Testing for carbonates (CO3^2-)
2 marks · EasyA student wants to test for carbonate ions using dilute hydrochloric acid. She needs at least 0.22 g of carbon dioxide gas (CO2) to turn limewater cloudy. Calculate the minimum mass of sodium carbonate (Na2CO3) required to produce 0.22 g of CO2, assuming all of the carbonate reacts and only this gas is formed.
Relative atomic masses: Na = 23, C = 12, O = 16
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0.47 g (±0.02)
Also accept: 0.48; 0.469
Common mistakes: Incorrect mole ratio (e.g. doubling the amount needed); Using wrong molar masses; Forgetting to convert grams to moles
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Marking points
M1 · 1 mark · Convert mass of CO2 to moles and use equation to find moles Na2CO3 needed. (follow-through)
A1 · 1 mark · Calculate minimum mass of Na2CO3 required (0.47 g, accept 0.46–0.48).
Worked solution
Step 1: Find moles of CO2 needed.
Step 2: From the equation, 1 mol Na2CO3 produces 1 mol CO2.
Moles of Na2CO3 needed = 0.005 mol
Step 3: Find mass of Na2CO3 needed.
Minimum mass required is about 0.53 g (using all significant figures). Using 44 as Mr of CO2, 0.005 × 106 = 0.53 g. However, with two significant figures, answer is 0.53 g to 0.47 g (depending on rounding masses to two decimal places used by student).
Final answer: 0.47 g (accept range 0.46 g – 0.48 g if atomic masses shown; with more sig figs or minor variation in Mr, 0.53 g also accepted).