Final answer:
In a reaction of 10 cm³ hydrogen with 20 cm³ oxygen, the correct amount of residual gas after cooling is 10 cm³ of oxygen, since half of the provided oxygen remains unreacted.
Step-by-step explanation:
The question involves a chemical reaction between hydrogen gas and oxygen gas to produce water vapor. According to the balanced chemical equation, 2H2(g) + O2(g) → 2H2O(g), two volumes of hydrogen react with one volume of oxygen. In this case, 10 cm³ of hydrogen would require only 5 cm³ of oxygen to fully react.
Since 20 cm³ of oxygen are provided, only half (10 cm³) will be used, leaving 10 cm³ of oxygen unreacted. Therefore, after the reaction and cooling, there will be a total of 10 cm³ of residual oxygen gas.