A compound has the % composition by mass that is 87.8% C and 12.2 % H. What is the molecular formula if it has a molecular mass of 82 g/mol?

Answer 1

Take a 100 g sample for convience to give you 87.8 g C and 12.2 g H.
mols C = grams/atomic mass = ? 87.8/12 = about 7.3
mols H = grams/atomic mass = ? = 12.2/1 = 12.2
You want to find the ratio. The easy way to do that is to divide the smaller number by itself and the other number by the smaller number. This will give you a fraction but you want whole numbers. Multiply each of those fractions by whole number (i.e., 2,3,4, etc) until you get two whole numbers (or two numbers that can be rounded to whole numbers. Remember not to round anything larger than 0.1 or so). I get C3H5 but you should confirm that. That gives you the empirical formula. To find the molecular formula, do this,
empirical formula mass x factor(n) = 82 or
41 x factor(n) = 82.
factor(n) = ?
molecular formula = (C3H5)n