Question
A student adds excess large pieces of magnesium carbonate, MgCO3, to dilute hydrochloric acid, HCl, and measures the volume of carbon dioxide gas, CO2, given off.
(a) Add the missing state symbols to the chemical equation for the reaction.
MgCO3 ….. + 2HCl….. → MgCl 2(aq) + H2O ….. + CO2 ….. [1]
(b) Complete the dot-and-cross diagram to show the electron arrangement of the ions in magnesium chloride.
The inner shells have been drawn.
Give the charges on the ions.[3]
(c) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of carbon dioxide.
Show outer shell electrons only.[2]
(d) The graph shows how the volume of carbon dioxide gas changes with time.
(i) Describe how the graph shows that the rate of this reaction decreases as time increases.[1]
(ii) Explain, in terms of particles, why the rate of this reaction decreases as time increases.[2]
(iii) The student repeats the experiment using powdered MgCO3 instead of large pieces.
All other conditions stay the same.
On the grid, draw the line expected when powdered MgCO3 is used instead of large pieces. [2]
(e) Determine the volume of CO2 gas given off when excess MgCO3 is added to 25.0cm3 of 0.400mol/dm3 HCl at room temperature and pressure.
MgCO3 + 2HCl → MgCl2 + H2O + CO2
Use the following steps.
- Calculate the number of moles of HCl in 25.0cm3 of 0.400mol/dm3 of acid.
mol
- Determine the number of moles of CO2 gas given off.
mol
- Calculate the volume of CO2 gas given off in cm3.
cm3 [3] [Total: 14]
Answer/Explanation
Ans:
2(a) MgCO3(s) + 2HCl(aq) → MgCl2(q) + H2O(l) + CO2(g)
2(b) eight crosses in second shell of Mg
7 dots and 1 cross in third shell of both Cl
2+ charge on Mg and – charge on both Cl ions on correct answer line
2(c) C atom double bonded to 2 O atoms
4 non-bonding e– on each O and no non-bonding e– on C and both octets complete
2(d)(i) gradient (of line) decreases
2(d)(ii) concentration of particles (of acid) decreases
lower rate of collisions of particles
2(d)(iii) a new line steeper than printed line and starts at origin and levels off earlier than printed line
levels off at the same volume
2(e) M1 mol HCl = 0.400 × 25.0 / 1000 = 0.01(00)
M2 mol CO2 = M1 / 2 = 0.0100 / 2 = 0.005(00)
M3 volume CO2 = M2 × 24000 = 120 (cm3)
Question
Many organic compounds contain carbon, hydrogen and oxygen only.
(a) An organic compound V has the following composition by mass.
C, 48.65%; H, 8.11%; O, 43.24%
Calculate the empirical formula of compound V.
empirical formula = …………………………
(b) Compound W has the empirical formula CH4O and a relative molecular mass of 32.
Calculate the molecular formula of compound W.
molecular formula = …………………………
(c) Compounds X and Y have the same general formula.
X and Y are both carboxylic acids.
Compound X has the molecular formula \(C_2H_4O_2\).
Compound Y has the molecular formula \(C_4H_8O_2\).
(i) Deduce the general formula of compounds X and Y.
(ii) Draw the structure of compound Y. Show all of the atoms and all of the bonds.
Name compound Y.
name …………………………………………………………………………………………………………………..
(iii) Give the name used to describe a ‘family’ of similar compounds with the same general
formula, similar chemical properties and the same functional group.
(d) Propene is an unsaturated hydrocarbon. The formula of propene is shown.
\(CH_3CH=CH_2\)
(i) State the colour change observed when propene is added to aqueous bromine.
from ……………………………………………………. to ……………………………………………………
(ii) Propene can be produced by cracking long chain alkanes.
Pentadecane, \(C_{15}H_{32}\), is cracked to produce an alkane and propene in a 1:2 molar ratio.
Complete the chemical equation for this reaction.
\(C_{15}H_{32}\) → ………………………………… + …………………………………
(iii) Propene can be converted into poly(propene).
