Limiting Reactant in a School Hydrogen Preparation
A student in Kozhikode is asked to prepare hydrogen gas using 5.0 g of zinc granules labelled ‘80% pure’ and 100 mL of 2.0 M hydrochloric acid. She calculates the expected volume from the acid and predicts 2.24 L of hydrogen at STP. She adds the zinc to the acid in a conical flask, collects the gas over water, and obtains 1.2 L at STP. The reaction is Zn + 2HCl → ZnCl2 + H2. She is surprised because she expected the acid to be completely used up. Her teacher asks her to check which reactant is actually limiting before blaming the apparatus. The zinc granules are known to contain sand and zinc oxide as impurities, and some gas escapes before the delivery tube is fitted.
1. Determine the limiting reactant and calculate the theoretical volume of hydrogen at STP.
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Pure Zn mass = 5.0 g × 0.80 = 4.0 g. Moles Zn = 4.0/65.4 = 0.0612 mol. Moles HCl = 0.100 L × 2.0 mol/L = 0.200 mol. From Zn + 2HCl → ZnCl2 + H2, 0.0612 mol Zn needs 0.1224 mol HCl. Only 0.200 mol HCl is present, so HCl is in excess; Zn is limiting. Moles H2 = moles Zn = 0.0612 mol. Volume at STP = 0.0612 × 22.4 L = 1.37 L (about 1.4 L). The student's 2.24 L would require 0.100 mol H2, which would need 0.100 mol Zn (6.54 g pure Zn) and 0.200 mol HCl; she does not have enough pure zinc. So the prediction based on acid alone was wrong.
2. The student obtained 1.2 L instead of 1.37 L. Give a reasoned judgement on the likely causes, distinguishing between calculation error and experimental loss.
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The difference is 0.17 L, about 12% below the theoretical value. This is an experimental yield problem, not a mole-ratio error, because the limiting-reactant calculation already accounts for the 80% purity. Likely causes: (i) the label ‘80% pure’ is approximate, so pure Zn may be less than 4.0 g; (ii) some hydrogen escaped before the delivery tube was fitted; (iii) gas collected over water is saturated with water vapour, so the measured volume at STP is affected; (iv) reaction may be incomplete if zinc granules are coated with oxide or if the acid is not fully mixed. The student should not blame the apparatus until she checks the purity and the gas-collection method. A yield of 1.2 L is plausible for a school setup.
3. A classmate says, ‘Just add more zinc and you will get 2.24 L.’ Evaluate this suggestion.
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Partly correct, but only up to the HCl limit. Zinc is limiting, so adding more pure zinc will increase hydrogen until the 0.200 mol HCl is used. That requires 0.100 mol Zn = 6.54 g pure Zn, i.e. about 8.2 g of the 80% material. The student started with 5.0 g, so adding about 3.2 g more of the same impure zinc could raise the yield towards 2.24 L. Beyond that point, extra zinc has no effect because HCl is exhausted. Also, adding more zinc does not fix gas leakage or incomplete reaction. The suggestion is therefore conditionally valid, not a general cure.