1. Empirical formula & Percentage Composition Find the empirical formula of a compound given its % composition Find the molecular formula given empirical formula and molar mass

2. 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.

3. Empirical Formula Is the simplest ratio of the different atoms present. In ethane, C 2 H 6 the ratio C:H is 1:3, the empirical formula is therefore CH 3 As the formula for an ionic compound states the ratio of atoms present the formula is empirical.

4. 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.

5. Find the empirical formula of a compound whose % composition is 85.7% C and 14.3% H M(C) = 12.0g/mol M(H) = 1.0g/mol Have a go…

6. 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..

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

8. Example If the molar mass of the compound with empirical formula CH 2 is 42.0g/mol, find the molecular formula. M(CH 2 ) = 12.0 + (2x1.0) = 14.0 g/mol 42.0g/mol = 3 14.0g/mol CH 2 x 3 = C 3 H 6