Determining a chemical formula

Question

Vitamin C has the formula C_xH_yO_z. You burn 0.275 g of the compound in a combustion analysis chamber and isolate 0.413 g of CO₂ and 0.112 g of H₂O. Enter an integer or a decimal number, such as 2 or 1.5 or 1.67.

What is the ratio of mol H to mol O in the sample?

Answer 1

Use CO2 to find moles of carbon

Use H2O to find moles of hydrogen

Find total mass of C and H and deduct from 0.275 go to obtain mass of oxygen (O), then convert to moles.

0.413 g CO2 x 1 mol CO2/44 g x 1 mol C/mol CO2 = 0.00939 moles C (x 12g/mol = 0.113 g C)

0.112 g H2O x 1 mol H2O/18 g x 2 mol H/mol H2O = 0.0124 moles H (x 1g/mol = 0.0124 g H)

mass of O = 0.275 g - 0.113 g - 0.0124 g = 0.1496 g O x 1 mol 16 g = 0.00935 moles O

We now have moles of C, H and O and we divide by the lowest value to get whole numbers...

C: 0.00939/0.00935 = 1.00

H: 0.0124/0.00935 = 1.3

O: 0.00935/0.00935 = 1.00

Ratio of moles H to moles O = 1.3

If you want to find the empirical formula, we can multiply all by 3 to get H to be a whole number and we end up with the empirical formula...

C₃H₄O₃