Law pf combining volumes: if ten volumes of dihydrogen gas react with five valumes of diaxygen gas, how many volumes of water vapor are produced ?
Strategy : According to the law of combining volumes, if the product is a gas (or vapor), its volume is related to the volume of the gaseous reactants by a simple ratio.

Explanation

Accroding to the balanced chemical equation,
`underset(2"volumes")(2H_(2)(g))+underset("1 volumes")(O_(2)(g))rarrunderset("2 volumes")(2H_(2)O(g))`
two volumes of dihydrogen gas always react with one volume of dioxygen gas to produce two volumes of water vapor. Thus, ten volumes of dihydrogen gas will react with five volumes of dioxygen gas to produce ten volume of water vapor.