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Mass Ratio Calculation

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Mass Ratio Calculation

Credit: Laura Guerin
Source: CK-12 Foundation

What are the similarities and differences between these two equations?

One of the fundamental laws of chemistry deals with the fact that we cannot (using chemical means) create or destroy matter.  When a reaction is run, the number of atoms of each specific type must be the same on both sides of the equation.  For some materials, it turns out that one element can combine with a second element in more than one ratio.  Carrying out mass ratio calculations helped establish the law of multiple proportions.

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.

Known

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  $\frac{4.08 \ \text{g Cu}}{2.28 \ \text{g Cl}}=\frac{1.79 \ \text{g Cu}}{1.00 \ \text{g Cl}}$

Compound B  $\frac{7.53 \ \text{g Cu}}{8.40 \ \text{g Cl}}=\frac{0.896 \ \text{g Cu}}{1.00 \ \text{g Cl}}$

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

$\frac{1.79 \ \text{g Cu (in compound A)}}{0.896 \ \text{g Cu (in compound B)}}=\frac{2.00}{1}=2:1$

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

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.

Credit: User:Chemicalinterest/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Copper_chloride_crystals.JPG

CuCl 2 . [Figure2]

Summary

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

Practice

Questions

1. What is the mass ratio?
2. What is the hydrogen:water mass ratio?
3. How many molecules of water per molecule of oxygen?

Review

Questions

1. What does the mass ratio tell us?
2. In the compound CH 4 , what is the carbon:hydrogen mass ratio?
3. Methane is CH 4 and ethane is C 2 H 6 . What is the mass ratio of carbon per gram of hydrogen in the two compounds?

1. [1]^ Credit: Laura Guerin; Source: CK-12 Foundation; License: CC BY-NC 3.0
2. [2]^ Credit: User:Chemicalinterest/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Copper_chloride_crystals.JPG; License: CC BY-NC 3.0

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