# PCAT Chemical Processes – Molecular and Empirical Formulas

One mole of an unknown compound is determined to have a molecular weight of 86 grams per mole. The mass percent of the compound is as follows: 55.81% carbon, 6.98% hydrogen, and 37.21% oxygen. What is the empirical formula and molecular formula, respectively, of the unknown compound.

1. C2H3O and C4H6O2
2. C2H3O and C6H9O3
3. C5H8O and C5H8O
4. C5H8O and C10H16O2
##### Explanation

To answer this question, you must determine the empirical formula of the unknown compound by treating the mass percent as the number of grams of each element out of a 100g sample. The number of moles of each element is determined by the following calculations:

Moles of C = (55.81g C) x (1mol C/12g C) = 4.65 mol C

Moles of H = (6.98g H) x (1 mol H/1g H) = 6.98 mol H

Moles of O = (37.21g O) x (1 mol O/16g O) = 2.33 mol O

Dividing each by the largest common denominator of 2.33 gives the empirical formula of the unknown compound: C2H3O

(4.65/2.33) mol C = 2 mol C

(6.98/2.33) mol H = 3 mol H

(2.33/2.33) mol O = 1 mol O

Since the molecular weight of the compound is 86 g/mol, the correct answer must be A.

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