# If the masses of all but one of the substances that take part in a chemical reaction are known, why is it possible to determine the unknown mass by subtraction?

Dec 1, 2016

Because mass is always conserved in a chemical reaction.......

#### Explanation:

So if I start with a 10 g mass of reactants, from all sources, AT MOST, I can get a 10 g mass of products. In practice, I am not even going to get that 10 g because losses always occur on handling.

All of this illustrates the principle of $\text{stoichiometry}$, i.e. $\text{GARBAGE IN EQUAL GARBAGE OUT}$.

For simple combustion reactions, e.g that of coal, I only need to know the starting mass of the coal. If the coal combusts completely, I know how much dioxygen is consumed as a reactant, and how much carbon dioxide is evolved:

$C \left(s\right) + {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right)$

If I combust 10 g of coal, how much carbon dioxide do I get out?

Is mass always conserved in a nuclear reaction? Why or why not?