PERCENT COMPOSITION

Percent Composition• Percent Composition – the percentage by mass of

each element in a compound

Percent = _______PartWhole

x 100%

So…Percent compositionof a compound or =molecule

Mass of element in 1 mol____________________Mass of compound 1 mol

x 100%

CALCULATING PERCENTAGE COMPOSITION

Step 1 : Calculate molar mass

Step 2 : Divide the subtotal for each element’s mass by the molar mass.

Step 3: Multiply by 100 to convert to a percentage.

Percent CompositionExample: What is the percent composition of Potassium Permanganate (KMnO4)?

Molar Mass of KMnO4K = 1(39.1) =

39.1Mn = 1(54.9) = 54.9O = 4(16.0) = 64.0

MM = 158 g

Percent CompositionExample: What is the percent composition of Potassium Permanganate (KMnO4)? = 158 g

% K

Molar Mass of KMnO4

39.1 g K158 g

x 100 =24.7 %

% Mn54.9 g Mn158 g

x 100 =34.8 %

% O64.0 g O158 g

x 100 = 40.5 %K = 1(39.10) = 39.1

Mn = 1(54.94) = 54.9

O = 4(16.00) = 64.0MM = 158

Percent CompositionDetermine the percentage composition of sodium carbonate (Na2CO3)?

Molar Mass Percent Composition

% Na =46.0 g106 g

x 100% =43.4 %

% C =12.0 g106 g

x 100% =11.3 %

% O =48.0 g106 g

x 100% =45.3 %

Na = 2(23.00) = 46.0C = 1(12.01) = 12.0O = 3(16.00) = 48.0 MM= 106 g

PERCENT COMPOSITION

What is the % composition of C6H12O6?

7

Carbon = 72.066 g C

6(12.011 )

Hydrogen = 12.0948 g H

12(1.0079g )

Oxygen = 95.994g O

6(15.999g )

180.155

72.066180.155

12.0948180.155

95.994180.155

x100%=40.0% C

x100%=6.7% H

x100%=53.3% O

% composition is 40.0% C, 6.7% H, and 53.3% O

EXERCISE

Morphine, derived from opium plants, has the potential for use and abuse. It’s formula is C17H19NO3. What percent, by mass, is the carbon in this compound?

a) 12.0 %b) 54.8 %c) 67.9 %d) 71.6 %

17 12.01 = 0.716 100 = 71.6% C

17 12.01 + 19 1.008 + 14.01 + 3 16.00

8

EXERCISEConsider separate 100.0 gram samples of each of the following: 

H2O, N2O, C3H6O2, CO2

Rank them from highest to lowest percent oxygen by mass.

H2O, CO2, C3H6O2, N2O

16/18 32/44 32/74 16/44 0.89 0.73 0.43 0.36

9

18g/mol 44g/mol 74g/mol 44g/mol

PERCENT COMPOSITIONThe percent composition can also be calculated

from experimental data.A student prepares a compound of tungsten chloride from

3.946 g of tungsten and 3.806 g of chlorine. Assuming the reaction goes to completion, calculate the percent composition.

%W = (mass W / mass WxCly )100 = (3.946 g / 7.752 g)100 = 50.9%

%Cl = (Cl / WxCly )100 = (3.806g / 7.752 g)100 = 49.1%

GROUP STUDY PROBLEMS

______1. A 5.016 g sample of chromium, Cr, combined with oxygen to give 7.333 g of an oxide compound. The percent composition of this compound is:

a) 59.1% Cr, 40.9% Ob) 24.4% Cr, 75.6% Oc) 68.4% Cr, 31.6% Od) 71.5% Cr, 28.5% O

2. Calculate the percent composition. K2CrO4 K =_____ Cr =_____ O = _____ K2Cr2O7 K =_____ Cr =_____ O = _____