Solution Concentration

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Solution Concentration

• Qualitative
• Quantitative
• -Mass Percent
• -Molarity
• -Mole Fraction
• -Molality

2
Definition of Concentration
Concentration refers to the amount of solute
present in a solution
We commonly use both qualitative and quantitative
terms to describe solution concentrations.
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Qualitative Descriptions of Concentration
Concentrated indicates the presence of a large
amount of solute
Dilute indicates the presence of a small amount
of solute
Saturated indicates that the solution contains
the maximum amount of solute possible
(Note that because some solutes are only slightly
soluble, it is possible for a solution to be
saturated and still be dilute.)
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Quantitative Descriptions of Concentration
Quantitative descriptions indicate the precise
amount of solute present in a specified amount of
solvent.
There are four commonly used units of
concentration
Molarity
Mass Percent
Mole Fraction
Molality
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Molarity
The most commonly used concentration unit,
molarity is defined as...
the number of moles of solute per liter of
solution.
moles of solute
molarity
liters of solution
n
M
V
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Molarity (continued)
It is important to note that the volume is of the
entire solution, not just the solvent, and that
it must be expressed in liters, not milliliters.
When preparing a solution with a specific
molarity, a volumetric flask is used (the image
shown is of a 500 mL flask). First the required
amount of solute is placed in the flask and
dissolved in a small amount of solvent.
After the solute has completely dissolved, enough
solvent is added so that the total volume is
correct as indicated by the mark on the neck of
the flask.
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Molarity (example)
How would 500.0 mL of 0.250 molar glucose
solution be prepared?
1. First determine the number of moles of glucose
needed using the molarity equation.
moles of solute
molarity
liters of solution
moles of solute
(molarity)
(liters of solution)
moles of solute
(0.250 M)
(.5000 L)
moles of solute 0.125 mols
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Molarity (example continued)
2. Find the mass of sucrose required by
multiplying by its molar mass (C6H12O6, 180.2
g/mol).
mass
(number of moles)
(molar mass)
mass
(0.125)
(180.2)
mass 45.0 grams
So, to make 500. mL of 0.250 M solution, place
45.0 g of sucrose in a 500 mL volumetric flask.
Add about 100 mL of distilled water and dissolve
the solid. Then fill the flask to the mark so
that there is 500.0 mL of solution (not of
solvent.)
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Mass Percent
Mass percent is simply the mass of the solute
divided by the total mass of the solution put in
percent form.
mass of solute
Mass Percent
x 100
mass of solution
(The mass percent of the solvent can also be
found)
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Mole Fraction
Mole fraction is the number of moles of one
component of the solution divided by the total
number of moles present. The symbol for mole
fraction is the Greek letter chi (?).
moles of one component
mole fraction
total moles in the solution
The mole fraction can be calculated for the
solute, for the solvent, or for any single
substance in a solution containing several
solutes.
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Mole Fraction (continued)
For instance, in a solution formed by dissolving
substance A in substance B, the mole fraction
of A would be ...
moles of A
mole fraction of A
total moles
nA
?A
ntotal
and the mole fraction of B would be...
moles of B
mole fraction of B
total moles
nB
?B
ntotal
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Molality
Molality is defined as...
the number of moles of solute per kilogram of
solvent.
moles of solute
molality
kg of solvent
n
m
mkg