Colligative Property of Water Containing Rock Salt
Background/Purpose:
If you have ever lived in a cold climate you might remember that during snow storms road crews apply a mixture of salt and sand to the roads. Have you ever wondered what is the reason for this application? Salt tends to lower the freezing point of water so water stays liquid at colder temperatures. This is an example of the colligative properties of water.
Colligative properties of solutions are properties that depend on the concentration of solute molecules or ions dissolved in a solvent but not on the chemical identity of the solute. These properties include boiling point elevation and freezing point depression.
Freezing point depression can be calculated using the formula ∆Tf = KfCmi where
∆Tf is freezing point depression
Kf is the freezing point depression constant that depends on the nature of solvent
(You can look this constant up in most chemistry textbooks)
Cm is concentration in units of molality. It can be calculated using the formula
Cm = Molality = moles of solute / kg of solvent
Remember; Moles = grams of solute / molecular weight (mass) of solute
i is the number of ions found in this salt
Remember; NaCl ---------> Na+ + Cl-
If you only know the volume of your solvent, you can convert it to mass using the density formula
Density = Mass / Volume or Mass = Volume x Density (you can look up the density of your solvent in most chemistry textbooks)
Density is usually given in units of g/ml. You will obtain the mass in units of gram. You need to convert grams to kilograms by the formula kilograms = grams / 1000
Safety, Handling,
Disposal:
Do not consume any food in laboratory. Do not use laboratory equipment for food consumption.
Wear you laboratory goggles at all times.
All excess solutions and products can be flushed down the drain with excess water.
Materials needed:
1/4 can of sweetened evaporated milk
1 teaspoon of vanilla
1/4 of a carton of egg substitute
2/3 of cups of heavy whipping cream
1 quart size plastic bag
1 gallon size or ½ gallon size ice cream bucket
Ice
150 g of rock salt
Thermometer
Balance that can weigh to 0.1 g
250 ml beaker
100 ml graduated cylinder
Drying oven or hot plate
Procedure:
1. Weigh out 150 grams of rock salt using a balance. Record the exact mass in data sheet.
2. Add the evaporated milk, egg substitute and heavy whipping cream into the quart size bag and add 1 teaspoon of vanilla extract and close the bag.
3. Put the bag inside of another bag and put it inside of the gallon size ice cream bucket. Pack the bucket with ice.
4. Measure the temperature using a thermometer. Record the temperature in data sheet.
5. Add the rock salt to the bucket and close the lid.
6. Roll the bucket between lab partners or shake it for about15 minutes until ice cream freezes.
7. Open the bucket and record the temperature of the salt water solution.
8. Obtain a 250 ml beaker and a watch glass.
9. Determine their mass using a balance.
10. Add 100 ml of saltwater from the bucket and re-weigh the beaker and watch glass and record the mass in data sheet.
11. Place the beaker covered with watch glass on top of a hot plate and let it boil until all the water has evaporated and solid salt is remaining. Or leave the beaker in a laboratory oven overnight.
12. Remove the beaker from the hotplate/oven and let it cool.
13. Re-weigh the beaker and watch glass to obtain the grams of salt and record in data sheet.
Calculate the molality of the saltwater solution.
Calculate the expected ∆Tf from molality, the Kf constant and number of ions.
Calculate the percent error using the following formula:
% Error = [(Observed ∆Tf - Calculated ∆Tf) / (Calculated ∆Tf)] x 100
Answer the questions at the end of the data sheet.
Now you can take the ice cream outside of the lab and consume it using provided plastic spoons.
Colligative Property of Water Containing Rock Salt
Data Sheet
Name ________________
1. Mass of salt added to bucket _______________
2. Temperature of ice before adding salt _______________
3. Temperature of saltwater after making ice cream _______________
4. Mass of empty 250 ml beaker and watch glass _______________
5. Mass of Beaker, watch glass + saltwater _______________
6. Mass of beaker, watch glass + salt (after drying) _______________
7. Mass of salt (line 6 - line 4) _______________
8. Mass of saltwater (Line 5 - line 4) _______________
9. Moles of salt (mass/formula weight) _______________
10. Mass of water in kg (Line 8 – line 7) /1000 _______________
11. Molality (moles of salt/kg of water) _______________
12. Kf of water _______________
13. Number of ions in salt _______________
14. Calculated ∆Tf _______________
15. Observed ∆Tf (line 3 - line 2) _______________
16. Percent Error _______________
Post Lab Questions
1. A 0.2536 g sample of an unknown substance was dissolved in 30 ml of water. The freezing point depression was 4.5oC. Calculate the molar mass of the unknown substance.
2. What would be the effect of the following on calculated change in freezing point temperature?
a. A foreign solute was already present in water.
b. Some salt water splattered while it was boiling.
c. The thermometer was not calibrated correctly. It always gave a temperature that is 1oC too low.
3. If 0.15 moles of a covalent compound is present in 1300 ml of water as a solute, what is the ideal melting point (freezing point) for the solution?