Moles and Molar Mass

Molar Mass Calculations: Practice Problems

What is the molar mass of calcium bromide?

Start by determining the chemical formula for calcium bromide. Calcium is in Group II of the periodic table. Bromine is in Group VII. That means that the chemical formula for the compound will be: CaBr2.

In other words, one mole calcium bromide consists of one mole of calcium atoms plus TWO moles of bromine atoms.

According to the periodic table: Ca = 40.08 grams/mole and Br = 79.04 grams/mole. 


Ca:
              40.08 grams Ca
1 mole Ca x  --------------- = 40.08 grams Ca
              1 mole Ca

Br:
              79.04 grams Br
2 moles Br x  -------------- = 158.08 grams Br
              1 mole Br
Now add up the masses: 40.08 + 158.08 = 198.16 grams/mole

What is the molar mass of water?
The chemical formula for water is H2O. That means that one mole of oxygen has combined with two moles of hydrogens.

From the periodic table: H = 1.01 gram/mole and O = 16.00 grams/mole


O:
              16.00 grams O
1 mole O x  --------------- = 16.00 grams O
              1 mole O

H:
              1.01 grams H
2 moles H x  --------------- = 2.02 grams H
              1 mole H
Now add up the masses: 16.00 + 2.02 = 18.02 grams/mole

What is the molar mass of sodium nitrate?

The chemical formula for sodium nitrate is NaNO3. This means that sodium nitrate consists of:
        1 mole of sodium (Na)
        1 mole of nitrogran (N)
        3 moles of oxygen (O)

From the periodic table, we see that:
        1 mole of sodium (Na): 22.99 grams/mole
        1 mole of nitrogran (N): 14.01 grams/mole
        1 moles of oxygen (O): 16.00 grams/mole

But - notice that sodium nitrate has THREE moles of oxygen. This means we need to multiply the mass (16.00 grams/mole) x 3 moles.

    Na: 1 mole x 22.99 grams/mole = 22.99 grams
    N: 1 mole x 14.01 grams/mole = 14.01 grams
    O: 3 moles x 16.00 grams/mole

    		 = 48.00 grams
So the molar mass of NaNO3 = (22.99 + 14.01 + 48.00) = 85.00 grams/mole

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