road surface
OR
specific example of damage to the ecosystem
OR
«outdoor» temperatures may go below effective levels for NaCl «to lower freezing point» so NaCl could be wasted
OR
roads can refreeze causing hazards

Do not accept “tyre damage”.

Do not accept “economic issues” OR “environmental issues” unless specified (eg accept “increase in costs for local councils road budgets” but not “cost” alone).

Do not accept “makes roads more slippery”.

d.
Scroll to Top