How do you solve empirical formula calculations?

Step 1 : Assume percentages as masses contained in a total of 100 grams.

Step 2 : Use #n = m /M# formula to calculate the number of moles in each atom.
#n# = number of moles
#m# = mass of substance
#M# = molar mass

Step 3 : Divide all number of moles by the smallest.

Step 4 : Round them to nearest whole numbers and assign as subscripts.

Example question: Calculate empirical formula of a compound that has #11%# hydrogen and #89%# oxygen.

Assume that 100 grams contains #11# #g# of #H# and #89# #g# of #O#.

To move on to step 2, you also need to know the molar mass of #H# and #O#. In that case, the molar mass of #H# is #1.0# #g#/#mol# and the molar mass of #O# is #16.0# #g#/#mol#.

Now you can find the number of moles for each atom.
#n(H)# = #11/1.0# = #11# moles
#n (O)# = #89/16.0# = #5.6# moles

Step 3, multiply the number of moles by the smallest.
#H# = #11 / 5.6 = 2.00#
#O = 5.6 /5.6= 1.00#
*Has been rounded to nearest whole number

Step 4, assign them as subscripts.
Since there is 2 Hydrogen and and 1 Oxygen, you know that the empirical formula is #H_2O#.