Question

What concentration of ethylene glycol is needed to raise the boiling point
of water to 105°C? (K) = 0.51°C/m)
a. 1.5 m
b. 2.5 m
c. 9.8 m
d. 205 m​

Answer 1

Answer- The above question is from the chapter 'Solutions'.

Concept used: 1) Elevation in boiling point (Ebullioscopy) is a colligative property.

When a non-volatile solute is dissolved in a solvent, the vapour pressure of the solvent decreases and thus, the boiling point increases.

It is represented by $\Delta T_b .$

$\Delta T_b \propto m \\\Delta T_b = K_b \times m$

where m = molality (Concentration term)

$K_b$ = Molal Elevation Constant or Ebullioscopic Constant.

2) Molality: It is the number of moles of solution in 1 kg of solvent.

Unit of Molality: Molal (m)

•When mass of solvent is in kilograms,

Molality = Moles of solute ÷ Mass of solvent in kilograms

•When mass of solvent is in grams,

Molality = Moles of solute × 1000 ÷ Mass of solvent in grams

Given question: What concentration of ethylene glycol is needed to raise the boiling point of water to 105°C? (K) = 0.51° C/m)

a. 1.5 m

b. 2.5 m

c. 9.8 m

d. 205 m​

Answer: We are given that

$\Delta T_b = 105^{\circ} C\\K_b = 0.51 \: C/m$

We need to find molality (m).

$\Delta T_b = K_b \times m\\105 = 0.51 \times m$

m = 105 ÷ 0.51

m = 205.88 m

m ≈ 205 m

∴ concentration of ethylene glycol is needed to raise the boiling point of water to 105°C is 205 m.

⇒ d. 205 m is the correct option.