An unknown substance contains 43.20% oxygen, 8.200% hydrogen and 48.60% carbon. Determine the empirical formula.
When we are given the composition by mass of a substance and we have are asked to determine the empirical formula we usually have to follow some steps.
1) We can suppose that we have 100 g of that substance. So we can convert those percentages into the mass of each element.
mass of sample = 100 g
% of O = 43.20 %
mass of O = 43.2 g
% of H = 8.200%
mass of H = 8.200 g
% of C = 48.60%
mass of C = 48.60 g
2) We have to find the number of moles of each element that we have in the sample.
molar mass of O = 16.00 g/mol
molar mass of H = 1.01 g/mol
molar mass of C = 12.01 g/mol
moles of O = 43.2 g/(16.00 g/mol)
moles of O = 2.7 moles
moles of H = 8.200 g/(1.01 g/mol)
moles of H = 8.12 moles
moles of C = 48.60 g/(12.01 g/mol)
moles of C = 4.05 moles
3) The definition of empirical formula is: "it is the simplest whole number ratio of atoms present in a compound". So if we want to find the simples number ratio we have to divide the number of moles of each element by the least of them.
O = 2.7/2.7 = 1
H = 8.12/2.7 = 3
C = 4.05/2.7 = 1.5
The empirical formula seams to be:
C₁.₅H₃O
4) We can't have fractions as subscripts, we need whole numbers. So we have to multiply the formula by 2.
Empirical formula = C₃H₆O₂
Answer: The empirical of the unknown substance is C₃H₆O₂.