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?

1 Answer
Dec 1, 2016

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


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 #"stoichiometry"#, i.e. #"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(s) + O_2(g) rarr CO_2(g)#

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?