Percent Composition

Percent Composition:
The percent 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

1. Calculate the molar mass, MM, of the compound
2. Calculate the total mass of each element present in the formula of the compound
3. Calculate the percent composition: % by weight (mass) of element = total mass of element present ÷ molar mass x 100

Examples:

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.

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By: Hikari Hoshika