| When calculating percent
composition, you are determining the relative percent mass that each
element contributes to the total mass of the compound. For example,
the gram formula mass of H2O is 18.0 g/mol.
I arrived at this number by adding the mass of oxygen, 16.0, and the
mass of two hydrogen, 2.0. Oxygen makes up 16.0 of the total 18.0
grams. Hydrogen is 2.0 of 18.0 grams. If you divide each elements
total mass by the compounds total mass and multiplying this result
by 100, you will see that water is 88.8% oxygen by mass and 11.1%
hydrogen by mass. |
Calculating a substance's percent composition
Calculate the percent composition of ammonium nitrate.
|
Description of Action |
Action |
| 1.
Write the formula for the given compound. |
NH4NO3 |
| 2.
Record the amount of each element in the compound. (Note: There are
2 total nitrogen, so record them together.) Multiply the amount of
each element by its atomic weight (measured to the tenths place).
Add the results to find the gram formula mass of the compound. |
N: 2 x 14.0
= 28.0
H: 4 x 1.0 = 4.0
O: 3 x 16.0 = 48.0
80.0 g/mol
|
| 3.
Divide the total mass of each element by the gram formula mass. For
these calculations your answer should have 3 places after the decimal
(round if necessary). |
N: 28.0 ÷
80.0 = 0.350
H: 4.0 ÷ 80.0 = 0.050
O: 48.0 ÷ 80.0 = 0.600
|
| 4.
Multiply each result by 100. Add the % symbol to your new result.
(If you were to add up your percentages the must equal 100.) |
N: 0.350 x 100
= 35.0%
H: 0.050 x 100 = 5.0%
O: 0.600 x 100 = 60.0%
100.0%
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