– the percent by mass of each element in a compound.

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Presentation transcript:

– the percent by mass of each element in a compound. Percent composition – the percent by mass of each element in a compound. % mass = grams of element x 100% grams of compound % mass = g of element in 1 mol of compound x100% molar mass (in g) of compound

Percent Composition from Formula If the formula is known, a two-step process is needed to determine the percent composition. Step 1: Calculate the molar mass (sum of atomic masses) Step 2: Divide the total mass of each element in the formula by the molar mass and multiply by 100.

Example: Calculate the percent composition of sodium chloride. Step 1: First calculate the molar mass of NaCl 23.0 g/mol Na + 35.5 g/mol Cl 58.5 g/mol NaCl Step 2: Now determine the percent composition. 23.0 g/mol Na x 100% = 39.3 % Na 58.5 g/mol NaCl 35.5 g/mol Cl x 100% = 60.7 % Cl 58.5 g/mol NaCl

Practice: Calculate the percent composition of Ca(NO3)2. Ca 40.1 40.1 / 164.1 = 24.4 % 14.0 x 2 N 28.0 28.0 / 164.1 = 17.1 % 16.0 x 6 O 96.0 96.0 / 164.1 = 58.5 % Total 164.1

Practice: Calculate the percent composition of K2CrO4. % Comp. 39.1 x 2 K 78.2 78.2 / 194.2 = 40.3 % 52.0 / 194.2 = 26.8 % Cr 52.0 16.0 x 4 O 64.0 64.0 / 194.2 = 33.0 % Total 194.2

Percent Composition from Experimental Data The percent composition can be determined from experimental data without knowing the formula of a compound. Step 1: Calculate the mass of the compound formed Step 2: Divide the mass of each element by the total mass of the compound and multiply by 100.

Example: When heated in air, 1.63 g of Zn combines with 0.40 g of O2 to form zinc oxide. Calculate the percent composition of the compound formed. Step 1: Calculate the mass of the compound formed 1.63 g Zn + 0.40 g O2 2.03 g ZnO Step 2: Divide the mass of each element by the total mass of the compound and multiply by 100. 1.63 g Zn x 100% = 80.3 % Zn 2.03 g ZnO 0.40 g O2 x 100% = 19.7 % O 2.03 g ZnO

Practice: 13.43 g Al 53.18 g Cl 66.61 g Total Aluminum chloride is formed by reacting 13.43 g aluminum with 53.18 g chlorine. What is the percent composition? 13.43 g Al 53.18 g Cl 66.61 g Total 13.43 g Al X 100 = 20.16 % Al 66.61 g 53.18 g Cl X 100 = 79.84 % Cl 66.61 g