Empirical formula & Percentage Composition

•Find the empirical formula of a compound given its % composition

•Find the molecular formula given empirical formula and molar mass

Percentage Composition

• Gives the mass of each component in 100g of substance.

• If a chemical substance has a % composition of 85.7% C and 14.3% H, this means that in every 100g of substance there is 85.7g of C and 14.3g of H.

Empirical Formula

• Is the simplest ratio of the different atoms present.

• In ethane, C2H6 the ratio C:H is 1:3, the empirical formula is therefore CH3

• As the formula for an ionic compound states the ratio of atoms present the formula is empirical.

Finding an empirical formula from % composition

1. Obtain molar mass M values for each atom from periodic table

2. Use formula n=m/M to calculate the # of moles of each atom present in 100g of compound.

3. Divide each n value by the smallest n value to get the ratio of moles of atoms present.

4. Use the ratio to write the empirical formula.

Find the empirical formula of a compound whose % composition is 85.7% C and 14.3% H

M(C) = 12.0g/molM(H) = 1.0g/mol

Have a go…

Example 2

• If 40.0g of compound contains 30.0g of C and 10.0g of H, find the empirical formula.

• M(C) = 12.0 g/mol M(H) = 1.0 g/mol• n(C)= 30.0g/12.0gmol-1 = 2.5mol

• n(H) =

• Divide each n value by the smallest n value to get the ratio..

Molecular Formula

• Gives the actual numbers of the atoms present in a molecule.

1. Find the empirical formula mass using periodic table

2. Find value of molar mass of compoundempirical formula mass

3. Multiply the numbers in the empirical formula by this value to give the molecular formula.

Example

If the molar mass of the compound with empirical formula CH2 is 42.0g/mol, find the molecular formula.

M(CH2) = 12.0 + (2x1.0) = 14.0 g/mol

42.0g/mol = 314.0g/mol

CH2 x 3 = C3H6