Limestone Purity and an Acid Spill
A technician at a quarry donates 2.00 g samples of crushed limestone to a school for a neutralisation demonstration. The label says the rock is 'at least 90% calcium carbonate'. A student adds 50.0 cm³ of 1.50 mol/dm³ hydrochloric acid, immediately fits a bung, and collects the carbon dioxide over water. At 25 °C and 101 kPa the molar volume of a gas is 24.5 dm³/mol. The student obtains 430 cm³ of gas. While fitting the bung, a few bubbles escape, and some gas dissolves in the water. The teacher wants to know whether the quarry's claim is supported and whether the limestone is suitable for treating a small acid spill on the concrete apron. A second student argues that 'excess acid proves the limestone is pure'.
1. Calculate the volume of carbon dioxide that would be expected if the 2.00 g sample were pure calcium carbonate, and use the measured volume to estimate the percentage purity. Show each step.
Read the worked answer
Equation: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + CO2(g) + H2O(l). Mr(CaCO3) = 40 + 12 + 3(16) = 100. Amount of CaCO3 if pure = 2.00 g / 100 g/mol = 0.0200 mol. Amount of HCl used = 0.0500 dm3 × 1.50 mol/dm3 = 0.0750 mol. This is more than the 0.0400 mol needed, so CaCO3 is limiting. From the equation, 1 mol CaCO3 gives 1 mol CO2, so expected CO2 = 0.0200 mol. Expected volume = 0.0200 × 24.5 dm3/mol = 0.490 dm3 = 490 cm3. Estimated purity = (430/490) × 100% = 87.8%. This assumes all the CO2 is collected and measured at the stated conditions.
2. The quarry claims at least 90% calcium carbonate. Does the result support the claim? Explain how the two sources of error affect your answer and describe one improvement.
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The direct estimate is 87.8%, below 90%, but the result does not prove the claim false. Escaped bubbles and dissolved CO2 both make the measured volume smaller than the true volume, so the true purity is higher than 87.8%. It could therefore be above 90%. To decide, improve collection: use a gas syringe, fit the bung before adding acid through a dropping funnel, or saturate the water with CO2 first. Repeat and average the results. The acid being in excess only shows that carbonate is present; it does not show purity, because excess acid was used deliberately.
3. The teacher wants to neutralise 2.0 dm3 of 0.50 mol/dm3 sulfuric acid spill with 1.0 kg of this limestone. Using the estimated purity, decide whether there is enough limestone. Include an equation and comment on one practical limitation.
Read the worked answer
Equation: H2SO4 + CaCO3 -> CaSO4 + CO2 + H2O. Amount of H2SO4 = 2.0 × 0.50 = 1.0 mol. This needs 1.0 mol CaCO3 = 100 g pure CaCO3. 1.0 kg at 87.8% contains 878 g CaCO3 = 8.78 mol, nearly nine times the amount required. So there is more than enough on paper. Practical limitation: calcium sulfate is sparingly soluble and can coat lumps, slowing neutralisation. The limestone should be powdered and mixed, and the spill contained. Safety: the acid is corrosive; use PPE.