Composition
Stoichiometry
Chapter 7
Imagine…
You are at GAP, a t-shirt that you love is marked
$24.99. Then, you see a 30% off sign above the shirts!
Based on the percentage listed, how much is the shirt
discounted?
What will be the final price for the shirt?
How much will you be charged for tax if KY sales tax is
6%?
Percent Composition by Mass
What is a percent?
A percent is part of a whole quantity.
WAIT!!!! So is a fraction!!!! How are fractions and percents related?
When using percents, we assume that the whole quantity is represented
by the number 100.
In chemistry, we often describe the composition, or make up of a
compound or molecule, by using percent by mass.
We can use the equation: mass of element in the sample x 100
total mass of sample
Percent Composition
Another way to find percent composition by mass is to use the
following equation:
mass of element in 1 mole of a compound
molar mass of compound
x 100= % of element
in compound
o
Let’s do an example.
o
Find the percent of each element by mass in copper (I) sulfide,
Cu2S.
Experimental Example
In the lab, you are given 2.5 grams of a sample of
copper (II) oxide. After it is heated strongly for 30
minutes, 1.95 g of red copper solid is left.
A) How much oxygen was in the sample?
B) What percentage of oxygen was in the sample?
C) What percentage of copper is in the sample?
Empirical Formulas
An empirical formula consists of the symbols for the
elements combined in a compound with subscripts
that show the smallest whole number ratio of the
atoms in the compound.
•
Example: The empirical formula for B2H6 is BH3.
•
To determine the empirical formula, you begin with
percentages of each element or masses of each element
present in the compound.
How to determine empirical
formulas
Step 1: Assume that you have 100 g of your sample.
Convert all percentages to grams.
Step 2: Convert each element to moles by dividing by
its molar mass.
Step 3: Divide all the molar amounts by the smallest
molar amount.
Step 4: The mole ratios become the subscripts in the
empirical formula.
Watch Out!
Sometimes the mole ratios will not be a whole number.
In this case, you must multiply by some value (start
with 2, then 3, then 4 and so on…) to get a whole
number.
Whichever number you choose must be multiplied by
all the subscripts in the formula.
Example: Quantitative analysis shows that a compound
contains 32.38% sodium, 22.65% sulfur and 44.99%
oxygen. Find the empirical formula of this compound.
Analysis of a 10.150 g sample of a compound known to
contain only phosphorus and oxygen indicates a
phosphorus content of 4.433 g. What is the empirical
formula of a compound?
Molecular (True) Formulas
Molecular formulas are the “true” formulas for a
molecule or compound. They consist of all the atoms
that would be present if you were to draw the Lewis
structure.
To find molecular formulas, you MUST have the
empirical formula first! It will either be given to you in
the problem, or you will be able to find it by converting
the percent or grams of elements to mole ratios.
Finding Molecular Formulas
Step 1: Determine the empirical formula.
Step 2: Add up the molar mass of the empirical
formula.
Step 3: Divide the actual mass (ALWAYS given in the
problem) by the empirical molar mass.
Step 4: Multiply each subscript from the empirical
formula by the number you got in step 3.
Example Problems
1) Determine the molecular formula of the compound
with the empirical formula CH and a formula mass of
78.110 amu.
2) If 4.04 g of N combine with 11.46 g O to produce a
molecule with a molar mass of 108.0 g/mol, what is
the molecular formula of this compound?
3) Chemical analysis of citric acid shows that it
contains 37.51% C, 4.20% H and 58.29% O. What is
its molecular formula if its molar mass is 192.12 g/mol?
Determining the Percent
Water in a Hydrate Lab
Hydrates are substances that have water molecules loosely
bound to them. Many ionic salts are hydrates.
Experimentally, we can determine the percent water lost
from a hydrate by heating the sample to “drive off” all the
water.
When we do this, we must mass the sample initially. Then
we heat it for 5 minutes. Once cooled, we mass it again.
Then we reheat for about 2 minutes to ensure that all the
water is gone. When all of the water has been removed, the
anhydrous salt is left.
We can be confident that all the water has been removed
when the last two mass measurements are nearly the same.
Determining the Percent
Water in a Hydrate Lab
To calculate the water lost, subtract the mass of the
anhydrous salt (after the final heating) from the initial
mass of the hydrate.
This will give you the mass of water in the sample.
To find the % water, divide the mass of water by the
mass of the whole sample (hydrate).
Compare this value to the percent of water in each of
the known hydrates. This will help you identify the
unknown sample’s identity.