Upload
marcia-baldwin
View
227
Download
1
Tags:
Embed Size (px)
Citation preview
The relative amounts of each element (Part) in a compound (Whole) are
expressed as the Percent Composition or the Percent by Mass of each element in
the compound.
The percent composition of a
compound has as many percent values as there are different elements
in the compound.
To calculate the percent composition of a
compound, use the chemical formula to
determine which elements and how many moles of
each are in the compound.
Divide the total mass of one of the elements in the compound by the molar mass of the compound then multiply by 100 to
convert to a percent.
Remember that the total mass of the element is the molar mass of the
element multiplied by its subscript in the formula.
Example: What is the percent composition of
FeSO4?
Fe = 100xg151.9
g55.8
S = 100xg151.9
g32.1
O = 100xg151.9
g64.0
= 36.7 %
= 21.1 %
= 42.2 %
The Percent By Mass in a compound is the
number of grams of the element divided by the number of grams of the compound multiplied by
100.
Sample Problem 7 - 10 p 189
An 8.20 g piece of magnesium combines with 5.40 g of
oxygen to form a compound. What is the Percent
Composition (Percent By Mass) of this compound?
% Mg = 100xcompoundofmass
Mgofmass
% Mg = %60.3100xg13.60g8.20
% O = 100xcompoundofmass
Oofmass
% O = %39.7100xg13.6g5.40
You can use the percent composition to determine the mass of
each element in a sample of a compound.
Example:Aluminum makes up
52.9 % of Al2O3. Calculate the mass of Aluminum in a 50.0 g
sample of Al2O3.
We first assume that we have a 100.0g sample of Al2O3. 52.9 % of 100.0 g
is 52.9 g. We can use 52.9 g divided by 100.0 g sample as a conversion
factor.
EMPIRICAL FORMULAIf the atoms are in a small whole number ratio, then the moles of atoms of each element in the substance will also be in a small whole number ratio.
Two Sources of Data From Which to Calculate the
Empirical FormulaExperimental Analysis - What
is the empirical formula of a compound if a 2.50 gram sample contains 0.900 g of calcium and 1.60 g of chlorine?
Two Sources of Data From Which to Calculate the
Empirical Formula
Percentage Composition - A compound has a percentage composition of 40.0% C, 6.71% H, and 53.3% O. What is the empirical Formula?
Determine the number of moles of each element in the compound.
What is the empirical formula for a compound if a 2.50 g sample contains 0.900 g of calcium and 1.60 g of chlorine?
To obtain the smallest whole number ratio, divide both numbers of moles by the smaller number of moles.
The next step is to determine the smallest whole number ratio.
The mole ratio is 1 mole of Calcium to 2 moles of Chlorine.
The Empirical formula is therefore CaCl2.
Example of Empirical Formula Calculation From
Percentage Composition:A compound has a percentage composition of 40.0% C, 6.71% H, and 53.3% O. What is the empirical formula?
Example of Empirical Formula Calculation From
Percentage Composition To calculate the ratio of
moles of these elements, we assume a 100 g sample . 40.0% of 100 g is 40.0 g.
Example of Empirical Formula Calculation From
Percentage Composition
Next we calculate the number of moles of each element present.
Example of Empirical Formula Calculation From
Percentage Composition
Cg12.0Cmol1
x1
Cg40.0
1.01gH1molH
x1
6.71gH
16.0gO1molO
x1
53.3gO
Mol C 3.33
Mol H 6.67
Mol O 3.33
The number of moles of each element calculated in step 2 were: 3.33 mol C, 6.64 mol H, and 3.33 mol O. Dividing by the smallest value we obtain the smallest whole number ratio of moles.
4 Determine the smallest Whole Number Ratio.
What is the empirical formula for a compound
if a 40.0 g sample contains 29.7 g of sodium
and 10.3 g of oxygen?
This ratio shows that there are 2 moles of
sodium for one mole of oxygen.
The empirical formula is therefore Na2O.
The Empirical Formula tells us the ratio of each
kind of atom present in a compound.
The Molecular Formula tells us the actual number of each kind of atom in a molecule of a compound.
Example:
A certain gas, having a molar mass of 78 g, has an empirical formula of CH. What is the molecular formula?
If we multiply the mass of the empirical formula
by an integer, the product must equal the
molar mass.
(CH)6 gives C6H6
By multiplying each subscript in the
empirical formula by the integer (6), we obtain the
molecular formula.
Problem:
The empirical formula of a compound is CHOCl and the molar mass is 129 g. What is the molecular formula?