Measuring the Rise in Boiling Point of Water with the Addition of Solutes

Purpose

The purpose of this lab was to see the rise in boiling point of water when adding NaCl and Caso4 to water until saturated. We used a thermometer to measure the temperature of the course of time and compared it to the normal boiling point of water. The effect the salt had on the water was called colligative properties.

Procedure:

25 ml of fresh water was put in a 50 ml beaker. The beaker was placed on a hot plate with a thermometer after each salt dissociated enough into the water to make it saturated. The temperature was recorded every 30 seconds until water stayed at an approximate constant temperature. This process was repeated with each both salts NaCl and CaSo4.

Data:

Part 1 & 2:

Mass of beaker Mass of beaker & solution Color and clarity Volume of solution

Saturated Nacl 28.35 55.79 clear 25ml

Saturated Caso4 28.54 51.97 cloudy 25ml

Part 2:

Trial 1:

Time (s) Temp. Water Temp. NaCl

0 28.25 41.61

30 49.77 41.61

60 69.81 65.13

90 85.85 79.70

120 97.96 94.37

150 100.18 103.71

180 100.49 105.27

210 100.65 105.75

240 100.63 106.20

270 100.63 106.2

300 100.63 106.45

330 100.63 106.4

Trial 2:

Time (s) Temp. water Temp. Nacl

30 24.4

60 39.78

90 59.69

120 74.98

150 89.70

180 101.70

210 104.57

240 105.41

270 105.81

300 106.21

330 106.35

360 106.55

390 107.55

Results: Part 1/2:

Mass of saturated NaCl in solution 3.38g

Density of NaCl solution .135 g/ml

Mass of saturated CaSo4 in solution 3.55g

Density of CaSo4 solution .142 g/m

Part 3: Trial 1

Van’t Hoff Factor i=2

Molality of NaCl solution 3.96

Change in temperature 5.77 c

Kb of water .729 c/mole

% Difference 42.38 %

Trial 2:

Van’t Hoff Factor i=2

Molality of solution 1.65

Change in temperature 1.25 c

Kb of water

% Difference

Sample calculations:

Mass of solution: 28.38g-25= 3.88g

Density of solution: 3.38g/25ml = .135

Molality of Nacl: NaCl= 58.449g

5.77g/ 58.449g= .099 moles

.099moles/.025 L = 3.96

Change in initial temp.: 106.4 c-100.63 c= 5.77 C

Boiling point elevation: 5.77c= 2(kb)3.96 kb= .729 c/mole

Percent difference: (.512-.729)/.512 x 100 = 42.38 %

Discussion questions:

All saturated solutions do not have the same density. If they did they would be the same substance. The higher the solubility of a salt the more it can be dissolved in an aqueous solution making it denser.

((1.8/2)/2)) x100 = 90%

I feel comfortable making the assumption the NaCl was dissociated because some was still left on the bottom of the beaker after the water had became saturated. This would affect kb by making the actual moles of salt dissolved lower than what was calculated.

ΔT = iKb*m

5.77c = 2 (0.512 °C /m) * m

M= .177 compared to my experimental value which was 3.96

I think that NaCl would make a better de-icer than CaSO4 because NaCl is more soluble in solution and thus a solution of NaCl will have more moles per volume of solvent than a solution of CaSO4. With more moles of NaCl that solution will melt ice faster than the lower concentrated solution of CaSO4.

Conclusion:

In this lab we observed the colligative property of boiling point elevation by adding salts to pure water solutions. The first part of the lab demonstrated that different salts with dissolve with different rates in an aqueous solution. The NaCl was more soluble in water than the CaSO4 and thus the solution of NaCl was denser and contained more moles of solute than the solution of CaSO4. In the second part of the lab we measured temperature changes and boiling points for a pure water solution and a solution of NaCl in order to obtain an experimental value for Kb, the boiling point elevation point of water. The pure water boiled at a lower temperature than the NaCl solution showing that the added salt raised the boiling point of water. By using the equation ΔT = iKb*m we calculated the Kb of water as 0.729 C/m for the first trial. This value was very far from the accepted value of 0.512 °C/m giving us a percent difference of 42.38% which is unexpectedly high. This poor results might have been due to inherent error in the lab, the fact that the van’t Hoff factor for NaCl dissociation is actually i=1.8 rather than i=2, or they might have been because of poor lab technique.