Question

Chemistry! help! Please!

1. The table below (ATTACHED) shows the freezing points of solutions of three nonvolatile solutes, X, Y, and Z, in water. Refer to the table to answer questions 2 and 3. The Kf for water is -1.86°C/m.

2. Which of the following statements is not true?
a. All three solutes are nonelectrolytes.
b. Solute X could be Ca(NO3)2.
c. Solute Y could be sucrose.
d. Solute Z could be KCl.

3. Which of the solutions described in the table would have the highest boiling point?
a. the solution of solute X
b. the solution of solute Y
c. the solution of solute Z
d. All three solutions would have the same boiling point.

Answer 1

2. a. All three solutes are nonelectrolytes.

3. a. the solution of solute X

Explanation:

The freezing point depression (ΔTf) and boiling point elevation (ΔTb) are colligative properties: they depend on the numbers of particles.

The formula for ΔTf is

ΔTf = iKf·b

i is the van’t Hoff factor: the number of moles of particles you get from a solute.

For sucrose,

Sucrose (s) ⟶ sucrose (aq)

1 mole sucrose ⟶ 1 mol particles i = 1

For NaCl

NaCl(s) ⟶ Na⁺(aq) + Cl⁻(aq)

1 mol 1 mol + 1 mol i = 2

For Ca(NO₃)₂

Ca(NO₃)₂(s) ⟶ Ca²⁺(aq) + 2NO₃⁻(aq)

1 mol 1 mol + 2 mol i = 3

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2. Freezing points

For a nonelectrolyte, i = 1.

Kf = 1.86 °C·kg⁻¹mol⁻¹

b = 1 mol/kg Calculate ΔTf

ΔTf = 1 × 1.86 × 1

ΔTf = 1.86 °C

Tf = Tf⁰ - ΔTf =0.00 °C – 1.86 °C = -1.86 °C

All the other solutions have lower freezing points, so the solutes must be electrolytes.

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3. Boiling points

The formula for ΔTb is

ΔTb = iKb·m

The solution with the highest boiling point will have the highest value of i.

In other words, the solution with the highest boiling point will be the one with the lowest freezing point.

That’s the solution of solute X.