Concentrations of Solutions
Solution = Solvent + Solute
Solvent – water, acid, organic solvent (acetone, benzene, etc)
Solute – salts, acids, organic compounds (sugar) /physical state - solid, gaseous, liquid/
There are a number of ways to express the relative amounts of solute and solvent in a solution.
Percent Composition (by mass) - C% - the grams of solute per 100 grams of solution.
We can consider percent by mass (or weight percent, as it is sometimes called) in two ways:
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The parts of solute per 100 parts of solution.
The fraction of a solute in a solution multiplied by 100.
We need two pieces of information to calculate the percent by mass of a solute in a solution:
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The mass of the solute in the solution.
The mass of the solution.
Use the following equation to calculate percent by mass:
Molarity – CM
Molarity tells us the number of moles of solute in exactly one liter of a solution. (Note that molarity
is spelled with an "r" and is represented by a capital M.)
We need two pieces of information to calculate the molarity of a solute in a solution:
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
The moles of solute present in the solution.
The volume of solution (in liters) containing the solute.
To calculate molarity we use the equation:
Molality – Cm
Molality, m, tells us the number of moles of solute dissolved in exactly one kilogram of solvent.
(Note that molality is spelled with two "l"'s and represented by a lower case m.)
We need two pieces of information to calculate the molality of a solute in a solution:


The moles of solute present in the solution.
The mass of solvent (in kilograms) in the solution.
To calculate molality we use the equation:
Dilution of Solutions
A solution is prepared by dissolving a solute in a solvent, for example, NaCl(aq) tells us that
NaCl was dissolved in water.
When a solution is diluted, more solvent is added to it.
c1 = n1 ÷ V1
c1 = initial concentration in mol L-1 before dilution
n1 = moles of solute
V1 = initial volume of solution in litres before dilution
Rearranging: n1 = c1 x V1
Diluting the solution means adding more solvent:
volume increases, V2=final volume of solution after dilution (L)
concentration decreases, c2=final concentration of solution after dilution (mol L-1)
moles of solvent remains the same, so, n1=moles of solute (mol)
For the diluted solution n1 = c2 x V2
Since the moles of solute = n1 both before and after dilution:
c1V1 = c2V2 = const
To calculate the new concentration (c2) of a solution given its new volume (V2) and its original
concentration (c1) and original volume (V1):
c2 = (c1 x V1) ÷ V2
To calculate the new volume (V2) of a solution given its new concentration (c2) and its original
concentration (c1) and original volume (V1):
V2 = (c1 x V1) ÷ c2