Chemical reactions often take
place in aqueous solutions. To perform stoichiometric
calculations in such cases the amounts of chemicals present in solution
– the concentration of solution 
must be known.
Concentration of a solution is a
measurement stating the amount of a solute present in a known amount of
solution:
Concentration = amount of solute / amount of
solution
The terms solute and solution are
usually used for liquid samples but they can be extended to gaseous and solid
samples.
The most common units of
concentration are given in Table I.1:
Common Units of
Concentration


Name

Symbol

Units

molarity

moles solute / liters of
solution

M

molality

moles solute / kg
solvent

m

normality

number of equivalent
weights of solute / liters of solution

N

formality

number of formal weights
of solute / liters of solution

F

weight %

g solute / 100 g of
solution

%
w/w

volume %

ml solute / 100 ml
solution

%
v/v

weighttovolume %

g solute / 100 ml
solution

%
w/v

parts per million

g solute / 10^{6}
g solution

ppm

parts per billion

g solute / 10^{9}
g solution

ppb

Note: Another way of describing
solution concentration is the mole
fraction (xi)
Molarity (M) is defined as the number of moles of solute per liter of
solution.
i.e by dissolving 0.1 mol NaOH in
1 l of H_{2}O gives a solution that contains 0.1 mol Na^{+} and
0.1 mol of OH^{} in 1 l. The concentration of the solution is
[Na^{+}] = 0.1 M and [OH^{}] = 0.1 M.
Since molarity depends on the
volume of the solution it changes slightly with temperature.
Another way of describing solution
concentration is molality
(m) which is the number of moles of solute per kilogram of
solvent.
Molality is independent of temperature since it depends on
mass.
In very dilute aqueous solutions
the molarity (M) and molality (m) are nearly the same.
Example #1
A solution of 1M H_{2}SO_{4} has density 1.04 g/cm^{3}. Calculate the (%w/w) concentration of the solution.
Given

[H_{2}SO_{4}] = 1M
d = 1.04
g/cm^{3}
P_{H2O} =
41 mmHg
MW
_{H2SO4} = 98 g/mole

Asked
for

(%w/w) = ?

From the definition of (%w/w) =
g solute / 100 g of solution
(1)
The mass of solute (g solute) is
unknown but it can be calculated.
Since [H_{2}SO_{4}] = 1M ⇒ 1000 cm^{3} of H_{2}SO_{4}
solution contain 1 mole “pure”
H_{2}SO_{4} (2)
The mass of 1 mole
“pure” H_{2}SO_{4} can be calculated as shown
below:
mass (g) = mole
* MW = 1 mole * 98 g/mole = 98
g (3)
From (2) and d =
m/V = 1.04 g/cm^{3}_{ } the mass of 1000 cm^{3} of
H_{2}SO_{4}can be calculated:
m = d * V = 1.04 g/cm^{3}_{ } * 1000 cm^{3} = 1040 g of H_{2}SO_{4}
solution (4)
From (2), (3) and (4):
Mass of 1040 g of
H_{2}SO_{4} solution contain 98 g of “pure” H_{2}SO_{4}
Mass of 100 g of H_{2}SO_{4} solution
contain x = ? g of “pure” H_{2}SO_{4}
x = 98 g “pure” H_{2}SO_{4} * (100g of H_{2}SO_{4} / 1040g of H_{2}SO_{4}) = 9.42 g of “pure” H_{2}SO_{4}
Therefore,
(%w/w) =
9.42
Provided that the theory and the
definitions of solution concentration units is understood a % solution calculator
can be used.
Meant to be used in both the
teaching and research laboratory, a %
solution calculator can be utilized to perform
a number of different calculations for preparing percent (%) solutions when
starting with the solid or liquid material.
An additional solved example regarding solutions and concentration is shown in the following video
Properties of solutions  Henry'slaw  Effect of Pressure on Solubility
An additional solved example regarding solutions and concentration is shown in the following video
Relevant Posts
Properties of solutions  Henry'slaw  Effect of Pressure on Solubility
References
David W. Oxtoby, H.P.
Gillis, Alan Campion, “Principles of Modern Chemistry”, Sixth
Edition, Thomson Brooks/Cole, 2008
Steven S. Zumdahl,
“Chemical Principles” 6^{th} Edition, Houghton Mifflin Company,
2009
Ralph H. Petrucci,
“General Chemistry”, 3^{rd} Edition, Macmillan Publishing
Co., 1982
Key Terms
solution, concentration,
composition, solutions, molarity M, molality m, normality N, formality F, %w/w,
%v/v, ppm, ppb, IB chemistry, Chemistry Net, how to solve solution concentration
problems, solution concentration molarity, solution concentration units, solution
concentration definition, solution concentration examples
No comments:
Post a Comment