combustion of CHO

Question

A 2.052 gram sample of an organic compound containing C, H and O is analyzed by combustion analysis and 5.176 grams of CO₂ and 0.9082 grams of H₂O are produced.

In a separate experiment, the molar mass is found to be 122.1 g/mol. Determine the empirical formula and the molecular formula of the organic compound.

1. Enter the elements in the order C, H, O

empirical formula =

molecular formula =

Answer 1

All the C --> CO₂ and all of the H --> H₂O so you can just use the mass stoichiometry to find g C and g H in the original compound. The grams of O can be obtained indirectly (see 2)

1) C: g CO₂ (12 g C/ 44 g CO₂) H: gH (2 g H/ 18 g H₂O (You should probably use 2 decimal places and 3 for H. Empirical formula calculations require accuracy.

2) g O = 2.052g - g C - g H

3) Find moles of C, moles of H, moles of O Whichever is smallest (let's say C, hypothetically) is divided into itself and the others to obtain the empirical formula C₁H_xO_y If x or y is a fraction, multiply all subscripts by the number that will make the formula only have integral subscripts.

4) Divide the molar mass by the empirical formula mass to obtain the multiplier n and the molecular formula will be C_nH_xnO_yn