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$2H_2(g)+O_2(g) rarr 2H_2O(l)$

$36*g$ of reactants, and $36*g$ of products..........

VERSUS.......

$H_2(g) + 1/2O_2(g)rarr H_2O(l)$

$18*g$ of reactants, and $18*g$ of products..........

Is mass conserved in each reaction? Well, clearly mass IS conserved. And while of course I cannot have half a molecule of dioxygen gas, I can certainly have a $16*g$ quantity of $O_2(g)$, i.e. a HALF A MOLE quantity of dioxygen gas. The use of such fractional coefficients often makes the arithmetic involved in a bit easier, which of course is why they are used.