Relative formula mass, #"M"_r#, is determined by multiplying the subscript of each element in the formula by its relative atomic mass, #"A"_r# on the periodic table. The results for each element are added together to calculate the relative formula mass. The unified mass unit or dalton, u or Da, is applied to the relative formula and atomic masses, though they actually have no unit.

**Example:** What is the relative formula mass for sodium sulfate, #"Na"_2"SO"_4# using whole numbers for the #"A"_r#.

#"M"_r " Na"_2"SO"_4"##=(2xx"23 u Na")+(1xx"32 u S")+(4xx"16 u O")="142 u Na"_2"SO"_4#

When working with moles, the relative atomic and formula masses are numerically equal the molar mass. Molar mass is the mass in grams of one mole of a substance. So the mass of one mole of sodium, #"23 u"#, would become #"23 g/mol sodium"#, and the mass of one mole of sodium sulfate, #"142 u"#, would be #"142 g/mol"#.