# How do you calculate the percent by weight of each element in Na_2SO_4?

$\text{% Element}$ $=$ "Mass of element"/"Molar mass of compound"xx100%.
So for %O, we calculate (4xx15.999*g*mol^-1)/(142.04*g*mol^-1)xx100% $=$ 45.05%.
When you calculate the other percentages, you will find that the sum of the percentages adds to 100%. Why should this be?