IB Chemistry HL 3.1.4 pH scale Question Bank

Practise IB Chemistry HL 3.1.4 by applying ph scale to exam-style questions.

[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 3.1.4—pH scale 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 ]

$\quad 90 \%$ ;

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3.1.4—pH scale question 1

Question (a)

Question (i)

Question (ii)