Question

Sodium hydroxide is a common ingredient in drain cleaners such as Drano. The mole fraction of sodium hydroxide in a saturated aqueous solution is 0.310. What is the molality of the solution?(a) 12.4 m
(b) 0.690 m
(c) 25.0 m
(d) 1.24 m
(e) 0.310 m

Answer 1

Answer:

The correct answer to this question is (c) 25.0 m

Explanation:

To solve this we list out the variables thus

mole fraction of sodium hydroxide = 0.310

Mole fraction = number of moles of a component ÷ total number of moles in the solution

Mole fraction = 0.310

In a saturaturated aqueous solution we have NaOH and water

∴ Number of moles of water molecules per unit = 1 - 0.310 = 0.690

However 0.690 moles of H₂O  weighs = 0.690 mole × 18.01528 g/mol =12.43 g = 0.01243 kg of H₂O

But the molality = number of moles per Kilogram of H₂O

therefore molality of NaOH in the sample of solution =

(0.310 mol of NaOH )÷(0.01243 kg of H₂O)

= 24.93 mol/kg or ≅ 25.0 m