20 % of acetic acid is dissociated when its 5 g is added to 500 mL of water. The depression in freezing point of such water is _ _ _ _ _ × 10 - 3 C ∘ . Atomic mass of C , H and O are 12 , 1 and 16 a.m

Question

20 % of acetic acid is dissociated when its 5 g is added to 500 mL of water. The depression in freezing point of such water is _ _ _ _ _ × 10 - 3 C ∘ . Atomic mass of C , H and O are 12 , 1 and 16 a.m.u. respectively. [Given : Molal depression constant and density of water are 1 . 86 K kg mol - 1 and 1 g cm - 3 respectively.

TestHub · Accepted Answer

Van't Hoff factor (i) for dissociation of acetic acid can be written as follows,

$i = 1 + (n - 1) α$

$n =$ Number of ions

$α =$ Degree of dissociation

$(i = 1 + 0.2 (2 - 1) = 1.2$

${ΔT}_{f} = {iK}_{f} m {ΔT}_{f} = Depression in freezing point K_{f} = Molar depression constant m = molality$

${ΔT}_{f} = 1.2 \times 1.86 \times \frac{5 \times 1000}{60 \times 500}$

${ΔT}_{f} = 0.372$

$Δ T_{f} = 372 \times 10^{- 3}$

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