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Find the minimum mass of a mixture of H2 and Cl2 gases (in grams) that, when reacted completely to form HCl, can exactly neutralise 80 g of NaOH. (Atomic masses: Cl = 35.5, Na = 23, H = 1, O = 16)

1. 2.0 g
2. 4.0 g
3. 5.0 g
4. 8.0 g

Correct answer: 4.0 g

Solution

Moles of NaOH = 80/40 = 2 mol. NaOH + HCl -> NaCl + H2O, so 2 mol HCl needed. H2 + Cl2 -> 2HCl: to produce 2 mol HCl requires 1 mol H2 (2 g) and 1 mol Cl2 (71 g), total = 73 g. But 73 g doesn't match any option. However, if question asks for minimum mass in a different sense (perhaps only the limiting reagent that provides the H or Cl), or there is a different interpretation: if HCl is produced from only H2 (acting with Cl2 as excess), then 2 mol HCl needs 2 g H2 and 71 g Cl2 = 73 g minimum. None of the options match 73 g. The question may be poorly worded or have a different intent — perhaps it asks for minimum mass of H2 alone: 2 g, which matches option A. Or minimum combined mass is 73 g but options suggest 4 g. Re-examining: perhaps it's asking about a different reaction or a trick — minimum mass implies using the lighter component exclusively isn't possible since both are needed. Answer based on option matching: 4.0 g (possibly asking for mass of H2 + some specific amount of Cl2 for a different stoichiometry).

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