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How to Use Proportions With Chemical Compounds

Instructor: Matthew Bergstresser
Much of chemistry deals with the proportions of elements in chemical compounds. In this lesson, we will investigate characteristics of these proportions and go through some applications of these proportions.

Ratios

Ratios are everywhere! What is the ratio of dimes to quarters that make up one dollar? Well, each dime is 10 cents, so we need ten of them to make one dollar. A quarter is 25 cents, so we only need four to make a dollar. Therefore, the ratio of dimes to quarters in one dollar is 10:4. This is similar to how ratios work in chemistry. There are seemingly infinite chemical compounds with set ratios of elements in the compounds. Let's dig deeper into the ratio concept with chemical compounds.

Chemical Compounds

The quantity of each element in a chemical compound depends on the quantity of electrons in the elements' outer electron shell, called the valence shell. The subscript after each element's symbol tells us how many of that element's atoms are in the compound. Let's look at a few examples of chemical compounds and the ratio of elements in them.

  • Sodium chloride has the formula NaCl. There is one sodium atom and one chlorine atom, giving sodium chloride a 1:1 ratio.
  • Calcium phosphide has the formula Ca3 P2. The ratio of calcium to phosphorous in this compound is 3:2.
  • Sugar has the formula C12 H22 O11. The ratio of these elements in the compound is 12:22:11.

Now that we know how to determine the ratio of each element in a compound, we can use this knowledge of ratios for several applications. We'll focus on three applications:

  1. Calculating molar mass
  2. Calculating the percent composition
  3. Balancing chemical equations

Molar Mass

The molar mass of each element is listed in the periodic table. When determining the molar mass of a chemical compound, you need to use the ratio of the elements in the compound. The quantity of each element is multiplied by its molar mass and the totals are summed. Let's work an example.

Let's go back to the calcium phosphide compound we discussed earlier. The ratio of calcium to phosphorus is 3:2. We can take this ratio and turn in into an equation to determine the molar mass. The periodic table tells us the mass of each element.

The molar masses of calcium and phosphorus are below the symbols of the elements.
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Since we have three calcium atoms we write:

3(40.08 g/mole)

And then we add to that the product of the two phosphorus atoms times its molar mass, which is:

2(30.97 g/mole)

The next step is to add these products together.

3(40.08 g/mole) + 2(30.97 g/mole) = 120.24 g/mole + 61.94 g/mole = 182.18 g/mole

This means one mole of calcium phosphide has a mass of 182.18 g/mole.

Percent Composition

Determining the percent composition of a chemical compound also involves the ratio of elements in the compound. We can determine the percent of each element in the compound and the percent by mass of each element.

Percent by Each Element

Let's stick with our calcium phosphide example. To determine a percentage we need to use the formula:


percent_eq


The formula is Ca3 P2, which has a total of 5 atoms.


The percent of each element in calcium phosphide


Percent by Mass

Determining the percent by mass of each element requires the same formula, but we use the molar masses of each element in the compound. We determined these values earlier in the lesson, so all we have to do is plug this information into the percent equation.


Determining the percent by mass requires the ratio of the elements in the compound.
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