·  True formula can only be found if the true molar mass is given (since it depends on the actual number of atoms)

·  In some cases, the empirical and molecular formulas are identical.

·  In other cases, the molecular formula is a whole-number multiple of the empirical formula.

·  If you know the molar mass simply divide it by the empirical mass to obtain a ratio by which the subscripts in the empirical formula can be multiplied in order to obtain the molecular formula (factor coefficient)

Factor coefficient, X = ______

Example 1: The empirical formula of a compound was found to be CH2. Its molecular mass was also found to be 42 g/mol. What is its molecular formula?

Step 1 Determine the empirical formula

already done for you in this case

Step 2 Determine the total molar mass of the empirical formula

1 X MC = g/mol

2 X MH = g/mol

g/mol

Step 3 Divide the molar mass by the empirical formula molar mass

Experimental Molar mass = Factor coefficient

Empirical formula molar mass

g/mol =

g/mol

This tells us the coefficient by which to multiply each of the subscripts in the empirical formula

\ multiply subscripts by ______

Example 2: Chemical analysis reveals that a compound is 28.64% sulphur and 71.36% bromine. The molar mass of the compound is 223.94 g/mol. What is its molecular formula?

Suppose a 100 g sample

Element / % or
Mass (g) / Molar Mass
(g/mol) / # moles
(mol) / Ratio of moles / Subscripts
(whole # ratio)
S
Br

*Because the subscripts are already whole numbers, you do not need to determine the least common multiple. The empirical formula is ______

Find the molar mass of the empirical formula:

MSBr=1Ms+1MBr

Find the factor coefficient:

x= Experimental Molar Mass = ______g/mol =

Empirical Formula Molar Mass g/mol

Therefore the molecular formula will be

Review Steps:

1. Convert % to mass

2. Find # moles

3. Find the empirical formula

4. Find the molar mass of the empirical formula

5. Find the multiplier

Mtiplier = molecular mass/empirical mass

6. Multiply the multiplier by the empirical formula

NB: Do not round in step 2 or 3