Question
Saturated calcium hydroxide solution is used to test for carbon dioxide. Calculate the pH of a 2.33 × 10–2 mol dm–3 solution of calcium hydroxide, a strong base. [2]
Answer/Explanation
Ans
Alternative 1:
[OH− ] = « 2 × 2.33 × 10−2 mol dm−3 =» 0.0466 «mol dm−3» «[H+] = = 2.15 × 10−13 mol dm−3» pH = « −log (2.15 × 10−13) =» 12.668
Alternative 2:
[OH− ] =« 2 × 2.33 × 10−2 mol dm−3 =» 0.0466 «mol dm−3» «pOH = −log (0.0466) = 1.332» pH = «14.000 – pOH = 14.000 – 1.332 =» 12.668
Question
The equations of two acid-base reactions are given below.
Reaction A \({\text{N}}{{\text{H}}_{\text{3}}}({\text{aq)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}} \rightleftharpoons \) \({\rm{NH}}_4^ + ({\rm{aq}}) + {\rm{O}}{{\rm{H}}^ – }({\rm{aq}})\)
The reaction mixture in A consists mainly of reactants because the equilibrium lies to the left.
Reaction B \({\text{NH}}_2^ -({\text{aq)}} + {{\text{H}}_2}{\text{O(l)}} \rightleftharpoons \) \({\rm{NH}}_3^{}({\rm{aq}}) + {\rm{O}}{{\rm{H}}^ – }({\rm{aq}})\)
The reaction mixture in B consists mainly of products because the equilibrium lies to the right.
Two acidic solutions, X and Y, of equal concentrations have pH values of 2 and 6 respectively.
For each of the reactions A and B, deduce whether water is acting as an acid or a base and explain your answer.
In reaction B, identify the stronger base, \({\text{NH}}_2^ – \) or \({\text{O}}{{\text{H}}^ – }\) and explain your answer.
In reactions A and B, identify the stronger acid, \({\text{NH}}_4^ + \) or \({\text{N}}{{\text{H}}_{\text{3}}}\) (underlined) and explain your answer.
Describe two different experimental methods to distinguish between aqueous solutions of a strong base and a weak base.
Calculate the hydrogen ion concentrations in the two solutions and identify the stronger acid.
Determine the ratio of the hydrogen ion concentrations in the two solutions X and Y.
Answer/Explanation
Markscheme
acid in both reactions;
because it loses a proton/hydrogen ion/\({{\text{H}}^ + }\) / proton/hydrogen ion/\({{\text{H}}^ + }\) donor;
Second mark can be scored if they do not identify it as an acid in both reactions.
\({\text{NH}}_2^ – \);
more readily accepts a proton / equilibrium lies to the right / takes \({{\text{H}}^ + }\) from \({{\text{H}}_{\text{2}}}{\text{O}}\);
If OH– chosen award [0]
\({\text{NH}}_4^ + \);
donates a proton more readily than \({\text{N}}{{\text{H}}_{\text{3}}}\) / equilibrium lies to the left;
If NH3 chosen award [0]
solutions of the same concentration;
pH meter;
strong base has a higher pH / weak base has lower pH;
indicator paper/U.I solution;
strong base has a higher pH/more purple / weak base has lower pH/blue not purple / OWTTE;
measuring conductivity (with conductivity meter);
strong base has a higher conductivity / weak base has lower conductivity;
comparing heat of neutralisation with acid;
strong base releases more heat / weak base releases less heat;
Award [4 max] for two correct methods with expected results.
X;
\({\text{[X]}} = {10^{ – 2}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{)}}\) and \({\text{[Y]}} = {10^{ – 6}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{)}}\);
\(10\,000/{10^4}:1\);
Ratio should be in form above.
Examiners report
This was the second most popular question. In (a) many candidates scored marks for their understanding of acid-base behaviour in terms of proton transfer and correctly identified \({{\text{H}}_{\text{2}}}{\text{O}}\) as acting as an acid. Identifying and explaining \({\text{NH}}_2^ – \) as the strongest base and \({\text{NH}}_4^ + \) as the strongest acid proved more problematic.
This was the second most popular question. In (a) many candidates scored marks for their understanding of acid-base behaviour in terms of proton transfer and correctly identified \({{\text{H}}_{\text{2}}}{\text{O}}\) as acting as an acid. Identifying and explaining \({\text{NH}}_2^ – \) as the strongest base and \({\text{NH}}_4^ + \) as the strongest acid proved more problematic.
This was the second most popular question. In (a) many candidates scored marks for their understanding of acid-base behaviour in terms of proton transfer and correctly identified \({{\text{H}}_{\text{2}}}{\text{O}}\) as acting as an acid. Identifying and explaining \({\text{NH}}_2^ – \) as the strongest base and \({\text{NH}}_4^ + \) as the strongest acid proved more problematic.
In (b), in spite of the wording in the question (“experimental methods”) many answers mentioned only a property, such as “a strong base has a higher pH than a weak base”, and several who chose an indicator to distinguish them picked one with only two colours, such as phenolphthalein. Most candidates omitted to mention that the solutions should be of the same concentration. Although most could describe one good method (either pH or conductivity), the second method often involved reaction rates or titrations and descriptions of how these were poor.
In (c), although most were able to convert pH values into \({\text{[}}{{\text{H}}^ + }{\text{]}}\) values, fewer were able to compare them as a ratio in the correct form –10,000:1. Some candidates had difficulty identifying the stronger acid.
In (c), although most were able to convert pH values into \({\text{[}}{{\text{H}}^ + }{\text{]}}\) values, fewer were able to compare them as a ratio in the correct form –10,000:1. Some candidates had difficulty identifying the stronger acid.