# Which sample has the greatest mass? (multiple choice)

## A. 1 mol of Na2S B. 3.0 x ${10}^{24}$ molecule of O2 C. 8.8 x 10 g of NaCl D. 1.2 x ${10}^{24}$ atoms of K The answer is B. 3.0 x ${10}^{24}$ molecule of O2, why?

Feb 17, 2016

#### Explanation:

A

To calculate the mass of 1 mol of $\text{Na"_2"S}$ we need just to multiply it by it's molar mass: 78,0452 g/mol. Therefore the result will be 78,0452 g.

B

To calculate the mass of ${\text{O}}_{2}$, we first need to divide the result by the Avogadro constant.

3.0 × 10^24color(white)(l) "molecules O"_2 × "1 mol O"_2/(6.022 × 10^23color(white)(l) "molecules O"_2) = "4.98 mol O"_2

This gives us the moles of ${\text{O}}_{2}$.

Then we multiply by the molar mass.

${\text{4.98 mol O"_2 × "32.00 g O"_2/"1 mol O"_2 = "160 g O}}_{2}$

C

For C we only have 88 g.

D

And for D the molar mass of $\text{K}$ is 39,0983 g/mol.

This means

1.2 × 10^24color(white)(l) "atoms" × "1 mol"/(6.022 × 10^23color(white)(l) "atoms") × "39.10 g"/"1 mol" = "80 g"