How do you calculate empirical formula of a compound?

Apr 25, 2016

If a question gives you a compound showing percentages , you have to find the simplest ratio of atoms in the compound.

Explanation:

For example, a question wants you to find the empirical formula that contains 11% hydrogen and 89% oxygen.

First step, assume that these substances add up to 100 grams in masses. Assume that 100 grams contains 11 grams of hydrogen and 89 grams of oxygen.

Second step, use $n = m \div M$ to calculate the number of moles. It is assumed that n = number of moles, m = mass of substance, and M = molar mass (equivalent to atomic weight on the periodic table).

number of moles of hydrogen = 11 grams $\div$1 g/mol = 11 moles
number of moles for oxygen = 89 grams $\div$16 g/mol = 5.56 moles

Third step, divide all number of moles by the smallest.

Hydrogen (H): 11 $\div$5.56 = 2.00
Oxygen (O): 5.56 $\div$5.56 = 1.00

*round it off to nearest whole number, then assign it as subscripts.

Therefore the empirical formula is ${H}_{2} O$.