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A 40 mL of a mixture of H2 and O2 was placed in a gas burette at 18 oC and  1 atm pressure. A spark was supplied so that the formation of water was complete . The remaining pure gas had a volume of 10 mL at 18 oC  and 1 atm pressure. If the remaining gas was H2 , what was the initial mole percentage of H2 in the mixture?

1. 25%
2. 50%
3. 35%
4. 75%