Final answer:
After sparking 10cm³ of hydrogen with 20cm³ of oxygen and cool down, the residual gas would be 15cm³ of oxygen, as only 5cm³ would react with the hydrogen provided, leaving 15cm³ unreacted. The correct option is c.
Step-by-step explanation:
When 10cm³ of hydrogen gas is sparked with 20cm³ of oxygen gas, this entails a chemical reaction where hydrogen and oxygen react to form water vapor (H₂O). The balanced chemical equation for this reaction is:
2H₂ + O₂ → 2H₂O
According to the stoichiometry of the reaction, 2 volumes of hydrogen react with 1 volume of oxygen. This means the 10cm³ of hydrogen will react completely with 5cm³ of oxygen (half of the hydrogen volume), as this would maintain the stoichiometric ratio.
Since there is 20cm³ of oxygen, which is in excess, and only 5cm³ is used, we are left with 15cm³ of unreacted oxygen gas after the reaction. Therefore, after the reaction and cooling to retrieve only the residual gas volume (since water has condensed), the volume of the residual oxygen gas would be 15cm³. The correct answer is C. 15cm³ of O₂. The correct option is c.