Question

When 19.5 g of F – CH₂ – COOH (Molar mass = 78 g mol⁻¹), is dissolved in 500 g of water, the depression in freezing point is observed to be 1°C. Calculate the degree of dissociation of F – CH₂ – COOH.

[Given: K_f for water = 1.86 K kg mol⁻¹]

Answer 1

Given: Mass = 19.5 g

Molar mass = 78 g mol⁻¹

ΔT_f = 1°C

K_f = 1.86 K kg mol⁻¹

ΔT_f = K_f × Molality × i

Molality = `"Moles of solute"/"Kg of solvent"`

= `(19.5/78)/(500/100)`

= 0.5 M

i = `(Δ"T"_"f")/("K"_"f" xx "Molality")`

= `1/(1.86 xx 0.5)`

i = 1.07

For F – CH₂ – COOH ⇒ i = 1 + α

α = i − 1

= 1.07 − 1

α = 0.07