1. Quantitative Chemistry (Part 1) Isotopes Standard Atom for Mass Relative Atomic Mass Formula Mass Percentage Composition

2. Isotopes Isotopes are atoms of the SAME element (i.e the same number of protons) which have different numbers of NEUTRONS. Examples: 1 1 H, 2 1 H, 3 1 H (1p, 0n, 1e) (1p, 1n, 1e) (1p, 2n, 1e) 12 6 C, 13 6 C, 14 6 C (6p, 6n, 6e) (6p, 7n, 6e) (6p, 8n, 6e)

3. Standard Atom for Mass The standard atom for mass is the 12 6 C isotope of carbon. 12 6 C = 12.000000 a.m.u. (a.m.u. = atomic mass units)

4. Relative Atomic Mass The relative atomic mass of an element is the average mass of an atom of the element compared to one twelfth of the mass of a 12 6 C atom – allowing for the abundances of the isotopes. The element chlorine has two isotopes: 35 17 Cl (75%), 37 17 Cl (25%)  Relative Atomic Mass (A r ) = (75 x 35) + (25 x 37) 100 = 35.5 a.m.u.

5. Formula (Molecular) Mass What is the formula mass of magnesium nitrate? Write the correct formula for magnesium nitrate: Mg(NO 3 ) 2 1 atom of magnesium = 1 x 24 = 24 2 atoms of nitrogen = 2 x 14 = 28 6 atoms of oxygen = 6 x 16 = 96 Total = 148

6. Percentage Composition What is the percentage by mass of each element in potassium sulphate? Write the correct formula for potassium sulphate: K 2 SO 4 M r for potassium sulphate = (2x39) + 32 + (4 x 16) = 174 % Potassium = (2 x 39) x 100 = 44.83% 174 % Sulphur = 32 x 100 = 18.39% 174 % Oxygen = (4 x 16) x 100 = 36.78% 174