Percent Composition!

Percent composition today! Hurray!

The percent composition of a compound is a relative measure of the mass of each different element present in the compound. To calculate percent composition, we first need to calculate the molecular mass of the compound. Then, calculate the total mass of each element present in the formula of the compound. Finally, determine the percent composition by mass of the element. Remember that percentages always add up to be 100!

That really is all there is to it! Now let's take up some examples.

EXAMPLES:
-What is the percent composition of the element Hydrogen in water?
 (2/18) = 0.1111... or 11.11%

-What is the percent composition of the element Oxygen in water?
(16/18) = 0.8889 or 88.89%

-Find the percentage composition of a compound that contains 1.94g of Carbon, 0.48g of Hydrogen, and 2.58g of Sulfur in a 5.00g sample of the compound.
C: 1.94g/5.00g = 38.8% C

H: 0.48g/5.00g = 9.6% H

S: 2.58g/5.00g = 51.6% S

-What is the percantage composition of (NH₄)₂CO₃?
N: 28/96.0 or 29.17% N

H: 8/96.0 or 8.33% H

C: 12.0/96.0 or 12.5% C

O: 48.0/96.0 or 50% O

-After answering the exercise above, how much oxygen is there in an unknown compound with a mass of 50g?
0.5 x 50 = 25g O

-A sample of an unknown compound with a mass of 0.847g has the following composition: 50.51 percent fluorine and 49.49 percent iron. When this compound is decomposed into its elements, what mass of each element would be recovered?
F: x/0.847 = 50.51/100
100x = 0.847(50.51)
100x/100 = 42.78197/100
x = 0.4278197g or 0.428g F

Fe: x/0.847 = 49.49/100
100x = 0.847(49.49)
100x/100 = 41.91803/100
x = 0.4191803g or 0.420g Fe







Next time: Empirical Formulas!

Density and Moles!

Density and Moles today! Hurray! Density is a measure of mass per volume and is measured in g/L or g/mL.

We use the formula d=m/V to determine density.

 


EXAMPLES:
-Water has a density of 1.0 g/mL. Determine the mass of 11.5 mL of water.


-An unknown compound has a molar mass of 65.0 g/mol. If 0.25 mol occupies a volume of 50 mL determine the compound's density.


-The density of Aluminum is 2.70g/mL. A solid piece of Aluminum has a volume of 45.0 mL. Determine the number of Aluminum atoms present.


DENSITY OF GASES (@STP)
The density of gases varies with temperature. At STP we can find density by:


EXAMPLES
-Calculate the density of O₂ at STP.
-An unknown diatomic gas has a density of 1.69 g/L at STP. Determine the molar mass and chemical formula.
Next time: Percent Composition!