Question

A 12. 2 g sample of X reacts with a sample of Y to form 78.9 g of XY. What is the mass of Y that reacted?

Answer 1

`"Mass"` `=` `78.0*g-12.2*g=?`

Explanation:

All chemical reactions CONSERVE mass. What does this mean?

It means that if I start with `10 *g` of reactants, from all source, AT MOST I can get `10*g` of product.

`"Garbage in equals Garbage out!"`

Here I finished with a `78.9*g` mass of `XY`. There were `12.2*g` of `X`. And the balance must represent the mass of `Y`.