Mass Ratio Calculations

Copper reacts with chlorine to form two compounds.  Compound A consists of 4.08 g of copper for every 2.28 g of chlorine.  Compound B consists of 7.53 g of copper for every 8.40 g of chlorine.  What is the lowest whole number mass ratio of copper that combines with a given mass of chlorine?

Step 1:  List the known quantities and plan the problem.


Compound A = 4.08 g Cu and 2.28 g Cl

Compound B = 7.53 g Cu and 8.40 g Cl

Apply the law of multiple proportions to the two compounds.  For each compound, find the grams of copper that combine with 1.00 g of chlorine by dividing the mass of copper by the mass of chlorine.  Then find the ratio of the masses of copper in the two compounds by dividing the larger value by the smaller value.

Step 2:  Calculate

Compound A  4.08 g Cu2.28 g Cl=1.79 g Cu1.00 g Cl

Compound B  7.53 g Cu8.40 g Cl=0.896 g Cu1.00 g Cl

Compare the masses of copper per gram of chlorine in the two samples.

1.79 g Cu (in compound A)0.896 g Cu (in compound B)=2.001=2:1

The mass ratio of copper per gram of chlorine in the two compounds is 2:1.

Step 3:  Think about your result.

The ratio is a small whole-number ratio.  For a given mass of chlorine, compound A contains twice the mass of copper as does compound B.

CuCl2. [Figure2]

Summary:The mass ratio gives the mass of an element that is found in combination with another element. 
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