Answer to Question #138177 in General Chemistry for Courtney Hirsch

Question

Complete combustion of 8.90 g of a hydrocarbon produced 28.4 g of CO2 and 10.2 g of H2O. What is the empirical formula for the hydrocarbon? Insert subscripts as necessary.

Answer 1

The overall reaction is:

C_xH_y + O₂ --> CO₂ + H₂O

$n(CO_2)=\frac{28.4g}{44.01g/mol}=0.645mol$

$n(H_2O)=\frac{10.2g}{18.02g/mol} =0.566mol$

Therefore, the moles of carbon atoms in the unknown hydrocarbon sample is $0.645mol$ , and the moles of hydrogen atoms = $0.566\times2=1.132mol$. The lowest whole number ratio of these quantities will determine the empirical formula. In this case the lowest whole number ratio of 0.645 : 1.132 is 4 : 7.

Answer: C₄H₇