State the element that you would expect to have chemical properties most similar to those of antimony.

Describe the relationship between \({\text{Sb}}{{\text{F}}_{\text{5}}}\) and \({\text{SbF}}_6^ - \) in terms of the Lewis theory of acids.

Explain the behaviour of HF in terms of the Brønsted–Lowry theory of acids.

The strength of the hydrogen–halogen bond.

The interaction between an undissociated hydrogen halide molecule and a water molecule.

Neelu and Charles decided to solve the problem by determining the volume of \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) sodium hydroxide solution needed to neutralize \({\text{25.0 c}}{{\text{m}}^{\text{3}}}\) of the acid. Outline whether this was a good choice.

Identify one indicator that could be used when titrating aqueous sodium hydroxide with both a strong acid and a weak acid, and outline the reason for your choice.

Indicator:

Reason:

Neelu and Charles decided to compare the volume of sodium hydroxide solution needed with those required by known \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) strong and weak acids. Unfortunately they chose sulfuric acid as the strong acid. Outline why this was an unsuitable choice.

Francisco and Shamiso decided to measure the pH of the initial solution, HQ, and they found that its pH was 3.7. Deduce, giving a reason, the strength (weak or strong) of the acid HQ.

Explain how the \({\text{p}}{K_{\text{a}}}\) could be determined from a graph of pH against the volume of \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) sodium hydroxide added.

Francisco and Shamiso found that the pH of the initial \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) solution was 3.7. However, this reading was inaccurate because they forgot to wash the pH probe. Calculate the \({\text{p}}{K_{\text{a}}}\) of HQ using the reading they obtained.

arsenic/As;

Accept bismuth/Bi.

\({\text{Sb}}{{\text{F}}_{\text{5}}}\) accepts an electron pair (from \({{\text{F}}^ - }\)) / \({\text{SbF}}_6^ - \) donates an electron pair (to \(H_2F^ +\));

\({\text{Sb}}{{\text{F}}_{\text{5}}}\) acts as a Lewis acid / \({\text{SbF}}_6^ - \) acts as a Lewis base;

one HF donates a \({{\text{H}}^ + }\)/proton and the other accepts a \({{\text{H}}^ + }\)/proton;

HF acts as both a Brønsted–Lowry acid and a Brønsted–Lowry base;

Award [1 max] for correct description of HF acting as a Brønsted–Lowry acid or base.

H–F bond stronger than H–Cl bond / H–Cl bond weaker than H–F bond;

H–F can hydrogen bond to water and H–Cl cannot;

not a good choice / poor choice;

requires same volume of base / the amount/volume to react/for neutralization does not depend on the acid strength;

phenolphthalein / phenol red;

pH at equivalence point 7 or above;

Accept pH range for colour change/end-point corresponds to rapid change in pH.

sulfuric acid is diprotic/dibasic/liberates two protons/\({{\text{H}}^ + }\);

Accept “reacts with 2 moles of alkali/base”.

weak;

strong \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) acid has a pH of 1/lower than that observed;

Accept “pH value of 3.7 means that it produces only 10^–3.7/2.0 \( \times \) 10^–4 [H⁺] in water”.

when volume of alkali is half equivalence volume/volume required for neutralization;

\({\text{p}}{K_{\text{a}}}\) is equal to the pH;

\([{{\text{H}}^ + }] = {10^{ - 3.7}} = 2.00 \times {10^{ - 4}}{\text{ }}({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}})\);

\({K_{\text{a}}} = \frac{{[{{\text{H}}^ + }][{{\text{Q}}^ - }]}}{{[{\text{HQ}}]}} = \frac{{{{(2.00 \times {{10}^{ - 4}})}^2}}}{{0.100}}\);

\( = 3.98 \times {10^{ - 7}}\);

\({\text{p}}{K_{\text{a}}} = 6.4\);

Award [4] for correct final answer.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.

Most students could identify an element which would be expected to have similar properties to antimony and the reaction between its fluoride and hydrogen fluoride was generally well interpreted in terms of acid-base theories, though hardly any students realized that HF is acting as both a base (to give \({{\text{H}}_{\text{2}}}{{\text{F}}^ + }\)) and an acid (to give \({{\text{F}}^ - }\) which complexes with \({\text{As}}{{\text{F}}_{\text{5}}}\)). The significance of the strength of the hydrogen-halogen bond on the strength of the hydrogen halides was often appreciated though very few seemed to realize that HF hydrogen bonds to water whereas HCl does not. Many students think that weak acids require a smaller volume of alkali for neutralization than strong acids of equal concentration, though most could correctly identify an appropriate indicator for the titration and justify their choice. Most realized that sulfuric acid was dibasic, were aware of the significance of the pH at the half equivalence point and correctly identified HQ as a weak acid, though justifying this proved more of a challenge. Quite a few students gained full credit the calculation of the \({\text{p}}{K_{\text{a}}}\) from the initial pH and many gained some marks for the calculation of the pH of the buffer system.