# How do you calculate the percent composition of water?

Apr 27, 2016 #### Explanation:

For water we have to calculate it % of Oxygen and % of Hydrogen

Its long calculation to do it here but this video will help

Apr 27, 2016

Since water is $\text{H"_2"O}$ (molar mass is $\text{18.015 g/mol}$), it implies we have a percent composition of $\text{H}$ and one for $\text{O}$.

Given that chemical formula, we have two equivalents of $\text{H}$, for which we can use its molar mass, ${M}_{\text{H" = "1.0079 g/mol}}$. We also have one equivalent of $\text{O}$, for which we can use its molar mass, ${M}_{\text{O" = "15.999 g/mol}}$.

The percent composition (by mass) of $\text{H}$ is written in terms of a unified ratio of the molar masses is

%"H" = (2xxM_"H")/(M_("H"_2"O"))xx100%

= (2xx"1.0079 g/mol")/("18.015 g/mol")xx100%

~~ color(blue)(11.19% "H")

Similarly for $\text{O}$, we have

%"O" = (1xxM_"O")/(M_("H"_2"O"))xx100%

= ("15.999 g/mol")/("18.015 g/mol")xx100%

~~ color(blue)(88.81% "O")

Thus, the percent composition of water, by mass, is 11.19% hydrogen and 88.81% oxygen.