Question

What is the formula weight?

Answer 1

This is simply a calculation of the mass of one mole of a given substance.

Explanation:

Let's take a random formula, `CaCO_3`.

1 mole of calcium has a mass of `40.1*g`; 1 mole of carbon has a mass of `12.01*g`; 3 mole of oxygen has a mass of `3xx15.999*g`. Sum them together, and we get a so-called `"formula mass"` of `100.12*g*mol^-1` (i.e. units are `g*mol^-1` or `g*("Avogadro's Number")^-1` of reagent).

From where did I get the atomic masses? Did I know them from memory? (In fact I did, and you'll get to know the common ones very rapidly.) This operation is something that you will do routinely and rapidly.