Percent Composition by Mass

Percent comp, empirical, molecular formula

By Mihir Paranjape

Percent Composition

You can find percent composition given a m​olecular formula.

e.g. Find the % composition of each element in CH₄?​

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​Element	​# atoms	​Atomic Mass	​Molar Mass	​% composition
​C	​1	​12	​12	​12/16x100 = 75%
​H	​4	​1	​4	​4/16 x 100 =25%
​Total	​	​	​16	​100%

Multiple Choice

What is the percent composition by mass of oxygen in the compound MgSO₄ (gram formula mass = 120 g/mol)?

• 20%
• 27%
• 46%
• 53%

Empirical Formula​

Given the % composition, you can find the Empirical Formula, this is smallest whole number of atoms that make up a compound. e.g. if we know a compound to be 75% C and 25% H.

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​Element	​%	​number	​atomic mass	​moles	​ratio	​whole number
​C	​75%	​75	​12	​75/12 = 6.25	​6.25/6.25	​1
H​	​25%	​25	​1	25/1 = 25​	​25/6.25	​4

Multiple Choice

Given the following: 42.07% Na, 18.89% P, and 39.04% O, determine the empirical formula.

•  NaPO2
• Na2PO3
• Na3PO4
• NaPO4

Multiple Choice

You are given the following percentages: 40.05% S and 59.95% O. Find the empirical formula for these elements.

• SO
• SO2
• SO3
• SO4

Molecular Formula​

Molecular Formula is the actual formula of a substance that occurs in nature. It is a multiple of Empirical Formula. Knowing Empirical formula and the Molar Mass we can find the multiple and therefore the Molecular Formula.

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Molecular Formula​

e.g. If we know that Empirical formula of a substance is CH₂, we can calculate the Empirical Mass as 1 x 12+ 2 x 1 = 14.

If we know that the Molecular Mass is 56 g/mole, then the multiple is

Molecular Mass/Empirical Mass = ​56/14 = 4.

Then the Molecular formula is (CH₂)₄ = C₂H_8​

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Multiple Choice

What is the molecular formula if the empirical formula is CH₂O and the molecular molar mass is 180.18?

• CH₂O
• C₂H₄O₂
• C₄H₈O₄
• C₆H₁₂O₆

Multiple Choice

What is the molecular formula for a compound with the empirical formula: K₂SO₄ and a molecular mass of 696g.

• K₂SO₄ 
• K₈SO₁₆
• K₈S₄O₈ 
• K₈S₄O₁₆