Thermochemistry measures energy PER MOLE, so every problem this chapter opens the same way: turn what you weighed into what you counted. The relation is $n = \dfrac{m}{M}$ — read aloud: n equals m over M. Letter by letter: $n$ is the amount of substance in moles, $m$ is the mass you actually weighed, in grams, and $M$ is the molar mass in grams per mole — a property of the substance, printed in the data table, never something your sample decides. Whichever of the three the question leaves blank is the one you solve for.

One rung further up sits $N = n\,N_A$ (N equals n N-A), where $N$ is the raw number of particles — molecules, atoms, ions — and $N_A$ is the Avogadro constant, $6.02 \times 10^{23}$ particles per mole. Note the subscript convention that runs all chapter: a letter subscript names a THING ($N_A$ is Avogadro's, $\Delta H_f$ is formation), while a number subscript names a STEP. So the ladder reads grams → moles → particles, and the mole is always the rung in the middle. Nobody crosses from grams to molecules in one jump — not on this paper, not in a real lab.