To convert mass into moles or moles into mass, we first need to learn how to calculate the molar mass of the compound.

To calculate molar mass, we take the number of atoms of each element and multiply it by the mass of the element found on the Periodic Table. We then add them together to get the molar mass.

Let's take a look at calculating molar mass for H2SO4. The molecule has H, S and O. There are 2 atoms of hydrogen, 1 atom of sulfur and 4 atoms of oxygen. Then, we look up the molar mass of each element on the Periodic Table and multiply by the number of atoms. Finally, we add it altogether to get the molar mass.

**Converting Grams to Moles**

To convert grams to moles, we need to divide the number of grams given by the molar mass.

**Example:** Convert 15 grams of H2SO4 to moles

We start with the given: 15 grams. Then we put multiplication sign and dash. On the bottom we put the units we want to get rid of, grams. On top, we put the units we want to get, moles. How do we know what numbers to use for this? Remember converting grams to moles or moles to grams involves molar mass. Molar mass always has a certain number of grams in 1 mole. Put 1 next to mole and the number of grams on the bottom. Then we divide 15 by 98.072 to get the answer.

**Converting Moles To Grams**

To convert moles to grams we multiply the number of grams by the molar mass.

**Example:** Convert 15 moles of H2SO4 to grams

Practice

Convert 10.0 g of ethanol, C2H5OH into moles

10.0 g C2H5OH (1 mol C2H5O/46.1 g C2H5OH) = **0.217 mol C2H5OH**

A chemist determines from the amounts of elements that 0.0654 mol ZnI2 can form. How many grams of zinc iodide is this?

0.0654 mol ZnI2 ( 319 g ZnI2 / 1 mol ZnI2) = **20.9 g ZnI2**