Expt 11 - Solution Preparation and Beers Law - 1033_F18 -...

SOLUTION PREPARATION AND
BEER’S LAW
ADDITIONAL READING
The concepts in this experiment are also discussed in sections 4.5, 4.9, 6.1– 6.2, 13.1 of Chemistry
and Chemical Reactivity by Kotz, Treichel, Townsend and Treichel, and in section 4.5, 6.1, 6.2, 13.1,
and 23.4c of Mindtap General Chemistry by Vining, Young, Day, and Botch A history of Kool-Aid
®
can
be found at: (checked 04-Jun-2018).
ABSTRACT
This is a two-part experiment. In part A, you and your lab partner will prepare solutions of your
assigned Kool-Aid
®
flavor(s). In part B, those solutions are used to construct standard Beer’s Law plots
of absorbance vs. concentration. The best-fit lines for those plots are then used to determine dye
concentrations in Kool-Aid
®
of known and unknown dilutions.
Background material includes topics not covered in your lecture text. The experiment employs the
Vernier colorimeter.
SOLUTION PREPARATION
For quantitative work we need to know the amount of solute in solutions. Concentration is most
commonly measured as molarity ( M ), which is defined as:
L
nsol'
Liters of solution
or M
n
Molarity
=
moles of solute
=
( 1 )
Molarity is a measure of the number of solute molecules present per liter of solution (NOT SOLVENT).
P
REPARATION OF A SOLUTION USING A PURE
,
SOLID SOLUTE
For 3-4 significant figure accuracy, weighed, dry solute is dissolved in less than the required volume of
solution in a volumetric flask (choose Class A for highest accuracy). Once dissolution (and adjusting to
near the glassware’s calibration temperature, typically 20°C) is complete, add solvent, and mix again.
Add more solvent with a transfer pipet to bring the solution level up to the fill line on the flask’s neck.
The bottom center of the liquid’s meniscus should be level with the top of the line. Stopper or cap the
flask; hold the top in place and support the base. Repeatedly invert the flask gently, swirl it, and revert
it until mixing is essentially complete (up to 10 times, at least a few times beyond when inhomogeneity
– streaking – is still visible).
How should 250 mL of 0.200 M (mol/L) CuSO
4
solution be prepared, given a 500 g bottle of
copper(II)
sulfate pentahydrate CuSO
4
· 5H
2
O (beautiful blue crystals) and deionized (DI) water?
We need the molar
mass of CuSO
4
· 5H
2
O.
copper 1 x 63.546 g/mol sulfur 1 x 32.065 g/mol oxygen 9 x 15.999 g/mol (4 from sulfate, 5
from 5H
2
O)
hydrogen 10 x 1.0079 g/mol (from the 5 waters of hydration) copper(II) sulfate
pentahydrate 249.681 g/mol
We can now use the molar mass to calculate the mass of solute required:

249.681 g/mole x .200 mole/L x 0.250 L = 12.5 g CuSO
4
· 5H
2
O
A top-loading balance is sufficient here.
Use a small (50-100 mL) beaker as the weighing vessel and a powder funnel to help quantitatively
transfer the weighed mass to a 250 mL volumetric flask (Class A: 250.00±0.12 mL). Dissolve and
rinse-in any adhering crystals with a DI-water squirt bottle. Using a beaker (tolerance typically ±5%) to
measure volume wouldn’t be accurate
enough. After adding the solid to the flask, add enough DI water to about two-thirds fill it, then swirl to
dissolve all of the solid. Add DI water to near the bottom of the neck, mix again, then add DI water with