Percent Composition (Percentage Composition)
Key Concepts
The percent composition (percentage composition) of a compound is a relative measure of the mass of each different element present in the compound.
To calculate the percent composition (percentage composition) of a compound:

Calculate the molar mass , M, of the compound

Calculate the total mass of each element present in the formula of the compound (that is, the mass of the element present in 1 mole of the compound)

Calculate the percentage compositon : % by weight (mass) of element
= (total mass of element present ÷ molar mass) x 100
Examples
Example 1
Calculate the percent by mass (weight) of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)
Calculate the molar mass (M): M = 22.99 + 35.45 = 58.44 g mol^{-1}

Calculate the total mass of Na present: 1 Na is present in the formula, mass = 22.99 g mol^{-1}

Calculate the percent by mass (weight) of Na in NaCl:%Na = (mass Na ÷ M) 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 g mol^{-1}

Calculate the percent by mass (weight) of Cl in NaCl:%Cl = (mass Cl ÷ M) 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 mass (weight) of each element present in sodium sulfate (Na_{2} SO_{4} ).
Calculate the molar mass (M): M = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04 g mol^{-1}

Calculate the total mass of Na present: 2 Na are present in the formula, mass = 2 x 22.99 = 45.98 g mol^{-1}

Calculate the percent by mass (weight) of Na in Na_{2} SO_{4} :%Na = (mass Na ÷ M) x 100 = (45.98 ÷ 142.04) x 100 = 32.37%

Calculate the total mass of S present in Na_{2} SO_{4} : 1 S is present in the formula, mass = 32.06 g mol^{-1}

Calculate the percent by mass (weight) of S present:%S = (mass S ÷ M) x 100 = (32.06 ÷ 142.04) x 100 = 22.57%

Calculate the total mass of O present in Na_{2} SO_{4} : 4 O are present in the formula, mass = 4 x 16.00 = 64.00 g mol^{-1}

Calculate the percent by mass (weight) of O in Na_{2} SO_{4} :%O = (mass O ÷ M) 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

Example 3
Calculate the percent by mass (weight) of each element present in ammonium phosphate [(NH_{4} )_{3} PO_{4} ]
Calculate the molar mass (M) of (NH_{4} )_{3} PO_{4} : M = 3x[14.01 + (4 x 1.008)] + 30.97 + (4 x 16.00) = 3 x [14.01 + 4.032] + 30.97 + 64.00 = (3 x 18.042) + 30.97 + 64.00 = 54.126 + 30.97 + 64.00 = 149.096 g mol^{-1}

Calculate the total mass of N present: 3 N are present, mass = 3 x 14.01 = 42.03 g mol^{-1}

Calculate the percent by mass of N present in (NH_{4} )_{3} PO_{4} :%N = (mass N ÷ M) x 100 = (42.03 ÷ 149.096) x 100 = 28.19%

Calculate the total mass of H present: 12 H are present in the formula, mass = 12 x 1.008 = 12.096 g mol^{-1}

Calculate the percent by mass of H present in (NH_{4} )_{3} PO_{4} :%H = (mass H ÷ M) x 100 = (12.096 ÷ 149.096) x 100 = 8.11%

Calculate the total mass of P present: 1 P is present in the formula, mass = 30.97 g mol^{-1}

Calculate the percent by mass P in (NH_{4} )_{3} PO_{4} :%P = (mass P ÷ M) x 100 = (30.97 ÷ 149.096) x 100 = 20.77%

Calculate the total mass of O present: 4 O are present in the formula, mass = 4 x 16.00 = 64.00 g mol^{-1}

Calculate the percent by mass of O in (NH_{4} )_{3} PO_{4} :%O = (mass O ÷ M) x 100 = (64.00 ÷ 149.096) x 100 = 42.93%
The answers above are probably correct if %N + %H + %P + %O =100, that is,
28.19 + 8.11 + 20.77 + 42.93 = 100

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