Mass of Mole of Substance

Theorem
Let $S$ be a substance with molecular weight $W_S$.

Then one mole of $S$ has a mass of $W_S$ grams.

Proof
Let $m_S$ grams be the mean mass of one molecule of $S$.

Let $m_C$ grams be the mass of one atom of carbon-12.

Let $M_S$ grams be the mass of one mole of $S$.

Let $M_C$ grams be the mass of one mole of carbon-12.

Then:

Before $2018$, the mole was defined as the number of atoms in $12$ grams of carbon-12.

While the mole is no longer so defined, the mass of $1$ mole of carbon-12 is still $12$ grams, for all practical purposes.

Thus we have: