Chemistry · May / June 2025 · Paper 61 · Question 1
0620
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The question as text
A student prepares some crystals of pure potassium sulfate by reacting aqueous potassium carbonate with dilute sulfuric acid. The equation for the reaction is shown. K2CO3(aq) + H2SO4(aq) K2SO4(aq) + H2O(l) + CO2(g) The student: • places 25.0 cm3 of aqueous potassium carbonate in a conical flask • adds a few drops of a suitable indicator to the conical flask • uses a burette to add dilute sulfuric acid to the conical flask until the end-point is reached and the indicator changes colour. Table 1.1 shows the burette readings the student obtains. Table 1.1 final burette reading / cm3 18.8 initial burette reading / cm3 4.2 volume of dilute sulfuric acid added / cm3
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Show the answer and mark scheme
1(a)[1]
14.6
1(b)[1]
(least concentrated solution) potassium carbonate
and
(because) a larger volume of potassium carbonate was used
1(c)[1]
(volumetric) pipette / burette
1(d)[2]
M1 methyl orange
M2 yellow to orange
1(e)[1]
swirl / mix (the contents of the flask)
1(f)[3]
M1 mix / add / combine together 25.0 cm3 aqueous potassium carbonate and 14.6 cm3 dilute sulfuric acid (without indicator) M2 (place in an evaporating basin) and warm until crystals start to form / warm until crystalising point / warm until saturated / half / most water gone
M3 cool (solution) (so that crystals form) / leave solution from M2 so that crystals form
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