AP Chemistry Question?

Question

Combustion of 0.5707 mg of a hydrocarbon produces 1.790 mg of CO2. What is the simplest formula of the hydrocarbon?

Answer 1

All of the carbon from the hydrocarbon is present in the CO2 formed. So you can calculate the amount of Carbon that was in the hydrocarbon by working out moles of C in the CO2.

moles = mass / molar mass

moles CO2 = 0.001790 g / 44.01 g/mol

= 4.0673 x 10-5 moles of CO2 are produced

1 molecule of CO2 has 1 C atom, therefore there are

4.0673 x 10-5 moles of Carbon in the CO2 produced, and thus in the hydrocarbon.

Now we know the moles of C, we can work out H by subtracting the mass of C from the total hydrocarbon weight.

mass of C = moles x molar mass

= 4.0673 x 10-5 moles x 12.01 g/mol

= 4.885 x 10-4 g

= 0.4885 mg of Carbon in the hydrocarbon

Mass of H in hydrocarbon = Total mass Hydrocarbon - mass Carbon

= 0.5707 mg - 0.4885 mg

= 0.0822 mg of H in the Hydrocarbon

Now, find the moles of H

moles H in hydrocarbon = mass / molar mass

= 0.0000822 g / 1.008 g/mol

= 8.155 x 10-5 moles of H in hydrocarbon

The ratio of moles C : moles H is therefore

4.0673 x 10-5 moles : 8.155 x 10-5 moles

Convert the ration to whole numbers by dividing both numbers by the lowest value.

C : H is

(4.0673 x 10-5/ 4.0673 x 10-5) : (8.155 x 10-5/4.0673 x 10-5)

1 : 2

So empirical formula is CH2