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Molar Mass Calculations
Let's look at one more step-by-step example of calculating molar mass and then apply what we know to chemical reactions.
Calculate the molar mass of CaSO4.
1. List the elements and how many of each there are:
Ca 1
S 1
O 4
2. Multiply each one by its mass on the periodic table.
Ca 1 x 40.1g = 40.1g
S 1 x 32.1g = 32.1g
O 4 x 16.0g = 64.0g
3. Add up all the totals to get the total molar mass for the compound.
Ca 1 x 40.1g = 40.1g
S 1 x 32.1g = 32.1g
O 4 x 16.0g = 64.0g
40.1g + 32.1g + 64.0g = 136.2 g/mol
The molar mass of CaSO4 is 136.2 grams/mole.
Verify Law of Conservation of Mass
Now let's apply what we know to chemical reactions, and use it to verify the Law of Conservation of Mass.
2H2 + O2 → 2H2O
Let's look at this chemical reaction. Notice there are coefficients to ensure that we have the same number of atoms on each side of the reaction. This is called a balanced chemical equation. Those coefficients actually represent the number of moles. So when we find the molar mass for each substance, we will have to multiple the mass by how many moles we have in the balanced reaction.
2CoefficientH2 + O2 → 2Coefficient H2O
Let's find the molar mass of both the reactants and the products. If the Law of Conservation of Mass is true, both sides should have the same total mass.
Let's start with the reactants:
H2
H 2 x 1.00g Remember, this number comes from the periodic table. = 2.00 g/mol
However, there are 2 total H2 moles (The 2 coefficient in front). Therefore we need to multiply our final answer by 2.
2.00g H2 x 2 moles H2 = 4.00g total
O2
O 2 x 16.0g Remember, this number comes from the periodic table. = 32.0g
So the total mass of our reactants is 4.00g + 32.0g = 36.0g.
Now let's do the products:
H2O
H 2 x 1.00g = 2.00g
O 1 x 16.0g = 16.0g
2.00g + 16.0g = 18.0 g/mol
However, notice the coefficient in front of H2O. We have 2 total moles of water in the products so we need to multiply our answer by 2. This means the total mass for our products in 18.0g x 2 = 36.0g.
The total mass of the reactants (36.0g) = the total mass of the products (36.0g) so we have been able to verify the Law of Conservation of Mass.
Use the Try-It to help you practice some math problems like these before moving onto the Task and Quiz for this lesson.