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# Conversions between Mass and Number of Particles

## Unit conversion techniques

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Conversions between Mass and Number of Particles

How much gas is there?

Avogadro was interested in studying gases.  He theorized that equal volumes of gases under the same conditions contained the same number of particles.  Other researchers studied how many gas particles were in a specific volume of gas. Eventually, scientists were able to develop the relationship between number of particles and mass using the idea of moles.

### Conversions Between Mass and Number of Particles

In the last lesson, you learned how to convert back and forth between moles and the number of representative particles.  Now you have seen how to convert back and forth between moles and mass of a substance in grams.  We can combine the two types of problems into one.  Mass and number of particles are both related to grams.  In order to convert from mass to number of particles or vice-versa, it will first require a conversion to moles.

Conversion from number of particles to mass or from mass to number of particles requires two steps

#### Sample Problem: Converting Mass to Particles

How many molecules is 20.0 g of chlorine gas, Cl 2 ?

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

Known

• molar mass Cl 2 = 70.90 g/mol
• 20.0 g Cl 2

Unknown

• number of molecules of Cl 2

Use two conversion factors.  The first converts grams of Cl 2 to moles.  The second converts moles of Cl 2 to the number of molecules.

Step 2: Calculate.

$20.0 \ \text{g} \ \text{Cl}_2 \times \frac{1 \ \text{mol} \ \text{Cl}_2}{70.90 \ \text{g} \ \text{Cl}_2} \times \frac{6.02 \times 10^{23} \ \text{molecules Cl}_2}{1 \ \text{mol} \ \text{Cl}_2}=1.70 \times 10^{23} \ \text{molecules Cl}_2$

The problem is done using two consecutive conversion factors. There is no need to explicitly calculate the moles of Cl 2 .

Step 3: Think about your result.

Since the given mass is less than half of the molar mass of chlorine, the resulting number of molecules is less than half of Avogadro’s number.

#### Summary

• Calculations are illustrated for conversions between mass and number of particles.

#### Practice

Read the material at the link below and then do practice problems on page 9 and the problem on page 17 (don’t peak at the answers until you have tried the problems).

#### Review

1. Why can’t we convert directly from number of particles to grams?
2. How many atoms of chlorine are present in the problem above?
3. The periodic table says the atomic weight of chlorine is 35.5. Why can’t I use that value in my calculations?

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