Testing for carbonates (CO3^2-)
3 marks · NormalA student tests 0.30 g of potassium carbonate ([K\ce{K2CO3}) powder with excess dilute hydrochloric acid. All of the carbonate reacts.
Write the chemical equation, including state symbols, for the reaction between potassium carbonate and hydrochloric acid.
Calculate the number of moles of potassium carbonate in 0.30 g. (Relative formula mass, , of \ce{K2CO3} = 138.0.)
Calculate the volume of carbon dioxide gas produced at room temperature and pressure (RTP), in cm. (1 mole of gas at RTP occupies 24 000 cm.)
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(a) \ce{K2CO3 (s) + 2HCl (aq) -> 2KCl (aq) + CO2 (g) + H2O (l)}
(b) 0.00217 mol
(c) 52.1 cm^3
Common mistakes: Incorrect formula or state symbols; Wrong significant figures; Error in ratio of moles
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Marking points
B1 (a) · 1 mark · Correct balanced equation with state symbols: \ce{K2CO3 (s) + 2HCl (aq) -> 2KCl (aq) + CO2 (g) + H2O (l)}
M1 (b) · 1 mark · Calculation: moles = mass/Mr; 0.30 / 138.0 = 0.00217 mol. Allow follow-through if incorrect Mr used. (follow-through)
A1 (c) · 1 mark · Calculates volume of \ce{CO2} using number of moles from (b): volume = moles = 52.1 cm (accept answers rounding to 52 cm). Allow error carried forward from (b). (follow-through)
Worked solution
(a) The balanced equation is:
(b) Moles of potassium carbonate:
(c) Volume of CO2: