Question

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

Answer 1

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 `("CaCl"_2")`?

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

`"Ca":` `(1xx"40.078 g/mol")="40.078 g/mol"`

`"Cl":` `(2xx"35.45 g/mol")="70.90 g/mol"`

`"CaCl"_2":` `"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.