Chemistry HL Paper 1B, specimen 2025

Deduce the oxidation state of chlorine in each of the three chlorine-containing species in the equation. $\ce{Cl2}$: $\ce{HOCl}$: $\ce{Cl-}$:

The pool operators add sodium carbonate to the water to raise its pH. Explain why a larger proportion of the dissolved chlorine reacts with water after the sodium carbonate has been added.

A student writing a report on chlorine as a disinfectant collects, from several websites and reference books, values for how much chlorine dissolves in pure water at various temperatures. Source Temperature / °C Solubility of chlorine A 5 1.18 g per 100…

Most pools, including this lido, are dosed with sodium hypochlorite solution, $\ce{NaOCl(aq)}$, instead of chlorine gas. Suggest why pool operators avoid dosing the water with chlorine gas itself.

Hypochlorous acid is a weak acid. In water it is in equilibrium with its conjugate base, the hypochlorite ion, $\ce{OCl-}$, which kills microorganisms far less effectively than the acid molecule does. The graph shows what percentage of the dissolved chlorine…

Bathers release nitrogen-containing compounds, including ammonia, into the pool water. Ammonia reacts with hypochlorous acid to form chloramines. (i) Deduce an equation for the reaction of ammonia with hypochlorous acid that forms trichloramine,…

Some users have asked for the water to be heated in winter. Suggest two reasons why leaving the water unheated is the better choice for the environment.

Identify two controlled variables in this investigation.

The student measures the hydrogensulfite concentration by titrating samples against a standard iodine solution, with starch as the indicator. Every three days, 25.00 $\mathrm{cm^3}$ of cordial is withdrawn from each flask, diluted to 250.0 $\mathrm{cm^3}$ with…

The following burette readings are recorded in one of the titrations. Final burette reading = 26.15 ± 0.05 $\mathrm{cm^3}$ Initial burette reading = 1.15 ± 0.05 $\mathrm{cm^3}$ Calculate the percentage uncertainty in the titre volume.

The student's results are plotted below. (i) Calculate the average rate at which the hydrogensulfite concentration in the 24 h flask decreased over the 12 days. Include units in your answer. (ii) Before starting, the student wrote this hypothesis: "The longer…

Suggest one way of extending the investigation so that it could lead to additional advice on storing cordial.

By drawing a suitable line on the graph, determine the rate at which carbon dioxide is released at the start of Experiment 1. Give your answer in $\mathrm{cm^3\,s^{-1}}$.

Estimate how long after the start of the experiment the piston would first move if a total mass of 750 g were loaded on it.

Suggest two reasons why less carbon dioxide is collected in the syringe when a greater mass rests on the piston.

Use the volumes collected at 120 s in Experiment 1 and Experiment 3 to calculate by what percentage the final volume of carbon dioxide falls for each 100 g added to the piston.

Sketch how the combined mass of the flask, the syringe and their contents would change with time during Experiment 1, and give a reason for the shape of your sketch. Label the axes 'Total mass' and 'Time'.