Name the type of polymerisation that occurs when propene is converted into poly(propene).
(iv) Complete the diagram to show a section of poly(propene).
Answer/Explanation
Answer:
(a) 48.65 / 12 8.11 / 1 43.24 / 16 (1)
OR evaluation
4.05 8.11 2.7(0)
divide all by smallest
OR 1.5 : 3 : 1
OR
6 : 3 : 2 (1)
\(C_3H_6O_2\) (1) ALLOW symbols in any order
(b) ( \(M_r\) of \(CH_4O\) = 32)
\(CH_4O\) (1)
(c) (i) \(C_nH_{2n}O_2\)
OR
\(C_nH_{2n+1} COOH\)
(ii) butanoic acid (1)
fully displayed carboxylic acid group (1)
correct structure of butanoic acid showing all atoms and bonds (1)
(iii) homologous series
(d) (i) brown to colourless
(ii) \(C_9H_{20}\) (1)
\(2C_3H_6\) (1)
(iii) addition
(iv) 
any one repeat unit (1)
both repeat units fully correct (1)
Question
This question is about salts.
(a) Salts that are insoluble in water are made by precipitation.
● Lead(II) iodide, \(PbI_2\), is insoluble in water.
● All nitrates are soluble in water.
● All sodium salts are soluble in water.
You are provided with solid lead(II) nitrate, \(Pb(NO_3)_2\), and solid sodium iodide, NaI.
Describe how you would make a pure sample of lead(II) iodide by precipitation.
Your answer should include:
● practical details
● a chemical equation for the precipitation reaction.
(b) Nitrates decompose when heated.
(i) When hydrated zinc nitrate is heated, oxygen gas is given off.
Describe a test for oxygen.
test ……………………………………………………………………………………………………………………..
observations ………………………………………………………………………………………………………..
(ii) Complete the equation for the decomposition of hydrated zinc nitrate.
\(2Zn(NO_3)_2•6H_2O → …..ZnO + …..NO_2 + O_2 + …..H_2O\)
(c) Some sulfates are hydrated.
When hydrated sodium sulfate crystals, Na2SO4•xH2O, are heated, they give off water.
\(Na_2SO_4•xH_2O(s) → Na_2SO_4(s) + xH_2O(g)\)
A student carries out an experiment to determine the value of x in \(Na_2SO_4•xH_2O\).
step 1 Hydrated sodium sulfate crystals are weighed.
step 2 The hydrated sodium sulfate crystals are then heated.
step 3 The remaining solid is weighed.
(i) Describe how the student can check that all the water has been given off.
(ii) In an experiment, 1.61g of \(Na_2SO_4•xH_2O\) is heated until all the water is given off. The mass of \(Na_2SO_4\) remaining is 0.71g.
\([M_r : Na_2SO_4,142; H_2O,18]\)
Determine the value of x using the following steps.
● Calculate the number of moles of \(Na_2SO_4\) remaining.
………………………… mol
● Calculate the mass of \(H_2O\) given off.
………………………… g
● Calculate the number of moles of \(H_2O\) given off.
………………………… mol
● Determine the value of x.
x = …………………………
Answer/Explanation
Answer:
(a) (add) water (to both salts) (1)
dissolve both salts / make solutions (1)
filter (lead(II) iodide)(1)
wash (residue of lead(II) iodide) with water AND dry e.g. with filter paper / description of washing and drying (1)
\(Pb(NO_3)_2 + 2 NaI → 2NaNO_3 + PbI_2\)
OR \(Pb^{2+} + 2I^– → PbI_2\) (1)
(b) (i) glowing splint (1)
relights / rekindles (1)
(ii) 2ZnO(s) and \(4NO_2\)(g) (1)
\(12H_2O\)(g) (1)
(c) (i) heat again and weigh again / repeat steps 2 and 3 (1)
until mass is constant (1)
(ii) 0.005 (1)
0.9 (1)
(0.9 ÷ 18 =) 0.05 (1)
(0.05 ÷ 0.005 =) 10 (1)