Solutions and Spectroscopy
CH202, Lab 2
Goals : To learn how to prepare a solution by dilution of a stock
solution or by weighing a solid.
To learn the proper use of pipets, volumetric flasks and the
analytical balance.
To use a spectrometer to measure the concentration of an
unknown solution.
Safety : CuSO4·5H2O is listed as toxic and an irritant. If the solid
contacts the skin, brush the solid off with a paper towel
and rinse with lots of water. If the solution contacts the
eyes or skin, rinse with lots of water.
Waste : Please discard waste in the container on the side shelf.
Solutions
mmol solute
mol solute
or
Molarity (M) =
mL solution
L solution
Two Methods to Prepare a Solution of Known Concentration
Method 1 :
Dissolve a weighed amount of
solid in a known volume of solution,
using a balance and volumetric
flask.
Method 2 :
Dilute a known volume of stock
solution in a known volume of
solution, using a pipet and volumetric
flask.
or
g o od to 0 . 0 0 1 g
good to 0.1% volume
good to 0.01 mL
good to 0.1% volume
Calculations Using a Solid
Example :
How many grams of Cu(NO3)2 are needed to prepare 250 mL of
a 0.0300 M solution?
0.250 L solution
.
0.0300 mol Cu(NO3)2
1 L solution
.
187.57 g Cu(NO3)2
1 mol Cu(NO3)2
= 1.41 g Cu(NO3)2
weigh paper
1.410 g
=
250 mL
7.50 mmol Cu2+
7.50 mmol Cu 2+
Calculations Using a Stock Solution
Example : How many of 1.20 M Cu(NO3)2 solution should be diluted to
prepared 250 mL of a 0.0300 M Cu2+ solution?
C1 V1 = Cf Vf
(1.20 M) (V1) = (0.0300 M) (250 mL)
mmol .
1.20
V1 = 7.5 mmol
mL
V1 = 6.25 mL
=
6.25 mL
7.50 mmol Cu 2+
250 mL
7.50 mmol Cu2+
Beer’s Law
Relating concentration to absorbance
A=εl c
A = absorbance = -log (Intensity out /Intensity in)
ε= molar absorptivity (M-1 · cm-1)
l = path length (cm)
c = concentration (M)
Intensity in
1 cm
Intensity out
Part A. Experimental Overview
Students prepare 4 solutions of Cu2+
Dilution of Cu2+ stock solutions
Example : 1.30 mL of a 0.503 M Cu2+ solution is added to 4.70 mL of water.
C1 V1 = C2 V2
.
(0.503 M) (1.30 mL) = C2 (1.30 mL + 4.70 mL)
0.115 M = C2
=
6.00 mL
1.30 mL
0.654 mmol Cu2+
0.654 mmol Cu2+
Part A. Experimental Overview
Students measure the absorbance of the solutions. Cu2+ is blue in color
and absorbs strongly at 620 nm.
Part A. Experimental Overview
Students generate a calibration plot.
[Cu2+ ]
Absorbance
0.225
0.274
0.115
0.140
0.318
0.387
0.426
0.519
Absorbance of Cu^2+ as a Function of
Concentration
The relationship
between Cu2+
concentration
and absorption
should be linear.
Absorbance
0.600
0.500
0.400
0.300
y = 1.19x + 0.0003
R2 = 0.9962
0.200
0.100
0.000
0.000
0.100
0.200
0.300
[Cu^2+]
0.400
0.500
Part A. Experimental Overview
Students will determine the [Cu2+] of an unknown solution.
From previous slide :
y = 1.19 x + 0.0003
Absorbance = (Molar Absorptivity · Pathlength)·concentration + offset
If the unknown has a measured absorbance of 0.322
0.322 = 1.19 · concentration + 0.0003
0.270 M = concentration
Part B. Experimental Overview
Students use CuSO4·5H2O solid and a volumetric flask to
prepare a solution with the same concentration as the unknown.
Part C. Experimental Overview
Students dilute a Cu2+ stock solution to prepare a solution with the
same concentration as the unknown.
Tips for Collecting Good Date
Double-check the function of the spectrometer by collecting a spectrum of
the reference sample. It should look like a flat line.
Measure all volumes carefully from the lowest point of the liquid meniscus
with the sample at eye level.
Follow instructions for collecting absorbance data carefully and check to
ensure the data is linear.