Finding Empirical formula

General information about various types of formulas follows:

  • In a chemical formula, the elements are represented by symbols, and a subscript number represents the number of each element.

Example: The chemical formula for ethane is C2H6.

  • An empirical (simplest) formula represents the relative number of atoms of each element in the compound. Example: The empirical formula for ethane is CH3. Or the lowest common multiple of the subscripts.
  • A molecular formula represents the actual number of atoms of each element rather than a ratio of atoms.

Example: The molecular formula for ethane is C2H6.

Another way of representing this is: Molecular formula = (empirical formula)n, where “n” is a whole number such as n = 1, etc. In this example, n = 2 C2H6 = (CH3)2

Steps in calculating empirical formula:

Q- a compound is found to contain the following % by mass: 69.58% Ba, 6.090% C, 24.32% O. What is the simplest (i.e. empirical) formula?

Step 1: Change your percentage so it is out of 100g of the substance. Thus, % will become mass in grams. E.g. 69.58 % Ba becomes 69.58 g Ba. (Some questions will give grams right off, instead of %)

Step 2: Calculate the # of moles for each.

Step 3: Divide the moles of each compound by the lowest number of moles calculated in your different types of compounds.

Step 4: Round the answer to the nearest whole number

Step 5: Write the simplest formula based on the whole number from the mol ratios.

 

Sample Problem:

If a compound has a composition of 40.0% carbon (C), 6.714% hydrogen (H), and 53.29% oxygen (O), determine the empirical formula of the compound.

Assume that there is a 100.0 g sample of the compound, in which case:

Step 1: 40.0% = 40.0g of C, 6.714g of H, 52.29g of O

Step 2: Calculate moles

No. of Moles of C = 40.0g/12.00g/mol = 3.33mol
No. of Moles of H = 6.714/1.00g/mol = 6.7mol
No. of Moles of O = 53.29/16.00g/mol = 3.33mol

Step 3: Divide by lowest mol number

C = 3.33mol/3.33mol = 1
H = 6.7mol/3.33mol = 2
O = 3.33 mol /3.33mol = 2

Step 4:

C1H2O2


Variation #2:

If a compound contains 71.65% chlorine (Cl), 24.27% carbon (C), and 4.07% hydrogen (H), determine the molecular formula if the molar mass is 98.96 g/mol.

Step 1: Assume that there is a 100.0 g sample of the compound, in which case:

Step 2:By dividing by the smallest number of moles, the ratio between the elements is C1H2Cl1, and the formula mass would then be 49.5 g/mol.

Step 3: If the molar mass is 98.96 g/mol, the molecular formula would be a multiple of the simplest formula, or The molecular formula is therefore C2H4Cl2.