Question

Complete combustion of 5.90 g of a hydrocarbon produced 18.8 g of CO2 and 6.75 g of H2O. What is the empirical formula for the hydrocarbon?

Answer 1

let in compound number of moles of C and H be x and y respectively

Number of moles of CO2 = mass of CO2 / molar mass CO2
= 18.8/44
= 0.4273

Number of moles of H2O = mass of H2O / molar mass H2O
= 6.75/18
= 0.375

Since 1 mol of CO2 has 1 mol of C
Number of moles of C in CO2= 0.4273
so, x = 0.4273

Since 1 mol of H2O has 2 mol of H
Number of moles of H = 2*0.375 = 0.75

Divide by smallest:
C: 0.4273/0.4273 = 1
H: 0.75/0.4273 = 1.75 = 7/8

multiply by 8 to get simplest whole number ratio:
C : 1*8 = 8
H : (7/8)*8 = 7
So empirical formula is:C₈H₇