Question

An unknown compound contains only C, H, and O. Combustion of 4.20 g of this compound produced 10.3 g of CO2 and 4.20 g of H2O.

What is the empirical formula of the unknown compound?

Answer 1

no of moles of CO2 = weight of CO2 / molar mass of CO2

= 10.3 / 44.01

= 0.234 moles CO2

no of moles of H2O = 4.2 / 18

= 0.24 moles H2O

all the carbons in unknown compound will convert in to CO2

we got 0.234 mole CO2 that means 0.234 mole of Carbon atoms are there

we can find out the mass of the carbon atom using following formula

moles = weight / molar mass

weight = 0.234 x 12 = 2.8 moles of C

there are two moles of H2O in water

so no of moles of H in unknown compound = 2 x 0.24 = 0.48 moles of H

mass of H = 0.48 x 1 = 0.48 grams

mass of the C + mass of H = 2.8 + 0.48 = 3.3 grams

mass of the compound taken = 4.2 grams

difference = 4.2 - 3.3 = 0.9 grams

which is the mass of the oxygen (O)

moles of Oxygen = 0.9 / 16 = 0.056 moles

now mole ratio

0.23 : 0.48 : 0.056

divide the ratio with smallest no

4.1 : 8.6 : 1

C4H8O is the empirical formula