Question

How do you calculate the molar mass of each of the following compounds?

`Cu(OH)2`
`MgO`
`Mg(NO_3)_2`
`NaCl`
`(NH_4)_2SO_4`
`H_2SO_4`
`Ca_3(PO_4)_2`
`NH_4NO_3`

Answer 1

I will answer one, and from the answer you can use the same method tom work out the rest: `Ca_3(PO_4)_2` = 310.177 g/mol

Explanation:

You need to look up the atomic weight of each element (these can be found by asking google for the atomic weights):

• Ca - calcium - 40.078
• P - phosphorus - 30.973762
• O - oxygen - 15.9994

`Ca_3` means there are three Ca; `PO_4` means there is one P and 4 oxygen, the `()_2` means there two `PO_4`. So, substituting in the values:

`Ca_3(PO_4)_2`
(3 x 40.078) + ((30.973762 + (4 x 15.9994)) x 2)
120.234 + ((30.973762 + 63.9976) x 2)
120.234 + (94.971362 x 2)
120.234 + 189.942724
310.177

So the answer is 310.177 g/mol

You should now be able to use the above approach to work out the answers for the other compounds.

Answer 2

Simply add the molar masses of the individual elements.

Explanation:

For calcium phosphate, `Ca_3(PO_4)_2` we do the sum: `{3xx40.08 (Ca),+2xx31.00 (P), +8xx16.00 (O)}*g*mol^-1` `=` `310.18*g·mol^-1`.

If we had lithium phosphate, `Li_3PO_4`, what would be the formula mass? I invite you to do the rest of the salts on your list. You will need a Periodic Table to get the individual molar masses of the elements.