South Pasadena • Honors Chemistry
Name
4 • Salts and Solutions
Period
4.2
NOTES
Date
– CONCENTRATION
Concentration
Dilute solution – a solution with less solute.
Concentrated solution – a solution with more solute.
Concentration of a solution can be seen by depth of color (in a colored solution), taste (for food chemicals),
or conductivity (for electrolytes).
Compare solutions of CuCl2: (these are 0.05 M, 0.10
M, and 0.05 M, respectively)
 Solution 1: 0.67 g CuCl2 in 100 mL flask.
Solution 2: 1.34 g CuCl2 in 100 mL flask.
Solution 3: 1.34 g CuCl2 in 200 mL flask.
 Conclude that concentration is directly related to
amount of solute, and inversely related to volume.
moles of solute
Molarity (M) – liters of solution
n
M=
V
Other Concentration Units
moles of solute
Molality (m) – kg of solvent
moles of solute
Mole fraction () – total moles
mass of solute
Mass percent – total mass × 100%
Percent mass by volume (% m/v) –
grams of solute
× 100%
mL of solution
Prepare a 1.5% m/v NaCl solution by adding 1.5 g
NaCl in 100 mL water.
mL of solute
Volume Percentage (% v/v) – mL of solution × 100%
Examples:
 Find the molarity if 0.25 moles of CaCl2 is dissolved to prepare a 1.20 L solution.
n 0.25 mol
M=?
M = V = 1.20 L = 0.21 M
n = 0.25 mol
We can write it this way: [CaCl2] = 0.21 M
V = 1.20 L
We say: “the concentration of calcium chloride is 0.21 molar.”

Find the total volume of a 0.30 M KBr solution if 0.72 mol KBr are dissolved.
n 0.72 mol
M = 0.30 M
V= =
= 2.4 L
M 0.30 M
n = 0.72 mol
V=?

Find the mass of NaCl needed to prepare 2.50 L of a 0.15 M solution.
0.15 mol
M = 0.15 M
n = V × M = (2.50 L) 1 L  = 0.38 mol


n=?
Molar Mass of NaCl = 1(22.99) + 1(35.45) = 58.44 g/mol
V = 2.50 L
58.44 g
m=?
m = 0.38 mol  1 mol  = 22 g


Preparing a Solution
 Obtain a volumetric flask for the volume of solution you want to prepare.
 Weigh out the mass of the solute using a balance.
 Transfer the solute into the volumetric flask.
 Fill the volumetric flask with distilled water.
 Stir the solution until the solute dissolves.
Dilutions
Performing a Dilution:
 Obtain a volumetric flask for the volume of solution you want to prepare.
 Transfer a small volume of the stock (concentrated) solution to the volumetric flask.
 Fill the volumetric flask with distilled water.
 Stir the solution.
In a dilution, amount of solute from transferred concentrated solution is the same as that in the diluted
solution, so nconcentrated = ndilute
Dilution Formula: Mconcentrated × Vconcentrated = Mdilute × Vdilute
Example: Find the volume of a 3.2 M HBr stock solution required to prepare 150 mL of a 0.40 M solution.
Mconcentrated = 3.2 M
Mconcentrated × Vconcentrated = Mdilute × Vdilute
Vconcentrated = ?
(3.2 M)(Vconc) = (0.40 M)(0.150 L)
Mdilute = 0.40 M
Vconc = 0.019 L or 19 mL
Vdilute = 0.150 L
Colorimetric or Spectrophotometric Analysis*
“measuring color” or “measuring the appearance of light”
Spectrophotometry – we want to use the degree of light that is absorbed in a colored solution to determine the
concentration of that solution. The darker the solution, the more light is absorbed, the greater the concentration.
When we perform this experiment, we want to use the wavelength of light that is best absorbed by the solution.
LIGH
Spectrophotometer
T
I0
Transmittance – the degree to which light passes
I
detector
through a solution.
Absorbance – “inversely” related to transmittance,
directly related to concentration.
I
T=I
0
A = − log T
Beer’s Law
A=εLc
A = absorbance (unitless)
ε = extinction coefficient or molar absorptivity (a property of the solution), (units of L·mol−1·cm−1)
L = length of cuvette (usually 1 cm)
c = concentration in molarity (units of mol·L−1)
Example: A 0.20 M solution of CoCl2 has an absorbance of 0.27, and a
0.50 M solution has an absorbance of 0.69. Assume the length of the
cuvette is 1 cm.
 What is the value of the molar absorptivity (extinction coefficient)
of CoCl2?
Using first data point:
A = 0.27
A=εLc
ε=?
(0.27) = ε (1 cm)(0.20 M)
L = 1 cm
ε = 1.35 L·mol−1·cm−1
c = 0.20 M
(Use graph to show the linear relationship.)
 What is the concentration of an unknown CoCl2 solution with an absorbance of 0.38?
A = 0.38
First estimate the concentration using the graph.)
−1
−1
ε = 1.35 L·mol ·cm
A=εLc
L = 1 cm
(0.38) = (1.35 L·mol−1·cm−1)(1 cm)(c)
c=?
c = 0.28 M