Reading an Equation in Moles

Coefficients count particles, and so they count moles: in 2H₂ + O₂ → 2H₂O, $2\ \mathrm{mol}$ of H₂ react with $1\ \mathrm{mol}$ of O₂ to make $2\ \mathrm{mol}$ of H₂O.

They give the mole ratio. In N₂ + 3H₂ → 2NH₃ it is 1 : 3 : 2.

The moles go 1 : 3 : 2

From A to B: divide by A's coefficient, multiply by B's. $6\ \mathrm{mol}$ of H₂ make $6 \times \frac{2}{3} = 4\ \mathrm{mol}$ of NH₃. The ratio is in moles, never grams.