# How do you determine the formula weight of "Na"_2"CO"_3" and "Ca"_3("PO"_4)_2"?

Jun 30, 2017

See the explanation.

#### Explanation:

To determine the formula weight of a substance, multiply each element's subscript by its atomic weight on the periodic table, then add the result. Reminder: no subscript is understood to be $1$.

Formula weight of $\text{Na"_2"CO"_3} :$

(2xx22.990 "u Na")+(1xx12.011 "u C")+(3xx15.999 "u O")="105.998 u Na"_2"CO"_3",

where $\text{u}$ is the symbol for unified mass unit.

Formula weight of ${\text{Ca"_3"(PO"_4)}}_{2} :$

(3xx40.078 "u Ca")+(2xx30.974 "u P")+(8xx15.999 "u O")="310.174 u Ca"_3"(PO"_4)"_2

If you need molar masses, replace $\text{u}$ with grams/mole, or g/mol.