IB Chemistry HL 3.1.4 Ph Scale Questions

Use real HL exam questions to calculate pH and hydrogen-ion concentration, compare acidity and report logarithmic results correctly.

Exam points

Calculate pH from a stated hydronium concentration using −log10[H3O+].
Calculate hydrogen-ion concentration from pH using [H+] = 10^−pH and compare ratios on the logarithmic scale.
Use pH and acid/base identity to compare acidity and report hydrogen-ion concentration with correct units and significant figures.

Learn this topic

Study 3.1.4—pH scale conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 3]

A group of students investigated the rate of the reaction between aqueous sodium thiosulfate and hydrochloric acid according to the equation below.

\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{NaCl}(\mathrm{aq})+\mathrm{SO}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(1)

The two reagents were rapidly mixed together in a beaker and placed over a mark on a piece of paper. The time taken for the precipitate of sulfur to obscure the mark when viewed through the reaction mixture was recorded.

Figure for Question IB Chemistry HL 3.1.4 Ph Scale Questions question 1 — IB Chemistry HL

Initially they measured out $10.0 \mathrm{~cm}^{3}$ of $0.500 \mathrm{moldm}^{-3}$ hydrochloric acid and then added $40.0 \mathrm{~cm}^{3}$ of $0.0200 \mathrm{~mol} \mathrm{dm} \mathrm{m}^{-3}$ aqueous sodium thiosulfate. The mark on the paper was obscured 47 seconds after the solutions were mixed.

(a)

The teacher asked the students to devise another technique to measure the rate of this reaction.

[ 3 ]

(i)

One group suggested recording how long it takes for the pH of the solution to change by one unit. Calculate the initial pH of the original reaction mixture.

[ 2 ]

$\left[\mathrm{H}^{+}\right]=0.5 \times \frac{10}{50}=0.1\left(\mathrm{moldm}^{-3}\right);$ $\mathrm{pH}\left(=-\log \left[\mathrm{H}^{+}\right]=-\log (0.10)\right)=1 ;$

(ii)

Deduce the percentage of hydrochloric acid that would have to be used up for the pH to change by one unit.

[ 1 ]

$\quad 90 \%$ ;

All question bank results loaded

Learn this topic

IB Chemistry HL 3.1.4 Ph Scale Questions question 1

Question (a)

Question (i)

Question (ii)