# How do you convert grams to moles and moles to grams?

Sep 28, 2014

You create a conversion factor from the molar mass and use it in your calculations.

Example 1.

Convert 25.0 g of water to moles of water.

We know that 1 mol H₂O = 180.2 g H₂O.

We can use this to create the conversion factors $\left(\text{1 mol H"_2"O")/("18.02 g H"_2"O}\right)$ and $\left(\text{18.02 g H"_2"O")/("1 mol H"_2"O}\right)$.

We choose the one that gives us the correct units for the answer. So we choose the first one and write

25.0 g H₂O ×$\left(\text{1 mol H"_2"O")/("18.02 g H"_2"O}\right)$ = 1.39 mol H₂O

Example 2.

Convert 1.39 mol of water to grams of water.

Here we use the other conversion factor.

1.39 mol H₂O × $\left(\text{18.02 g H"_2"O")/("1 mol H"_2"O}\right)$ = 25.0 g H₂O