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

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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)

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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)

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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.

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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

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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

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