Question

At constant temperature and pressure, how do the VOLUMES of gases relate to the number of molecules each volume contains?

Answer 1

At constant temperature, and constant pressure, equal VOLUMES of GAS contain the same number of molecules.

Explanation:

It follows that a DOUBLE volume of gas contains TWICE AS MANY molecules AS a SINGLE volume of gas so long as their temperature and pressure are equivalent (God knows why I am capitalizing CERTAIN words!).

So for the reaction,

`2H_2(g) + O_2(g) rarr 2H_2O(g)`

VOLUME thus corresponds to number of molecules. Accordingly, 2 volumes of dihydrogen REACT with 1 VOLUME of dioxygen to give 2 VOLUMES of WATER GAS, i.e. 200 mL as required. Is this clear?