You just learned that the atomic mass that appears underneath the chemical symbol for an element on the periodic table is the mass in grams of one mole of atoms of that element. To calculate the molar mass of a compound, multiply the molar mass of each element by the number of moles of each element, which is represented by the subscript in the formula.
Once you know the molar mass of a substance, you can convert moles of that substance to mass, or mass to moles. This conversion is an important step in many problems in chemistry. Read each tab to learn how this is done.
If you are given the number of moles of a substance, and you want to know the mass of the substance in grams, you can use the molar mass to set up a conversion factor with moles in the denominator, so moles will be eliminated, leaving grams.
Converting Moles to Mass
To convert from moles of a substance to the mass of that substance, multiply the number of moles by:
\( \frac{\text{molar mass (g)}}{1 \text{ mol}} \)
Look at this example:
Convert 4.00 moles of Mg to grams of Mg.
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Step 1: Write the given number and unit. |
4.00 mole Mg |
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Step 2: Set up a conversion factor so the given units cancel. |
The molar mass of magnesium is 24.30 \( \frac{\text{g}}{\text{mol}}. \) 4.00 mol Mg \( \times \frac{24.30\ g\ Mg}{1 \text{ mol } Mg} \) |
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Step 3: Cancel units and solve. |
4.00 |
If you are given the mass of a substance in grams, and you want to know the number of moles of the substance, you can set up a conversion factor with grams in the denominator, so grams will be eliminated, leaving moles.
Converting Mass to Moles
To convert from mass of a substance to the moles of that substance, multiply the mass by:
\( \frac{1 \text{ mol}}{\text{molar mass (g)}} \)
Look at this example:
Convert 1.71 g of C to moles of C.
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Step 1: Write the given number and unit. |
1.71 g C |
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Step 2: Set up a conversion factor so the given units cancel. |
The molar mass of carbon is 12.01 \( \frac{\text{g}}{\text{mol}} \). 1.71 g C \( \times \frac{1 \text{ mol } C}{12.01\ g\ C} \) |
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Step 3: Cancel units and solve. |
1.71 |
The relationship between the molar mass of an element or compound and 1 mol set as a fraction is the conversion factor used to convert between a given mass in grams and number of moles.
Let's Practice
See how well you can calculate the number of moles in a given mass of a substance, and the mass of a given number of moles of a substance by completing this activity. Answer the questions on each tab, then check your answer.
If you need a periodic table, click below to open an interactive periodic table or to download a PDF.
How many moles of Ne are in a sample with a mass of 18.94 g?
0.9386 mol Ne
If you need help arriving at this answer, click the Solution button.
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Step 1: Write the given number and unit. |
18.94 g Ne |
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Step 2: Set up a conversion factor so the given units cancel. |
The molar mass of neon is 20.18 \( \frac{\text{g}}{\text{mol}} \). 18.94 g Ne \( \times \frac{1 \text{ mol } Ne}{20.18\ g\ Ne} \) |
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Step 3: Cancel units and solve. |
18.94 |
Notice that in step 2 the molar mass in grams is in the denominator of the conversion factor so that in step 3 grams are eliminated, leaving the number of moles.
How many moles of NO3 are in a sample with a mass of 267.9 g?
4.320 mol NO3
If you need help arriving at this answer, click the Solution button.
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Step 1: Write the given number and unit. |
267.9 g NO3 |
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Step 2: Set up a conversion factor so the given units cancel. |
The molar mass of NO3 is \( 62.01 \) \( \frac{\text{g}}{\text{mol}} \). 267.87 g NO3 \( \times \frac{1 \text{ mol}}{62.01\ g \ NO_{3}} \) |
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Step 3: Cancel units and solve. |
267.87 |
Notice that in step 2 the molar mass in grams is in the denominator of the conversion factor so that in step 3 grams are eliminated, leaving the number of moles.
What is the mass of 8.55 mol of H2O2?
291 g H2O2
If you need help arriving at this answer, click the Solution button.
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Step 1: Write the given number and unit. |
8.55 mol H2O2 |
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Step 2: Set up a conversion factor so the given units cancel. |
The molar mass of H2O2 is \( 34.02 \) \( \frac{\text{g}}{\text{mol}} \). 8.55 mol H2O2 \( \times \frac{34.02\ g \text{ H}_{2}O_{2}}{1 \text{ mol} \text{ H}_{2}O_{2}} \) |
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Step 3: Cancel units and solve. |
8.55 |
Notice that in step 2 the number of moles is in the denominator of the conversion factor so that so that in step 3 moles are eliminated, leaving grams.
