Question

How do you calculate formula mass?

Answer 1

o calculate formula mass, multiply the subscript of each element in the formula by the element's atomic weight (relative atomic mass) found on the periodic table.

Explanation:

To calculate formula mass, multiply the subscript of each element in the formula by the element's atomic weight (relative atomic mass) found on the periodic table.

The unit for formula mass is either u or Da (Daltons). The amu has been replaced with u, even though it is still commonly used.

Example
What is the formula mass of potassium nitrate, `"KNO"_3"`.

`(1xx39.0983" u K")+(1xx14.007 " u N")+(3xx15.999" u O")="10.102 u"`

If you are asked to determine the molar mass of a formula, all you have to do is change u to g/mol.