A 2.87g sample of ascorbic acid (containing C, H, and O only) produces 4.31g CO2 and 1.17g H2O upon combustion. What is the empirical formula of ascorbic acid?

C₃H₄O₃

Explanation:

The empirical formula is the simplest whole-number ratio of atoms present in a molecule. To solve this question we must find the moles of CO2 = Moles of Carbon and moles of H2O = 1/2 moles H.

With the difference in masses of the atoms we can find the mass of ascorbic acid and its moles as follows:

Moles CO2 = Moles C -Molar mass: 44.01g/mol-

4.31g CO2 * (1mol / 44.01g) = 0.0979 moles C * (12.01g/mol) = 1.18g C

Moles H2O -Molar mass: 18.01g/mol

1.17g H2O * (1mol / 18.01g) = 0.0650 moles H2O * (2mol H / 1mol H2O) = 0.130 moles H = 0.13g H

Moles O:

Mass O: 2.87g-0.13g H - 1.18g C = 1.56g O

Moles O:

1.56g O * (1mol/16g) = 0.0975 moles O

Dividing each number of moles of atoms in the moles of Oxygen (The lower number of moles:

O/O = 0.0975 moles O/0.0975 moles O = 1

H/O = 0.130 moles H/ 0.0975 moles O = 1.333

C/O = 0.0979 moles C/ 0.0975 moles O = 1

As empirical formula requires whole-numbers we need to multiply this ratio 3 times:

O = 1*3 = 3

H = 1.333*3 = 4

C = 1*3 = 3

Empirical formula of ascorbic acid is: