Chemistry mole question

Question

A compound has a percentage composition of 43.1% C, 7.7% H, 8.2% O, and 41.0% Br. If the molar mass of the compound is 195g, what is the molecular formula for the compound?

Answer 1

Empirical/Molecular formulae:

To find the empirical formula, find moles of each element. Assume 100 g of unknown so % = gms.

mol C = 43.1 g x 1 mol C/12 g = 3.59 mole C

mol H = 7.7 g H x 1 mol H/1 g = 7.7 mole H

mol O = 8.2 g x 1 mol O/16 g = 0.513 mol O

mol Br = 41 g x 1 mol Br/79.9 g = 0.513 mol Br

Note the ratio of the moles of each. O and Br are the same and H is about 2x C, etc. We can get a better estimate by dividing all moles by the lowest value to try to get whole numbers. Thus, let's divide all by 0.513..

mole C = 7

mole H = 15

mole O = 1

mole Br = 1

Empirical formula = C₇H₁₅OBr

To find the molecular formula, we need to determine how many empirical formulae it takes to make a molecular formula. The molar mass of the empirical formula = 84 + 15 + 16 + 80 = ~195 g/mol

This happens to be equal to the molar mass of the molecular formula, so they are the same.

Molecular formula = C₇H₁₅OBr