Question

7) As solute particle size increases, the rate of dissolving a) increases. b) decreases. c) remains the same. 8) Which of the following solutions will have the lowest freezing point? Why? (2 points show calculations) a) 2.5 g of KCl in 100 g of water b) 2.5 g of NH4CI in 100 g of water c) 2.5 g of NaOH in 100 g of water d) All the solutions will have the same freezing point.

Answer 1

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PART 7:

As solute particle size increases the rate of dissolving decreases.

Because for a given amount of solute, as the particle size increases, the surface area decreases and hence rate of dissolving decreases.

So the correct option is Option (b) decreases.

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PART 8:

We know that, Depression of freezing point of a solution is given by:

AT, = Kym

Where, K_f is freezing point depression constant for water.

is molality of solute which is given by:

m = no. m =- no.of moles of solute Mass of solution

Here, mass of solution remain constant (100 g), but no.of moles of solute changes.

Given mass of each solute is same (2.5 g), but molar mass is different. The compound with lower molar mass will have highest no.of moles.

From the given compounds, NaOH has the highest molar mass. Therefore no.of moles of NaOH will be the highest and so the molality is highest for NaOH.

Therefore, NaOH solution will have the highest Depression of freezing point or the lowest freezing point.

So the correct option is Option (c) 2.5 g of NaOH in 100 g of water.

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