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# Percent Composition by Mass

 Total mass of the Element  (Part) x 100% = Percent Composition Total mass of compound (Whole) Example 1 Calculate the percent by weight of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl) Calculate the molecular mass (MM): MM = 22.99 + 35.45 = 58.44 Calculate the total mass of Na present: 1 Na is present in the formula, mass = 22.99 Calculate the percent by weight of Na in NaCl: %Na = (mass Na ÷ MM) x 100 = (22.99 ÷ 58.44) x 100 = 39.34% Calculate the total mass of Cl present: 1 Cl is present in the formula, mass = 35.45 Calculate the percent by weight of Cl in NaCl: %Cl = (mass Cl ÷ MM) x 100 = (35.45 ÷ 58.44) x 100 = 60.66% The answers above are probably correct if %Na + %Cl = 100, that is, 39.34 + 60.66 = 100. Example 2 Calculate the percent by weight of each element present in sodium sulfate (Na2SO4). Calculate the molecular mass (MM): MM = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04 Calculate the total mass of Na present: 2 Na are present in the formula, mass = 2 x 22.99 = 45.98 Calculate the percent by weight of Na in Na2SO4: %Na = (mass Na ÷ MM) x 100 = (45.98 ÷ 142.04) x 100 = 32.37% Calculate the total mass of S present in Na2SO4: 1 S is present in the formula, mass = 32.06 Calculate the percent by weight of S present: %S = (mass S ÷ MM) x 100 = (32.06 ÷ 142.04) x 100 = 22.57% Calculate the total mass of O present in Na2SO4: 4 O are present in the formula, mass = 4 x 16.00 = 64.00 Calculate the percent by weight of O in Na2SO4: %O = (mass O ÷ MM) x 100 = (64.00 ÷ 142.04) x 100 = 45.06% The answers above are probably correct if %Na + %S + %O = 100, that is, 32.37 + 22.57 + 45.06 = 100