Chemistry: how to find the molar mass
Tutoring high school chemistry, the mole is a central theme. The tutor shows, explicitly, how to find the molar mass of a compound.
Back on Sept 15 I opened the discussion about what a mole is. Whether or not students ever get comfortable with it, they need to use the concept. Calculations in chemistry center around moles; here’s an example that shows why:
2C2H6 + 7O2 → 4CO2 + 6H2O
The equation above means “2 moles C2H6 with 7 moles O2 reacts to produce 4 moles carbon dioxide and 6 moles water.” Reactions happen based on ratios of moles. Therefore, when predicting the output (or required input) of a reaction, moles are central.
Concerning moles, a primary skill a chemistry student needs to develop is finding the molar mass. Let’s see how it’s done in the following example:
Example: Find the molar mass of (NH4)3PO4
Solution: From the periodic table, we find the mass of each atom present, then multiply by its frequency in the compound. Afterward, we sum up the contributions to arrive at the molar mass:
atom | mass(g) | mulitplicity | mass contribution(g) |
N | 14.0 | 3 | 42.0 |
H | 1.01 | 12 | 12.12 |
P | 31.0 | 1 | 31.0 |
O | 16.0 | 4 | 64.0 |
MOLAR MASS(M.M.) | 149.12 |
So, apparently, the molar mass (aka M.M.) of ammonium phosphate (aka (NH4)3PO4) is 149.1g.
Even when finding M.M. on paper, using a tabulated format helps you keep track of more involved calculations:)
Source:
Mortimer, Charles E. Chemistry. Belmont: Wadsworth, Inc., 1986.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
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