# Why do we need to convert mass to moles in stoichiometry problems?

Aug 16, 2016

Because we need to establish equivalence. Reacting molecules have specific masses.

#### Explanation:

We represent a chemical reaction of the basis of molecules, i.e.:

${H}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow {H}_{2} O \left(l\right)$

But we perform a chemical reaction on the basis of grams and litres. Here $2 \cdot g$ of dihydrogen gas reacts with $16.0 \cdot g$ dioxygen gas to give $18.0 \cdot g$ water. Moles to mass and mass to moles conversions are thus vital to ideas of equivalence.

The starting point is still the chemical equation, but the reactants and products have definite and measurable masses.