DP Chemistry (last assessment 2024)

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Question 18M.3.sl.TZ1.2c.i

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Date May 2018 Marks available [Maximum mark: 1] Reference code 18M.3.sl.TZ1.2c.i
Level sl Paper 3 Time zone TZ1
Command term Deduce, Justify Question number c.i Adapted from N/A
c.i.
[Maximum mark: 1]
18M.3.sl.TZ1.2c.i

Students were asked to investigate how a change in concentration of hydrochloric acid, HCl, affects the initial rate of its reaction with marble chips, CaCO3.

They decided to measure how long the reaction took to complete when similar chips were added to 50.0 cm3 of 1.00 mol dm−3 acid and 50.0 cm3 of 2.00 mol dm−3 acid.

Two methods were proposed:

(1)     using small chips, keeping the acid in excess, and recording the time taken for the solid to disappear

(2)     using large chips, keeping the marble in excess, and recording the time taken for bubbles to stop forming.

(c.i)

Deduce, giving a reason, which of the two methods would be least affected by the chips not having exactly the same mass when used with the different concentrations of acid.

[1]

Markscheme

method 2 AND marble is in excess «so a little extra has little effect»

OR

large chips AND marble is in excess «so a little extra has little effect»

OR

method 2 AND HCl is limiting reagent «so a little extra marble has little effect»

OR

large chips AND HCl is limiting reagent «so a little extra marble has little effect»

 

Accept, as a reason, that “as the mass is greater the percentage variation will be lower”.

[1 mark]

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