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