Name___________________
Comparing the heating and cooling curves for pure water, with those for
sodium acetate solutions.
The purpose of this lab is study the heating and cooling curves for water, use
these to determine melting and boiling temperatures, and then to compare the melting and
boiling temperatures of water with that of concentrated solutions of sodium acetate.
Collect repeated temperature data, and make other observations of the properties of these
solutions as they are heated at a constant rate.
Safety Reminders: You will be working with hot plates and boiling water. Stay
at your own lab station and collect data continuously while the water heats. Sodium
acetate is a mild base, which could damage your eyes if it gets on your face. Remove
benches, stools and chairs from the lab area so that you can avoid accident if boiling
water splashes toward you. Goggles and close toed shoes must be worn while sodium
acetate is out and hotplates are in use.
Do not leave dry glassware on a hotplate. Do not leave thermometers
unsupported in the water. Do not transfer hot glassware to the counter until it is cool
enough to handle with bare hands.
Caution with the capillary tubes. They are sharp
Prelab Instructions: Go on-line to learn some of the commercial uses of sodium
acetate, and sodium acetate trihydrate. List and describe at least five commercial uses of
this compound: ________________________________________
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Sodium acetate trihydrate has the chemical formula C2H3O2(H2O)6
Calculate its molar mass. Show your work:
Calculate the number of moles of sodium acetate trihydrate for the following masses.
You will be assigned one of these six amounts to dissolve in 100 grams (0.1 kg) of water
for your own experiment. Calculate the molal concentration of each solution.
molal concentration is defined as:
molality= #moles / water mass(in kg)
mass
calculate #moles
calculate “molal”
concentration
13.61 grams
27.22 grams
40.83 grams
54.44 grams
68.05 grams
(more prelab instruction on the reverse of this page)
During the experiment, you will be heating a sodium acetate solution to boiling, and
make repeated measurements of the following type: time, temperature, and capillary
force(in millimeters). Prepare a data table in your notebook to collect these
measurements. Prepare a second data table to collect the same measurements for distilled
water.
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Day one: Each group will be assigned a certain concentration of sodium acetate solution
to prepare, from the table of solution concentrations calculated during the prelab.
Weigh the assigned mass of sodium acetate trihydrate.
Add the massed compound to 100 grams of distilled water, in a ~250 mL beaker
Measure the temperature of the water before, during and after the dissolution
process.
Make written observations before, during, and after the dissolution process.
Stir to make sure dissolution is complete. If stirring alone is not adequate to
dissolve all the crystals, begin heating them gently on a hot plate.
Once the crystals are entirely dissolved, prepare a second beaker with distilled
water, and turn the hotplate power up to about 80% max.
Each lab partner is responsible for monitoring the temperature and capillary force
of one of these two beakers, minute by minute throughout the heating process.
Measure the temperature by stirring the solution briefly (so that all parts of the
solution are at equal temperature) then record the temperature with a thermometer (to the
nearest half degree) near the center of the solution.
Measure the capillary force of the solution (in millimeters) by sticking a capillary
tube in the top of the beaker, and measuring with a ruler the highest the solution climbs
from the water’s surface. Take at least 8 capillary measurements between room
temperature and 85 oC. Be cautious not to burn yourself at the higher temperatures.
Record other types of observations of the solution in the margins of your data
chart, such as the temperature when steam first appears, when convection currents appear
in the water, when bubbles first appear, and when the solution reaches a rolling boil. At
the rolling boil stage, you may turn off the hot plate.
Label a large test tube with your group’s initials. Once your solution is cool to
the touch, transfer to a labeled test tube, and cover with parafilm.
If you finish early, begin graphing your data. See the instructions at the end of
this assignment.
Day 2:
Your solution has been placed in the freezer overnight. Place the tube in a room
temperature water bath, and record the temperature of the solution as it begins to melt,
then as it begins to climb toward room temperature.
Analysis and conclusions:
(Day 1)Report the concentration and boiling temperature of your solution to the class, by
writing these values on the board.
(Day 2)Report the concentration and freezing/melting temperature of your solution to the
class by writing these values on the board.
Each lab partner should prepare a heating curve graph for their own data (time vs
temperature) Include labels along the time axis highlighting other important
observations. Include labels on the temperature axis showing boiling point temperature.
Draw a graph of temperature vs capillary force, which includes at least 8 points
scattered across the time of the experiment.
Using the data from the entire class, prepare a pair of graphs of solution
concentration vs. melting point, and solution concentration vs. boiling point.
Write at least two paragraphs of summary conclusions. At minimum, this should
describe:
The relationship between solution concentration and boiling point.
The relationship between solution concentration and melting point.
The relationship between temperature and capillary force.
An explanation for the observed relationships (above) in terms of intermolecular
forces, and changes in the strengths of those forces with changing temperature. This last
part should demonstrate your understanding of the topics covered in chapter 14 of the
text.
Finally, comment on the quality of your data, and the degree to which it may limit
the strengths of your conclusions. What changes would you suggest for improving the
quality of the data?