Your Name
(1 pt)
Lab Partners
Date
(1 pt)
/ 30 pts
(1 pt)
Determining the Melting Point and Heat of Fusion of Lauric Acid (a wax)
Discussion: This experiment investigates the transitions that occur in a substance when it is heated or cooled. The
temperature at which a transition from solid to liquid occurs is called the melting point, and from liquid to solid is
the freezing point.
In order to melt a solid, enough energy must be supplied to overcome the forces that have been holding the molecules in
fixed positions. When a solid melts, its molecules lose their 3-dimensional order. Melting & freezing occur at the
same temperature for a substance. The temperature of a melting substance cannot rise above the melting point
until all of the solid has become a liquid. Impure solids or mixtures melt over a range of temperatures however.
Therefore, melting point data can be used to determine the purity of a solid.
The energy required to convert 1.0 gram of a pure substance from solid to liquid state is called the substance’s heat of
fusion, expressed in Joules/gram of the substance.
Objectives:
 to determine the freezing point temperature of lauric acid, a type of wax.
 to determine the heat of fusion of lauric acid.
Equipment & Materials
 ring stand
clamp
test tube
 calorimeter cup
lab balance
thermometer
lauric acid
water
Procedure, Day 1 – Determining the Melting Point of Lauric Acid
1. Obtain a test tube & weigh it. Fill the tube with lauric acid & find the new mass. Calculate the mass of lauric acid
that is present.
2. Place the test tube into a boiling water bath (at front bench), and melt the lauric acid completely. When the wax is
o
all melted, gently place a thermometer into it. Continue to heat the tube until the lauric acid is about 50 C.
3. Take the tube of melted wax back to your lab bench. Clamp the tube to the ring stand. Observe the temperature
of the lauric acid every 1 minute while the lauric acid cools & begins to solidify. Record your temperature vs. time
on the chart on the back of the data sheet. Label the temperature at which a cloudiness or crystals first appear in
the lauric acid.
4. Allow the lauric acid to completely solidify, while continuing to observe the temperature every minute. Once the
lauric acid appears to be completely solidified, continue to observe its temperature once/minute until it cools to
o
about 38 C.
5. Reheat your lauric acid in the boiling water bath at the front until it remelts and its temperature reaches about
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50 C. Remove the thermometer & wipe it clean for later use.
Procedure, Day 2 – Determining the Heat of Fusion of Lauric Acid
1. Obtain a large styrofoam calorimeter cup. It is not necessary to weigh the cup. Use a large graduated cylinder to
add enough water to the cup that the lauric acid in the test tube will be completely submerged once you put the
test tube in – but don’t put the test tube in yet! Record the volume of the water in mLs as the mass of the
water in grams.
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2. Find the temperature of the water to the nearest 0.1 C. This temperature is Tinitial.
3. Your lauric acid must still be completely melted & clear at this point. If it’s already started to crystallize (get
cloudy), reheat the lauric acid until it’s completely clear.
4. As soon as you see the first hint of cloudiness appear in your lauric acid, quickly place the test tube into the water
in the calorimeter cup. Gently swirl the tube in the water to evenly distribute the heat. Monitor the temperature of
the water as the lauric acid cools & solidifies. Record the highest temperature that you observe as T final.
Data and Calculations
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(recording temps to 0.1 C – 2 pts)
Day 2 (all masses recorded to nearest 0.01 g, except for mass of water in cup – 2 pts)
(calculations done correctly – 8 pts)
Day 1
1. Mass of empty test tube
g
2. Mass of test tube + lauric acid
g
3. Mass of lauric acid
g
3
4. Mass of water in calorimeter cup
g
4
5. Tinitial of water in cup
o
6. Tfinal of water in cup
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7. Change in temp. of water in cup (T)
o
8
8. Total heat absorbed by water
Qabsorbed = mass of water x 4.184 x T
J
Time in minutes
0
1
o
Temp, C
2
5
6
9
10
11
12
13
14
C
C
C
9. Heat lost by glass of test tube
J
Qglass = masstube x 0.84 x Tglass (where Tglass = melting pt. of lauric
acid – Tfinal)
10. Heat lost by lauric acid
J
Q lost by lauric acid = Q gained by water – Q lost by the glass of the
test tube.
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11. Heat of fusion of lauric acid
Hfus = Q lost by lauric acid
mass of lauric acid
J/g
Conclusion Questions
1. Prepare a graph of your freezing point data from the table at the left.
Plot time in 1 minute intervals along the X-axis, and temperature
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in 1 C intervals along the Y-axis. Your graph must include a title,
both axes must be labelled properly including quantity and units
(ex: time in minutes), and points plotted accurately. (6 pts)
2. From your data, what is the melting / freezing point temperature of
lauric acid? (1 pt)
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3. Explain why the lauric acid’s temperature remains constant while the
lauric acid is solidifying. (2 pts)
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4. The theoretical heat of fusion of lauric acid is 183.6 J/g. Calculate
your % error. Show your calculation. (1 pt for doing calculation correctly + 3
pts for accuracy)
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5. The molecular formula of lauric acid is C12H24O2. How much heat
would be required to melt 1 mole of lauric acid? Show your
calculation & express your answer with units of J/mole. This is
the molar heat of fusion of lauric acid. (2 pts)