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A mixture of H2 and O2 gases with a total volume of 55 mL is ignited in a eudiometer tube. After cooling, a contraction of 45 mL is observed. What is a possible composition of the original mixture?

1. 30 mL H2 and 25 mL O2
2. 10 mL H2 and 45 mL O2
3. 35 mL H2 and 20 mL O2
4. None

Correct answer: 35 mL H2 and 20 mL O2

Solution

Reaction: 2H2 + O2 -> 2H2O(l). Gas volume decreases = H2 consumed + O2 consumed = h + h/2 = 3h/2 when H2 is limiting. If O2 is limiting: O2=o, H2 consumed = 2o, contraction = 2o + o = 3o. Case A: h=30, O2=25. H2 limiting: O2 needed = 15 mL < 25. Contraction = 30+15=45. VALID. Case C: h=35, O2=20. O2 limiting: H2 needed = 40 mL > 35. H2 is limiting: O2 needed = 17.5 mL < 20. Contraction = 35 + 17.5 = 52.5 mL != 45. NOT valid. Case B: h=10, O2=45. H2 limiting: O2 needed = 5 < 45. Contraction = 10+5=15 != 45. NOT valid. So the answer is A: 30 mL H2 and 25 mL O2.

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