Question
If \({\text{20 c}}{{\text{m}}^{\text{3}}}\) samples of \({\text{0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) solutions of the acids below are taken, which acid would require a different volume of \({\text{0.1 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}\) sodium hydroxide for complete neutralization?
A. Nitric acid
B. Sulfuric acid
C. Ethanoic acid
D. Hydrochloric acid
▶️Answer/Explanation
B
Out of these, only Sulfuric acid has 2 valency, while rest three have 1 valency. Hence it will require different volume of sodium hydroxide for complete neutralization.
Question
What occurs when solid sodium hydrogen carbonate reacts with aqueous sulfuric acid?
A. Bubbles of sulfur dioxide form.
B. Bubbles of both hydrogen and carbon dioxide form.
C. Bubbles of hydrogen form.
D. Bubbles of carbon dioxide form.
▶️Answer/Explanation
D
H2SO4 + 2 NaHCO3 → Na2SO4 + 2 CO2+ 2 H2O
Bubbles of carbon dioxide forms.
Question
A student carried out a titration to determine the concentration of an acid and found that his value had good precision but poor accuracy. Which process explains this outcome?
A. Consistently overshooting the volume of solution from the burette into the flask.
B. Collection of insufficient titration data.
C. Reading the meniscus in the burette at a different angle each time.
D. Forgetting to rinse the flask after one of the titrations.
▶️Answer/Explanation
A
The accuracy of titration requires precise measurement of the volume of materials in use. But markings on a burette can be easily misread. One way to misread the volume is by looking at the measurement on an angle. From above, it can seem like the volume is lower, while from below, the apparent volume looks higher. Another source of measurement error is looking at the wrong spot. A solution forms a concave curve and the bottom of the curve is used to measure the volume. If the reading is taken from the higher sections of the curve, the volume measurement will be in error.
The end point of a titration is when the reaction between the two solutions has stopped. Indicators, which change color to indicate when the reaction has stopped, do not change instantly. In the case of acid-base titration, the indicator may first lighten in color before changing completely. Also, each individual perceives color slightly differently, which affects the outcome of the experiment. If the color has changed slightly, too much of the titrant, which comes from the burette, can be introduced into the solution, overshooting results.
Here, precision is good but accuracy is poor which means the student is overshooting the volume of solution from the burette into the flask.
Question
Which solutions have a pH less than 7?
I. \({\text{N}}{{\text{a}}_2}{\text{C}}{{\text{O}}_3}({\text{aq)}}\)
II. \({\text{[Fe(}}{{\text{H}}_2}{\text{O}}{{\text{)}}_6}{\text{]C}}{{\text{l}}_3}{\text{(aq)}}\)
III. \({{\text{(N}}{{\text{H}}_4}{\text{)}}_2}{\text{S}}{{\text{O}}_4}{\text{(aq)}}\)
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
C
Sodium carbonate is a basic salt as it is formed when a strong base (sodium hydroxide) is partially neutralized by a weak acid (carbonic acid). It has pH greater than 7.
The reaction between the sulphuric acid and the ammonium hydroxide produces ammonium sulfate. Sulphuric acid is a strong acid and ammonium hydroxide is a weak base. Therefore, the salt formed by ammonium sulphate is acidic. Hence, it has pH less than 7.
Solutions containing 3+ hexaaqua ions tend to have pH’s in the range from 1 to 3.