EXPERIMENT 2 - DOC by 03y0NED6

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									          EXPERIMENT 2.3
      SOLUTION CONCENTRATION
INTRODUCTION

In this experiment, you will prepare a sodium chloride solution and compare
its boiling point to that of pure water. The objectives of the experiment are to
practice the calculation of concentration using several unit systems and to
observe the colligative property of boiling point elevation. Colligative
properties are properties of solutions that vary with the numerical amount of
solute particle concentration but do not depend on the identity or chemical
nature of the solute.

The solution will be prepared by mass. That is, the amounts of salt and
water added together will be weighed. The density of the resulting solution
will then be determined, using a pycnometer. Pycnometers are glass vessels
designed so that when they are filled with liquid samples, the samples will
consistently have the same volume. By finding the mass and volume of a
sample of your solution, you will be able to calculate its density. This, along
with the original masses of solute and solvent will provide sufficient
information for you to calculate the concentration of the salt in units of
percent by mass, molality, molarity and mole fractions. Each of these units is
defined in the class text book.

By measuring the boiling point temperature of the solution and comparing it
to that of pure water, you will observe whether or not the boiling point will
have risen due to the addition of the solute to water. Further, you can
comment on the apparent accuracy of predicting the boiling point of your
solution using the boiling point elevation equation

                                  Tb = iKbm

where Tb is the amount of increase in the boiling point over that of the pure
solvent, i is the effective number of independent solute particles per formula
unit dissolved, Kb is a constant for the solvent, and m is the molality of the
solute formula units.

The mathematical laws for the colligative properties of solutions are all
estimates, making some idealized assumptions about solution behavior. Our
experimental method will have some uncertainties due to instrument and
operator limitations. You will be asked to consider how each of these may
contribute to imprecision between the calculated and measured boiling point
values.
MATERIALS AND SAFETY

The materials used are limited to sodium chloride and water so there are no
special hazards involved. Nevertheless, wear goggles at all times to avoid the
high salt concentration and other materials in the lab environment. The
pycnometers are expensive and very fragile. Handle one with care and do not
lose the small cap associated with it. At the end of the procedure, rinse the
pycnometer with distilled water and return it to the instructor or to the
equipment cart.

PROCEDURE

Clean and dry a 100 mL beaker. Weigh the beaker on the electronic balance
and record the mass in the data section of the report. Use your graduated
cylinder to measure about 60 mL of distilled water and add it to the beaker.
NEVER ADD LIQUIDS TO CONTAIN-ERS WHILE THEY ARE ON THE
BALANCE. Weigh and record the mass of the beaker and water. Next,
measure about 12 grams of sodium chloride in a plastic “weighing boat” and
add it to the beaker (away from the balance). Weigh and record the mass of
the beaker and solution.

Obtain a 50 mL pycnometer from the equipment cart. Please treat it with
extreme care. If it appears to be wet inside, use a very small portion of
acetone (red squirt bottles) to rinse the interior and allow the acetone to
evaporate. When it appears dry inside and out, weigh the pycnometer and
record its mass. Use a distilled water squirt bottle to fill the pycno-meter to
near the top of its neck. Have a lab tissue at hand and insert the holed cap
into the neck. The level of water should be high enough and the insertion
done carefully so that excess water comes out the hole and no air is trapped
below the cap. If you see substantial air bubbles or the water does not rise to
the top of the hole, take the cap out, add water and try again. Use the tissue
or paper towel to dry the outside and briefly pass the tissue over the top of
the hole to take off excess water bulging out above the hole. Weigh and
record the mass of the pycnometer and water.
The ground glass joint should make it possible to insert the cap to the same
depth any time the pycnometer is filled. Thus, if liquid fills to the same point
at the top of the hole in the cap, the volume will be consistent from sample to
sample.

