The concentration of hydrogen peroxide may be given in moldm-³ or as 'volume strength'. You will determine the concentration of hydrogen peroxide in moldm-³ and in 'volume strength' by a gas collection method. Hydrogen peroxide decomposes to form water and oxygen. The reaction is much faster in the presence of a catalyst such as manganese(IV) oxide. 2H2O2(aq) → 2H2O(l) + O2(g) 'Volume strength' is defined as the volume of oxygen in cm³ produced from the decomposition of 1.0 cm³ of hydrogen peroxide at room temperature and pressure. For example, 1.0cm³ of '100 volume' hydrogen peroxide will produce 100 cm³ of oxygen. FA 1 is a solution of hydrogen peroxide, H2O2. FA 2 is manganese(IV) oxide, MnO2.

1. (a)Method Read the whole method before starting any practical work. The diagram below may help you in setting up your apparatus. [Figure X.X] • Fill the tub with water to a depth of about 5 cm. • Fill the 250 cm³ measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. • Remove the paper towel and clamp the inverted measuring cylinder so that the open end is in the water just above the base of the tub. • Rinse the 50 cm³ measuring cylinder with a little FA 1 then use it to transfer 150 cm³ of FA 1 into the reaction flask labelled X. • Check that the bung fits tightly in the neck of flask X, clamp flask X and place the end of the delivery tube into the inverted 250 cm³ measuring cylinder. • Remove the bung from the neck of the flask. Tip FA 2 into the hydrogen peroxide and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. Swirl the flask occasionally until no more gas is given off. Replace the flask in the clamp. • Measure and record the final volume of gas in the measuring cylinder in the space below. Keep FA 1 for use in Question 2. Result[2]
2. (b)(i)Use the information on page 2 to calculate the 'volume strength' of FA 1. 'volume strength' of FA 1 =[1]
3. (b)(ii)Calculate the number of moles of oxygen collected in the measuring cylinder. [Assume 1 mole of gas occupies 24.0 dm³ under these conditions.] moles of O₂ =[1]
4. (b)(iii)Using your answer to (ii) calculate the number of moles of hydrogen peroxide in the volume of FA 1 added to flask X. moles of H₂O₂ =[1]
5. (b)(iv)Calculate the concentration of hydrogen peroxide, FA 1, in moldm⁻³. concentration of H₂O₂, FA 1 = mol dm⁻³[1]
6. (c)(i)A source of error in this experiment is that some oxygen escapes before the bung can be inserted. Suggest a change to the practical procedure given in (a) to reduce this source of error. You may draw a diagram as part of your answer.[1]
7. (c)(ii)The error in reading a 50 cm³ measuring cylinder is ±0.5 cm³. Calculate the maximum percentage error in the volume of hydrogen peroxide added to flask X in (a). maximum percentage error in volume of H₂O₂ = %[2]
8. (c)(iii)Explain why the presence of 20 cm³ of air in the 250 cm³ measuring cylinder before the start of the experiment would decrease the accuracy of the results obtained in (a).[1]
9. (d)If you repeated the method described using half the mass of FA 2, what volume of gas would you expect to collect? Explain your answer.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.