Converting between Mass and Number of Moles
Many calculations you will be required to do in chemistry involve using a conversion factor that allows you to convert between one unit and another. You'll find doing chemistry problems much easier if you remember and recognize what conversion factor you'll need for a particular question, instead of trying to memorize a lot of different mathematical formulas.
Here are some examples of conversion factors you are likely familiar with, expressed in different ways:
| 12 eggs per doz |
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| 1000 mL = 1 L |
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| 1 kg = 1000 g |
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Probably the most common type of conversion you'll need to do is to convert between the number of moles of a substance (the symbol n represents number of moles), and mass (which has the unit g for grams).
The conversion factor you'll need for these calculations is molar mass.
Molar mass is the mass, in grams,
of one mole of a substance
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The molar mass of a substance is numerically equal to the atomic or molecular mass of the element or compound
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You will need a periodic table to find molar mass in order to look up atomic masses.
Generally - if a chemistry question concerns mass, you'll need to determine the molar mass of the substance!
| The unit for molar mass is g·mol-1, or |
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Sample Questions
1. Find the mass of 2.50 mol of water, H2O
Since the question concerns mass, your conversion factor will be the molar mass of water.
Use a periodic table to find the atomic masses of hydrogen (1.0) and oxgyen (16.0) and multiply these by the number of atoms of each element in the compound.
| 2 H = 2 atoms × 1.0 = 2.0 g·mol-1 |
| 1 O = 1 atom × 16.0 = 16.0 g·mol-1 |
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| H2O = 18.0 g·mol-1 |
Use unit analysis to find the desired unit, g (the unit for mass):
g |
= |
18.0 g
mol
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× |
2.50 mol |
= |
45.0 g |
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answer |
2. How many moles are present in 137 g of sodium chloride, NaCl?
Again we will need to determine the molar mass of our substance, NaCl because the question involves mass.
| 1 Na = 1 atom × 23.0 = 23.0 g·mol-1 |
| 1 Cl = 1 atom × 35.5= 35.5 g·mol-1 |
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| NaCl = 58.5 g·mol-1 |
Use unit analysis to find the desired unit, moles:
mol |
= |
1 mol
58.5 g |
× |
137 g |
= |
2.34 mol |
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answer |
