Question

How many moles are there in 397 grams of `Na_2SO_4`?

Answer 1

2.80

Explanation:

A mole is a unit-molecular weight (grams/mole in this case) of an Avogadro's number of molecules or atoms. It is calculated for a molecule by adding up the individual atomic weights of its constituent atoms.

For `Na_2SO_4` this is `2 x 23 = 46` for the sodium, plus `4 x 16 = 64` for the oxygen, plus `32` for the sulfur. That adds up to a molecular weight of `142 (g/"mole")`. Now we divide the given number of grams by this value to derive the number of moles.
`(397/142)(g/(g/"mol")) = 2.80 "moles"`

Answer 2

`n_(Na_2SO_4)=2.79 mol`

Explanation:

Molar mass of `Na_2SO_4 =142.04g/(mol)`

Use the equation,

`n_(Na_2SO_4)=m/M`

where,

`n=` number of moles
`m=`mass (in grams)
`M=`molar mass (in `(grams)/(mol)`)

`n_(Na_2SO_4)=(397g)/(142.04(g/(mol)))`

`n_(Na_2SO_4)=2.79 mol`