Names:
Some Like It Hot!!
Boiling Point Elevation
Objective: To determine the boiling point elevation constant for water.
Procedure
1. Weigh an empty 250 mL beaker, then fill to the 100 mL mark on the beaker and reweigh.
Calculate the weight of the water in the beaker in kg.
2. Put the beaker on the hot plate and heat to boiling.
3. While the water is heating, weigh out 10 g* of NH4NO3 (wts. to 2 decimal places!!) into each of
the three weighing boats at your station. Make sure you know which one is which.
4. Once the water is boiling, measure the boiling temperature with the thermometer. Do not allow
the tip of the thermometer to touch the bottom of the beaker.
5. Add the contents of weighing boat #1 to the beaker and stir with the stirring rod until the
ammonium nitrate dissolves. Add water to the beaker if the level of the water has dropped below
the 100 mL mark.
6. Measure the boiling point of the solution once it begins to boil again.
7. Repeat steps 5 and 6 for the contents of weighing boats 2 and 3 (the molality of the solution
increases with each portion of NH4NO3).
8. Once you have your final boiling point, unplug the hotplate and clean up your lab station.
*I’ll deduct 5 points from your lab score if any of your weights are exactly 10.00g.
Data and Calculations
Mass of water in kg:
Weighing
Boat
Weight of
NH4NO3 (g)
Boiling point of water:
Moles of
NH4NO3 (mol)
Soln. molality
(mol/kg)
Boiling point of
solution (C)
T
(C)
#1
#2
#3
1. Plot T (y-axis) vs. the molality of the solution (x-axis) on a piece of graph paper. Use a ruler to
draw a single straight line through the data points (don’t connect the dots please). Be sure to
label your axes and to give your graph a title.
2. Record the slope of the line here:
C/m
3. Attach your graph to this lab sheet.
Questions
1. If the equation for calculating the boiling point elevation of a solution is T=i K b m ( i = 2 f or
N H 4 N O 3 , and m is the molality of the solution), what is the boiling point elevation constant, Kb?
2. Given that the accepted value for the Kb for water is 0.51 C/m, what is your %error?