Transcript Concentration and Dilution

CONCENTRATION AND DILUTION
DEFINITIONS
Solute-the minor component of a solution;
dissolved in the solvent; usually solid or liquid
 Solvent-the component of a solution that
dissolves the solute; much larger amount than
the solute; typically a liquid
 Concentration-amount of solute in a certain
amount of solvent or solution
 Molarity (M)- one way to describe the
concentration of a solution

molarity = moles of
solute
liters of
solution
DEFINITIONS CONTINUED
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Dilution – lab procedure for preparing a less
concentrated solution from a more concentrated one.
**Does not alter the number of moles present
moles of solute after dilution = moles of solute before dilution
M1V1 = M2V2
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Standard Solution – a solution whose concentration
is definitively known
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Concentrated
Make dilutions from this standard
SAMPLE PROBLEM 1: MOLARITY

Tyipcal blood serum is about 0.14 M NaCl. What
volume of blood contains 1.0 mg NaCl?
SAMPLE PROBLEM 2: CONCENTRATION OF
IONS

Give the concentration of each type of ion in the
following solution:
0.50 M Co(NO3)2
SAMPLE PROBLEM 3: CONCENTRATION OF
IONS
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Calculate the number of moles of Cl- ions in 1.75
L of 1.0 x 10-3 M ZnCl2.
SAMPLE PROBLEM 4: DILUTION
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What volume of 16M sulfuric acid must be used
to prepare 1.5 L of a 0.10 M H2SO4 solution?
**Remember: always add acid to water! Think
about what this would look like in the lab…
SPECTROPHOTOMETERS-DEFINITIONS
Transmittance-fraction or percentage of light
that passes through the solution relative to the
light that passes through the solvent (incident
light)
 Absorbance-the amount of light that is absorbed
by the solution.
 Blank-Sample that contains everything in the
solution except the solute to be measured.
(Therefore the concentration of the solute is 0.)
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SPEC-20 INSTRUCTIONS
Instructions:
1) Plug in turn on machine, and allow to warm up for at least 10
minutes.
2) Set the wavelength to the pre-determined wavelength for the
particular sample using the wavelength knob. This may be constant
for the entire lab or may change depending on what you are
measuring.
3) Fill a cuvette with a portion of the solvent. Wipe the sides of the
cuvette with a Kim wipe.
4) Place the cuvette in the sample holder and close the cap. Adjust the
light control knob so the transmittance is 100% (and the absorbance
is 0).
5) For each sample, place a portion in a cuvette, wipe the cuvette with a
Kim wipe, and place in the sample holder, close the cap, and take
measurement.
IMPORTANT THINGS TO REMEMBER:
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Avoid touching the sides of the cuvette and
always wipe the sides before placing in the spec
20
Make sure to blank the machine before taking
measurements, and when you change
wavelengths or solutions measured.
Always take measurements with the cap down
over the sample.
BEER’S LAW PLOT
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States a liner
relationship between
the absorbance and
concentration of a solute
It is no longer linear at
high concentrations.
WHY??
The goal is to create a
plot by analyzing a
series of dilutions. Then,
test an unknown to
determine the
concentration.
A = ebc
where:
A = absorption at a given wavelength of
light,
e = molar absorptivity, unique to each
molecule and varying with wavelength,
b = the path length through the solution
that the light has to travel, and
c = the concentration of the solution in
moles per liter (molarity).