As with all with these problems, we assume a mass of 100⋅g, and we address the elemental makeup by dividing thru by the atomic mass of each component.
Moles of carbon=64.65⋅g12.011⋅g⋅mol−1=5.38⋅mol.
Moles of hydrogen=13.52⋅g1.00794⋅g⋅mol−1=13.41⋅mol.
Moles of oxygen=21.62⋅g15.999⋅g⋅mol−1=1.351⋅mol.
They are spoon-feeding you a bit here, in that combustion analysis gives %C,H,N. %O would normally NOT be measured, and determined by the difference (100−%C−%H−%N)%. And now we divide thru by the SMALLEST MOLAR QUANTITY, that of oxygen to give C,H,O
C:5.38⋅mol1.351⋅mol=3.98≅4
H:13.41⋅mol1.351⋅mol=9.93≅10
O:1.351⋅mol1.351⋅mol=1
And thus an empirical formula, the simplest whole number defining constituent atoms in a species of C4H10O.
