A balance weighs grams. Chemistry reacts in whole numbers of particles. The mole is the bridge, and crossing it costs one multiplication in each direction.

$n = \dfrac{m}{M}$ — read aloud n equals m over M. $n$ is the amount of substance in moles, $m$ is the mass you weighed in grams, and $M$ is the molar mass in grams per mole, a property of the substance that you look up rather than measure. The unit does the arguing for you: grams divided by grams per mole leaves moles, and nothing else on the page divides that way.

The second rate is $N = n\,N_A$ — N equals n times N-A. $N$ is the number of particles, a bare count, and $N_A$ is the Avogadro constant, $6.022 \times 10^{23}$ particles per mole, fixed by definition since 2019. Note the direction: $N_A$ multiplies a number of moles. It has never multiplied a mass, and a mass fed into it comes back meaningless by a factor of the molar mass.

Scale is the point of this chapter, so get a feel for it. A 25 kg sack of caustic soda is 625 moles, and it is also about $3.8 \times 10^{26}$ formula units. Nobody counts the second number, and every dosing calculation on the plant runs on the first.

The nugget worth carrying: a mole is a count, not a mass. A mole of caustic soda is 40 g and a mole of limestone is 100 g, and they contain exactly the same number of formula units. That is the whole reason stoichiometry works in moles and never in kilograms.