1. Percentages How do you calculate your percentage score in a test if you correctly answered 34 out of 47 questions? Answer: 33/47 x 100/1 = ? If 18 out of 32 Chemistry students did their homework (obviously not at Nayland!), what percentage is that? Answer: 18/32 x 100/1 = ?

2. Percentage composition Its just the same for compounds. For example: Calculate the percentage of oxygen in carbon dioxide. Use molar mass values from the periodic table and use the formula: Molar mass of element x number of atoms ÷ molar mass of compound x 100/1

3. Molar mass of oxygen = 16 gmol -1 Oxygen 2 x 16 gmol -1 Carbon dioxide 2 x 16 + 12 32 x 100 = 73 % 44 1 There are 2 O atoms in CO 2

4. Now try these: Calculate the percentage composition of all elements in the following compounds 1.CaCO 3 2.H 2 SO 4 3.C 12 H 22 O 11 4.(NH 4 ) 3 PO 3 5.C 2 H 5 OH

5. Answers 1.Ca = C = O = 2.H = S = O = 3.C = H = O = 4.N = H = P = O = 5.C = H = O = 52% 12%48% 2%33%65% 42%6% 40% 32%9% 23% 36% 52%13%35%