a solution was prepared by dissolving 2.151 g of a salt into 25.000 g of water.

Question

a solution was prepared by dissolving 2.151 g of a salt into 25.000 g of water. It was observed that the solution temperature increased when Ehe salt dissolved. The so treezing point of the solution was found to be 2.9 degrees lower Chan the freezing point of pure solvent (water) lution was then put into an ice bath and the 1, what is the molality of the solution formed? 2. Using the answer to question 1, and the magnitude of the decrease in the freezing point, what is the value of the van 't, Hotf factor, i, for this solution. For water: Kf 1.860g kg mol-1. 3. When the solution was initially made by adding the salt to the water, the temperature of the solution was observed to increase. This allows us to conclude that delta H for the solution process is (b) equal to zero (c) less than zero (a) greater than zero 4. Since the solid actually did dissolve in the water, we can conclude that the solution process for this reaction is (a) spontaneous (b) not spontaneous 5. Using the answers to queetions 3 and 4, what can we conclude aboat delta S f,r the solution process? [Hint: refer to the table found on the tirst page of the background section of the experiment in your coursepack.] (a) It is positive. (b) It is equal to zero (c) It is negative. (d) It could be either positive or negative.

Explanation / Answer

1.

Moles of salt = mass / molar mass = 2.151 / 118 = 0.01823 mol

Mass of water = 25.00 g. = 0.02500 kg.

Molality = moles of solute / weight of solvent in kg

m = 0.01823 / 0.02500

m = 0.7292 m

2.

Depression in freezing point = i * Kf * m

2.9 = i * 1.86 * 0.7292

i = van't Hoff's factor = 2.138

3.

(c) Less than zero

Increase in temperature indicates the exothermic reaction. For exothermic reactions deltaH = - ve.

4.

(a) Spontaneous

5.

(d) It could be either positive or negative.

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