A compound contains only carbon, hydrogen, nitrogen, and oxygen. Combustion of a 2.18 g sample burns in excess oxygen yields 3.94 g of CO2 and 1.89 g of H2O. A separate experiment shows that a 1.23 g sample contains 0.235 g of N. Calculate the moles of C in the sample.
1. Calculate the moles of N for a 2.18 g sample (the sample size for the other analyses).
2. Calculate the moles of O in the sample.

The answers are

C = 0.0895 moles

1. 0.02975 moles of N

2. 0.0299 moles O

Explanation:

Mass of sample = 2.18 g

Combustion yields 3.94 g of CO₂ and 1.89 g of H₂O

1.23 g of sample contains 0.235 g of N

3.94 g of CO₂ which contains one mole of C and two moles of O

Molar mass of CO₂ = 44 mass fraction of C = 12/44 = 0.273, Therefore mass of  carbon in 2.18 g = 0.273×3.94 = 1.075 g

number of moles of C in sample = mass/(molar mass) = 1.075 g/(12.01 g/mol) = 0.0895 moles of C

(1) Mass of nitrogen, in 1.23 g of sample = 0.235

therefore mass in 2.18 g =0.235×2.18/1.23 = 0.4165 g

= 0.02975 moles of N

(2) To find the number of moles of oxygen in the sample we note that total mass of sample  = 2.18 g containing only nitrogen, hydrogen, oxygen and carbon

Mass of hydrogen can be fround from 2/18×1.89 = 0.21 g

Mass of oxygen therefore = 2.18 -(0.21+0.4165+1.075) = 0.4785  

number of moles = 0.475/16 = 0.0299 moles

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