Download Percent Composition

Chemistry 11
Sec 5.4-5.5
Sec 5.4
Percent Composition
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
numerical values, in the form of percents, that show the composition of a
compound
calculated by:
the mass of the element ÷ by the mass of the compound x 100
example;
H2O is composed of 2 moles of hydrogen and 1 mole oxygen.
Calculate the percent composition by mass of H2O.
Example; Calculate the percent composition by mass of K2Cr2O7
Percent composition can be used to determine the mass of an element in a certain sample
size.
Ex; Calculate the mass of Carbon in a 25.0g sample of CO2
Pg 91 # 44 b, e, g, 45a
Chemistry 11
Sec 5.4-5.5
Percent Composition Practice
Whiteboard
1. Calculate the percent composition of the elements in the compound CH3COOH
2. Calculate the percent composition of the elements in the compound (NH4 )2 CO3
3. Calculate the percent composition of Carbon in the compound Glucose, C6H12O6
4. Calculate the mass of Na in 125.0g sample of the compound Na3 PO4
Chemistry 11
Sec 5.4-5.5
Sec 5.5
Empirical Formula
The term empirical means experimentally determined.
Determination of the empirical formula of a substance is a common method of revealing
the simplest formula of a substance from a chemical analysis.
There are 4 basic steps in assigning the simplest formula to a substance.
1. the percent composition of the compound can be the mass of each element,
2. determine the number of moles of each element.
3. Determine the smallest whole number relationship of the moles of the elements in the
compound. To do this, divide the number of moles of each element by the smallest
number of moles.
4. multiply the mole values to convert them to a whole number, these whole numbers
are the subscripts in the formula of the compound.
Here is a little rhyme that will help…don’t laugh at me…this works!
Percent to mass
Mass to mole
Divide by small
Multiply 'til whole
Example 1.
A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What is the
empirical formula?
(1) Percent to mass
(2) Mass to moles
(3) Divide by small (4) Multiply 'til whole:
and the formula of the compound is ______________________________
Chemistry 11
Sec 5.4-5.5
Example 2.
A compound is analyzed and found to contain 68.54% carbon, 8.63% hydrogen, and
22.83% oxygen. What is the empirical formula?
(1) Percent to mass
(2) Mass to moles
(3) Divide by small (4) Multiply 'til whole:
The empirical formula of the compound is
Example 3.
Analysis of a compound revealed the following composition by weight : zinc-52.0%;
carbon-9.6%; and oxygen-38.4% . Calculate its simplest formula.
Pg 93 # 46c, d, g, m
Chemistry 11
Sec 5.4-5.5
Empirical Formula Practice
Whiteboards
1. A compound is composed of 7.20g of carbon, 1.20g of Hydrogen and 9.60g of
Oxygen. What is the empirical formula of the compound?
2. A Charcoal briquette composed of carbon has a mass of 43.2g. When burned it
combines with oxygen and the resulting compound has a mass of 159.0g. What is the
empirical formula of the compound?
3. A 15.53g sample of iron oxide is heated and the oxide driven off. Once this process is
complete the pure iron that remains has a mass of 10.87g. What is the empirical formula
of the compound?
Chemistry 11
Sec 5.4-5.5
Molecular Formula
The molecular formula is simply a multiple of the empirical formula and is the
CORRECT formula of the compound.
Empirical
HO
CH
CH
NO
NO2
Molecular
H2O2
C2H2
C6H6
NO
N2O4
Example 1.
A compound has an empirical formula of ClCH2 and a molecular weight of 98.96 g/mol.
What is its molecular formula?
Example 2
Given that the empirical formula of a certain compound is CH and the molar mass is
104 g/mol, calculate the molecular formula.
Chemistry 11
Sec 5.4-5.5
Now it is time for the difficult questions. In these questions, you will not be given the
molar mass of the molecular formula and you will be asked to find the molar mass given
different pieces of information.
REMEMBER THE UNITS FOR MOLAR MASS IS g/mol
For example:
1. from moles and grams
Find the molar mass if 0.0250 moles of X has a mass of 1.775g
2. from the density of a gas at STP
Find the molar mass if the density of gas X is 1.43 g/L at STP.
3. from the mass and volume of a gas at STP
Find the molar mass if 0.0425L of gas X at STP has a mass of 0.135g
pg 95 # 47, 48, 49, 52, 54