Name

Period

Date

Molal Boiling Point Constant Lab

OBJECTIVE

• To study the boiling point elevation of a solvent as more solute is added.

INFO

• You are going to measure the boiling point of pure water and of four different saltwater solutions. You will see how much the boiling point is raised for each solution, and from that data calculate the molal boiling point elevation constant: K_b.

MATERIALS

• 250 mL beaker, thermometer, hotplate, balance
• 125 g (mL) distilled water
• NaCl salt, aluminum foil

DATA

1. boiling point pure water = ^oC
2. boiling point solution 1 = ^oC
3. boiling point solution 2 = ^oC
4. boiling point solution 3 = ^oC
5. boiling point solution 4 = ^oC

PROCEDURE

1. Mass the NaCl to the hundredths place (2 decimal places). Mass out four 4.00-g samples.
1. mass out Al foil
2. Record the temperature to the tenths place (one decimal place).
3. Add 125 mL (0.125 kg) distilled water to the 250-mL beaker.
4. Set the 250-mL beaker and water on hot plate with Al foil covering the beaker.
1. Why do we use the Al foil?
5. Heat the water to boiling and record the temperature.
1. Do not have the thermometer resting on the bottom of the beaker
2. The bottom can get hotter than the boiling point.

6. Remove the beaker from the heat and dissolve 4.00 g of salt into the water. Place it back onto the heat and measure the new boiling point. Record that temperature.

a. CAUTION Add the salt SLOWLY or it will violently boil over.

7. Add another 4.00 g of salt (total of 8) and repeat this procedure until you have made boiling point measurements with 4.00 g, 8.00 g, 12.00 g, and 16.00 g of salt added. Make sure to record your data for each step.

8. Pour your salt water down the drain and clean up.

9. Calculate the ion molality and then the K_b for each salt solution.

CALCULATIONS

1. ∆T_b for solution 1 =
2. m_i for solution 1 =
3. K_b for solution 1 =
4. ∆T_b for solution 2 =
5. m_i for solution 2 =
6. K_b for solution 2 =
7. ∆T_b for solution 3 =
8. m_i for solution 3 =
9. K_b for solution 3 =
10. ∆T_b for solution 4 =
11. m_i for solution 4 =
12. K_b for solution 4 =