How do you calculate the percent composition by mass of #O# in picric acid (#C_6H_3N_3O_7#)?
Since one mole of picric acid contains seven moles of oxygen atoms, this accounts for 48.9% of the total mass.
Start by determining the molar mass of picric acid = 229 g
Next, noting that the molecule contains seven oxygen atoms, we can reason that one mole of picric acid must contain seven moles of oxygen atoms.
Find the mass of 7 moles of
Now, since 112 g out of the total mass of 229 g of picric acid is due to oxygen, express this as a percentage:
That's it! (You can of course, repeat this process for as many elements as you need in the formula.)