Lesson 10
Home Up (8) Formula and Formula Weight (9) Composition

 

(9) Composition

Obj. 9.  From the name of a compound, determine the composition (weight percent).

To determine the composition of a compound you need to expand on what you just did in the previous exercise. That involves figuring out the formula, then the formula weight and then the percentage of each one.

 

Exercises

What is the composition (in weight percent of each element) of the following compounds?

a. calcium nitrate
b. sodium sulfide
c. carbon dioxide
d. iron(III) chloride
e. iron(III) sulfide
f. aluminum oxide
g. ammonium carbonate

 

Worked-Out Example (a)

Calcium nitrate consists of calcium ion (+2) and nitrate ion, which has a -1 charge. So the formula for calcium nitrate, is Ca(NO3)2. That formula contains one calcium, two nitrogens, and six oxygens. The atomic weight for calcium is 40.1; the atomic weight for nitrogen is 14.0, so times two gives 28.0; and the atomic weight for oxygen is 16.0 times six of them gives 96.0. The sum of all of those atomic weights is 164.1. So, one mole of calcium nitrate weighs 164.1 grams. Of that, 40.1 grams is calcium; 28.0 grams is nitrogen; and 96.0 grams is oxygen.
Ca(NO3)2
Element Number of atoms Atomic weight Weight
Ca 1 40.1 40.1
N 2 14.0 28.0
O 6 16.0 96.0
Formula weight 164.1

 

To calculate the percentages, take each of those parts and divide by the total. So the percentage for calcium is 40.1 grams of calcium over 164.1 grams total and that comes out to be 24.4%. For nitrogen, it's 28.0 grams of nitrogen divided by 164.1 grams total and that comes out to be 17.1%. For oxygen, we have 96.0 grams of oxygen divided by 164.1 grams total and that comes out to be 58.5%. As a double check on the mathematics, add up the percentages to make sure they come up to 100%.
Element Fraction Percent
Ca 40.1
164.1		 24.4%
N 28.0
164.1		 17.1%
O 96.0
164.1		 58.5%
total 100.0%

 

Answers to Exercises

What is the composition (in weight percent of each element) of the following compounds?

a. calcium nitrate - Ca(NO3)2

24.4% Ca
17.1% N
58.5% O

b. sodium sulfide - Na2S

58.9% Na
41.1% S

c. carbon dioxide - CO2

27.3% C
72.7% O

d. iron(III) chloride - FeCl3

34.4% Fe
65.6% Cl

e. iron(III) sulfide - Fe2S3

53.7% Fe
46.3% S

f. aluminum oxide - Al2O3

52.9% Al
47.1% O

g. ammonium carbonate - (NH4)2CO3

29.1% N
8.4% H
12.5% C
49.9% O


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