`20 g` of hydrogen and `128g` of oxygen are contained in a `20 L` flask at `200^(@)C`. Calculate the total pressure of the mixture. If a spark ignites the mixture, what will be the final pressure?

Explanation

Moles of `H_(2)=(W)/(Molar weight)=(20)/(2)=10 mol`
Moles of `O_(2)=(W)/(Molar weight)=(128)/(32)=4 mol`
Total pressure
`P=P_(H_(2))+O_(O_(2))`
`=(n_(H_(2))+n_(O_(2)))(RT)/(V)`
`=(10+4)((0.082xx473))/(20)`
`=27.15 atm`
`2H_(2)(g)+O_(2)(g) to 2H_(2)O(g)`
at the temperature of the spark.
Hence, `4 mole` of oxygen will react with `8 mol` of hydrogen to give `8 mol` of water(g), and `2 mol` of hydrogen will remain unreacted.
Final pressure `=(n_(H_(2))+n_(O_(2)))(RT)/(V)`
`=((2+8)(0.082)xx473)/(20)=19.39 atm`