Freezing Point Depression

 

Goals

• Measure freezing point depression.
• Determine the Van't Hoff factor for two different solutes

Safety

• The substances we are using today are household chemicals (table salt and sugar). However, the usual lab precautions apply.

Equipment and materials

• From your drawer: 50 mL beaker, 100 mL beaker 
• On your bench: temperature probe
• At the balances: sucrose (s), sodium chloride (s)
• For task 4: sieve, calorimeter
• At a station: ice

Background

A colligative property is when a solute affects a solution property like its freezing or boiling point, and the effect depends only on the amount of particles dissolved, no matter which substance you use. In this lab we will investigate the effect of the different solutes on the freezing point of aqueous solutions. 

You should observe that as the solute concentration increases the freezing point \(T_{fp}\text{(solution)}\) decreases (the freezing point depression \(\Delta T\) increases), and this can be described by the following equation:

\[\begin{align} \Delta T & = i \cdot k_{f}\cdot\frac{n_\mathrm{solute}}{m_\mathrm{solvent}}  \\ &\text{where} \nonumber \\ i & = \text{Van't Hoff Factor} \nonumber \\ k_{f} & = \text{freezing point point constant} \nonumber \\ n & = \text{amount} \nonumber \\ m & = \text{mass}  \nonumber  \\ & \text{and} \nonumber   \\ \nonumber \Delta T & = T_{fp}\text{(pure solvent)}-T_{fp}\text{(solution)} \nonumber \end{align}\]

The value of k_f for the solvent water is 1860 °C g/mol, or 1.86 °C kg/mol. Check your units to decide which format is more convenient in this case.

Experimental Design Considerations

We are making the assumption that at the instance the ice melts the mixture of ice and water is at the freezing point. There can be multiple reasons why this is wrong, and we will discuss this in our pre-lab discussion.

Experimental Procedures

Record all observations in your lab notebook, and summarize your calculations and interpretations in this worksheet.

Part 1: Freezing point determination, pure water

1. Obtain a temperature probe and turn it on. Measure the mass of a clean, dry 100 mL beaker.
2. Pour approximately 25 mL of DI water into the 100 mL beaker
3. Place temperature probe into pure water in beaker
4. Add around 20 mL of fresh crushed ice to a 50 mL beaker.
5. Quickly add the ice to the 100 mL beaker, gently swirl the temperature probe. Record the lowest temperature, and the temperature just as the last bit of ice melts.
6. Weigh the mass of the beaker with the water and melted ice (so now just water). Record to the precision of the instrument. Clean and dry the 100 mL beaker and record the empty mass to the precision of the instrument.
7. Discard the water in the sink.
8. In your further calculations, use the lowest recorded temperature as the freezing point of water. The temperature probes are not calibrated, but are very good at measuring temperature differences. Don't be worried if the reading on your temperature probe is different from that of your neighbor, and don't swap temperature probes.

Part 2: Freezing point determination, sucrose solution

You can do part 2 and part 3 in any order, depending which balance is available.

1. Weigh approx 0.05 mol of sucrose (molar mass is 342.3 g/mol), record the exact mass, and transfer to the clean and dry 100 mL beaker.
2. Add approximately 25 mL water.
3. Place temperature probe into the solution in the beaker and swirl and stir to dissolve the sucrose completely.
4. Add around 20 mL of fresh crushed ice to the 50 mL beaker.
5. Quickly add the ice to the 100 mL beaker, gently swirl the temperature probe. Record the lowest temperature, and the temperature just as the last bit of ice melts.
6. Weigh the mass of the beaker with the sucrose solution and melted ice (so now a diluted sucrose solution). Record to the precision of the instrument. Clean and dry the 100 mL beaker and record the empty mass to the precision of the instrument. Then, calculate the mass of water (solvent) from the available data.
7. Calculate the van't Hoff factor for sucrose and record your work in the lab summary.

Part 3: Freezing point determination, sodium chloride solution

1. Weigh approx 0.05 mol of sodium chloride (molar mass is 58.44 g/mol), record the exact mass, and transfer to the clean and dry 100 mL beaker.
2. Add approximately 25 mL water.
3. Place temperature probe into the solution in the beaker and swirl and stir to dissolve the sodium chloride completely.
4. Add around 20 mL of fresh crushed ice to the 50 mL beaker.
5. Quickly add the ice to the 100 mL beaker, gently swirl the temperature probe. Record the lowest temperature, and the temperature just as the last bit of ice melts.
6. Weigh the mass of the beaker with the sodium chloride solution and melted ice (so now a diluted sodium chloride solution). Record to the precision of the instrument. Clean and dry the 100 mL beaker and record the empty mass to the precision of the instrument. Then, calculate the mass of water (solvent) from the available data.
7. Calculate the van't Hoff factor for sodium chloride and record your work in the lab summary.

Part 4: Try a different experimental strategy

With a partner, measure the freezing point of a solution of sodium chloride again, using a different experimental strategy based on the calorimeters we built last semester. This method is a bit faster but requires more equipment (work with a calorimeter instead of a beaker to minimize heat transfer, or use the minimum temperature in the calculation, quickly removing the remaining ice by pouring the slush through a sieve). As you are going through the steps, reflect on the pros and cons of this and the orginal method.

1. Weigh approx 0.05 mol of sodium chloride (molar mass is 58.44 g/mol), record the exact mass, and transfer to calorimeter cup.
2. Add approximately 25 mL water from a 100 mL beaker.
3. Place temperature probe into the solution in the beaker and swirl and stir to dissolve the sodium chloride completely. Place the calorimeter cup into the calorimeter.
4. Add around 20 mL of fresh crushed ice to the 50 mL beaker.
5. Quickly add the ice to the calorimeter cup, and gently stir with the temperature probe. Once the temperature stops decreasing (make sure there is still ice present), record the temperature, remove the temperature probe, and pour the mixture through a sieve into a dry 100 mL beaker. Discard the ice in the sieve.
6. Weigh the mass of the beaker with the sodium chloride solution. Record to the precision of the instrument. Clean and dry the 100 mL beaker and record the empty mass to the precision of the instrument. Then, calculate the mass of water (solvent) from the available data.
7. Calculate the van't Hoff factor for sodium chloride and record your work in the lab summary.