Percent Composition (CHEM 1405)

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Percent Composition is the percent of each element, by mass, present in a chemical compound.  It is probably one of the most important pieces of information that can be determined about a chemical compound.  Percent composition data is usually gathered experimentally and it can be used for a variety of things including confirming the identity of a new chemical!  Percent composition can also be used to…  determine if a mine has a rich enough deposit of a mineral to make it worth the mining expense…  determine the amount of elements in foods and nutrients…  determine if two samples are of the same chemical compound, etc.  Please study the examples I have provided and then pursue some percent compositions on your own!

 

Example One:

From laboratory measurements, a sample of a pure compound is known to have a mass of 3.74 grams.  Analysis of the sample shows 1.10 g calcium, 0.880 g sulfur, and 1.76 g oxygen.  What is the percent composition of this compound?

$\frac{1.10gCa}{3.74g}\times100\%=$ 29.4% calcium

$\frac{0.880gSulfur}{3.74g}\times100\%=$ 23.4% sulfur

$\frac{1.76gOxygen}{3.74g}\times100\%=$ 47.1% oxygen

 

Example Two:

Calculate the percent composition of copper (II) nitrate, Cu(NO₃)₂, from its formula.

$1moleCu\times(\frac{63.55gCu}{1molCu})=63.55gCu$

$2moleN\times(\frac{14.01gN}{1molN})=28.02gN$

$6moleO\times(\frac{16.00gO}{1molO})=96.00gO$

Molar Mass:  $63.55g+28.02g+96.00g=187.57g/mol$

$\frac{63.55gCu}{187.57gCu(NO_3)_2}\times100\%=$ 33.9% Cu

$\frac{28.02gN}{187.57gCu(NO_3)_2}\times100\%=$ 14.9% N

$\frac{96.00gO}{187.57gCu(NO_3)_2}\times100\%=$ 51.2% O

 

You should note that the basic premise behind both examples of calculating percent composition is that:

 

 

Please show complete work.  Don’t forget to label your work with units and substances.

1.  A 9.14 gram sample contains 4.77 g carbon, 1.19 g hydrogen, and 3.18 g oxygen.  What is the percent composition, by mass, of this compound?

 

_________ % carbon   

_________ % hydrogen           

_________ % oxygen

 

 

 

2.  A 2.85 gram sample contains 0.82 g magnesium, 1.62 g oxygen and the remainder is carbon.  What is the percent composition, by mass, of this compound?

 

_________ % magnesium        

_________ % oxygen  

_________ % carbon

 

 

 

3.  Calculate the percent composition of aluminum sulfate from its formula, Al₂(SO₄)₃.

 

_________ % aluminum          

_________ % sulfur               

_________ % oxygen

 

 

 

4.  Calculate the percent composition of ammonium chloride from its formula.

 

_________ % nitrogen 

_________ % hydrogen

_________ % chlorine

 

 

 

Now, calculate percent composition and use it to answer the question…

5.  Two samples containing only iron and oxygen were analyzed.  Sample #1 weighed 56.0 grams of which 43.5 grams was iron and the rest oxygen.  Sample #2 weighed 177.0 grams of which 123.7 grams was iron and the rest oxygen.  Are the two samples the same compound?  Why/why not?

 

Sample #1 __________ % iron                                                             

Sample #2 __________ % iron

 

 

 

6.  A mining chemist has been sent a sample of copper-containing ore and has been told that due to mining costs, the sample must contain at least 30 % copper to make the mine turn a profit.  He analyzes the 56.4 gram sample of ore and finds it contains 11.3 grams of sulfur, 22.6 grams of oxygen, and the rest is copper.  Based on his analysis, should the mining operation begin?  Why/why not?

 

__________ % copper

 

 

 

7.  Sodium phosphate is often an ingredient in detergents.  Calculate the percent composition of sodium phosphate and then determine how many grams of phosphorus are contained in a 908 gram sample of sodium phosphate.

 

__________ % sodium                        

__________ % phosphorus                              

__________ % oxygen

 

__________ grams of phosphorus