# How do you determine the mass of a substance when given the moles of the substance?

Nov 1, 2017

Multiply the number of moles of the substance by its molar mass in grams/mole (g/mol).

#### Explanation:

Example:

What is the mass of $3.25$ moles of calcium chloride $\left(\text{CaCl"_2}\right)$?

Multiply the number of moles of the substance by its molar mass in grams/mole (g/mol).

The molar mass of calcium chloride is determined by multiplying the subscript for each element by its molar mass (atomic weight in g/mol), and adding the results. You can also look up the molar mass.

Molar masses

$\text{Ca} :$(1xx"40.078 g/mol")="40.078 g/mol"

$\text{Cl} :$(2xx"35.45 g/mol")="70.90 g/mol"

$\text{CaCl"_2} :$$\text{40.078 g/mol + 70.90 g/mol"="110.978 g/mol}$

Determine mass in grams

3.25color(red)cancel(color(black)("mol CaCl"_2))xx(110.978"g CaCl"_2)/(1color(red)cancel(color(black)("mol CaCl"_2)))="361 g CaCl"_2" rounded to three significant figures.