# How many molecules are there in 39 grams of water?

Nov 16, 2015

$\text{39 g H"_2"O}$ contains $1.3 \times {10}^{24} \text{molecules H"_2"O}$.

#### Explanation:

One mole of water has a molar mass of $\text{18.015 g/mol}$ and $6.022 \times {10}^{23} \text{molecules H"_2"O}$. Therefore,

$\text{1 mol H"_2"O}$$=$$\text{18.015 g H"_2"O}$$=$$6.022 \times {10}^{23} \text{molecules H"_2"1O}$

Multiply the given mass of water times the conversion factors that cancel grams and moles of water, leaving molecules of water, as demonstrated below.

39cancel("g H"_2"O")xx(1cancel("mol H"_2"O"))/(18.015cancel("g H"_2"O"))xx(6.022xx10^23"molecules H"_2"O")/(1cancel("mol H"_2"O"))=1.3xx10^24"molecules H"_2"O"