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Assignment
Practice 1.

• A compound contains C, H, N. Combustion of 35.0mg of the compound produces 33.5mg CO2 and 41.1mg H2O. What is the empirical formula of the compound?

We need to figure the masses of the 3 elements in the sample.
Carbon constitutes 12/44 = 0.2727 of CO2 by mass, and Hydrogen constitutes 2/18 = 0.1111 of H2O by mass, so:
33.5 mg * 0.2727 = 9.14 mg of C in the sample
41.1 mg * 0.1111 = 4.57 mg of H in the sample, and
35.0 mg - 9.14 mg - 4.57 mg = 21.29 mg of N in the sample.
Now calculate the number of mmoles of the elements:
9.14 mg ÷ 12 mg/mmol = 0.762 mmol C
4.57 mg ÷ 1 mg/mmol = 4.57 mmol H
21.29 mg ÷ 14 mg/mmol = 1.521 mmol N
Now divide through by the lowest number (0.762)
0.762 mmol C ÷ 0.762 mmol C = 1.0 C
4.57 mmol H ÷ 0.762 mmol C = 6.0 H
1.521 mmol N ÷ 0.762 mmol C = 2.0 N
Therefore the emperical formula is:
CH6N2

Practice 2.
• Caffeine contains 49.48% C, 5.15% H, 28.87% N and 16.49% O by mass and has a molar mass of 194.2 g/mol. Determine the molecular formula.

We will first determine the mass of each element in 1 mole (194.2 g) of caffeine,
Mass of C = 49.48/100 x 194.2 = 96.09 g/mol C
Mass of H = 5.15/100 x 194.2 = 10.0 g/mol H
Mass of N = 28.87/100 x 194.2 = 56.07 g/mol N
Mass of O = 16.49/100 x 194.2 = 32.02 g/mol O
Now we will convert to moles,
No of moles of C = 96.09/12.01 = 8.001 mol C/mol caffeine.
No of moles of H = 10.0/1.008 = 9.92 mol H/ mol caffeine.
No of moles N = 56.07/14.01 = 4.002 mol N / mol caffeine.
No of moles of O = 32.02/16.00 = 2.001 mol O/ mol Caffeine.
\the molecular formula of caffeine can be given as: C8H10N4O2