Pour the water out of the pycnometer and into a beaker and measure the
water temperature. Before leaving the lab, refer to the water density chart
posted on the wall
near the barometer between the storeroom windows that face into the two
labs. Record the density of water at the measured temperature.

Rinse the pycnometer with one or two very small portions of the salt solution
you have prepared. Then fill it with salt solution and insert the cap as
before. The rinsing is done to substitute traces of your solution for the water
so that water from the previous filling will not change the concentration of
the sample of your solution. There should be more than enough solution to
fill the pycnometer. Save the excess in the beaker. Weigh and record the
mass of the pycnometer and salt solution. After weighing, pour the solution
sample back into the 100 mL beaker with the excess.

While one team member is carrying out some of the weighings, the other
should set up an iron ring and insulated wire mesh pad to support a beaker
over a Bunsen burner. Place about 60 mL of distilled water in a different
100mL beaker and heat to a moderate steady boil. Use a thermometer to
measure an estimate of the boiling point temperature. Support the
thermometer by hand as opposed to clamping or standing in the beaker, and
hold it well above the bottom of the beaker so it does not contact the glass
from which heat is being transferred to the water. Record the boiling point of
distilled water.

After the pycnometer weighing of the salt solution have been completed and
the solution, less the amount used for rinsing, has been returned to its 100
mL beaker, place this beaker over the burner and measure its boiling point.
Try to match the extent of boiling as closely as possible to that of the distilled
water sample boiled earlier. Record the solution boiling point.

When the data section is complete, rinse all of the glassware exposed to the
salt and return all items to your drawer or the equipment cart as
appropriate.
DATA: Create the following data table in your lab notebook as part of you
prelab.

a) Mass of empty 100 mL beaker________________

b) Mass of beaker plus distilled water________________

c) Mass of beaker plus solution of salt and water________________

d) Mass of empty pycnometer________________

e) Mass of pycnometer plus distilled water________________

f) Temperature of distilled water________________

g) Density of distilled water________________

h) Mass of pycnometer plus solution of salt and water________________

i) Approximate boiling point of distilled water________________

j) Approximate boiling point of solution of salt and water________________


Prelab Questions
1. A solution made to be 20.2% HCl by mass in water has a density of 1.096
   g/mL. Calculate the molarity and molality of the HCl in the solution.

2. Calculate the expected boiling point of a solution of 238 g of KBr added to
   3.00 kg of water. For water, Kb is equal to 0.512 °C/m.

3. As a solution of KBr and water boils, only the water molecules will
   vaporize. What should happen to the boiling point temperature over time
   as such a solution boils?

4. As a KBr and water solution begins to freeze, water molecules will tend to
   become organized into the pure ice crystal structure excluding the K+ and
   Br- ions which stay mixed with the remaining liquid water. Eventually,
   the remaining liquid water will be saturated with KBr and solid KBr will
   precipitate. Thus the freezing process, if performed slowly, may separate
   the water from the KBr. What should happen to the freezing point
   temperature during the early stages of the process?
Postlab

For each calculation, show detailed work steps in the space between the
statement of the objective and the blank result line. In contrast to many of
the experiment reports of Chemistry 151, the results have not been broken
down into as many intermediate reported steps. This is intended to give you
more flexibility as well as responsibility to determine an appropriate
calculation method for each objective.

1. Find the Percent by mass of NaCl in solution
2. Find the Molality of NaCl in solution
3. Find the Density of solution
4. Find the Molarity of NaCl in solution
5. Consider the solution to consist of three distinct particles: water
molecules, sodium ions and chloride ions. Find mole fraction of water in the
solution.
6. Difference in measured boiling points of water and solution
7. Expected T from calculation
8. Learning and Errors

CONCLUSIONS

Was boiling point elevation observed in the measurements? Comment on the
agreement between the measured and expected boiling point differences.
State the main trend in accuracy of the calculation as solute concentration
varies. Also identify the greatest source or sources of uncertainty in the
measurement.

								
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