Quick questions on The mole and chemical formulae: N(A)-Level Combined Science Chemistry

Atoms are far too small and light to weigh one at a time, so chemists compare their masses. The relative atomic mass ($A_r$) of an element tells you how heavy its atoms are compared with other atoms. You read these values from the periodic table, for example $\text{H} = 1$, $\text{C} = 12$, $\text{O} = 16$.

A mole is simply a fixed, very large number of particles, in the same way that a dozen is $12$ of something. The clever part is that one mole of any element has a mass in grams equal to its relative atomic mass. So one mole of carbon weighs $12\ \text{g}$, and one mole of oxygen atoms weighs $16\ \text{g}$. This lets us count atoms by weighing.

The relative formula mass ($M_r$) of a compound is found by adding up the relative atomic masses of all the atoms in its formula. Remember to multiply by the small numbers in the formula:

The link between mass, moles and relative formula mass is:

The formula of a compound tells you exactly which atoms it contains and how many of each. The small numbers (subscripts) apply only to the symbol just before them: