Vanillin is a common flavouring agent used in foods..

Question

Vanillin is a common flavouring agent used in foods. This compound consists of 63.15 % C, 5.30 % H, 31.55 % O. Its molar mass is 152 g/mol. Determine both its empirical formula and its molecular formula.

Answer 1

First we will solve for the empirical formula. When given the percent masses, we are allowed to assume that we have a 100 gram sample of the compound. This assumption allows us to say that we have 63.15g of C, 5.30g of H, and 31.55g of O.

When solving for an empirical formula, convert grams to moles for each element:

carbon: 63.15g / 12.01 g/mol = 5.26 mol

hydrogen: 5.30g / 1.008 g/mol = 5.26 mol

oxygen: 31.55g / 16.00 g/mol = 1.97 mol

Next, divide each number of moles by the smallest number of moles:

carbon: 5.26 mol / 1.97 mol = 2.67

hydrogen: 5.26 mol / 1.97 mol = 2.67

oxygen: 1.97 mol / 1.97 mol = 1.00

Of course, an empirical formula must have whole numbers. So now let's multiply by 3:

carbon: 2.67 X 3 = 8.01 = 8 (yes, 8.01 may be rounded to the whole number)

hydrogen: 2.67 X 3 = 8

oxygen: 1.00 x 3 = 3

The empirical formula is C₈H₈O₃.

Now, to determine the molecular formula, we divide the molar mass by the mass of the empirical formula:

(152 g/mol) / (152.144 g/mol) = 0.999 = 1

When the molar mass and the mass of the empirical formula are the same, that means that the molecular formula is the same as the empirical formula.