The normal boiling point of a liquid 'A' is 350 K. $DeltaH_(vap)$ at normal boiling point is $"35 kJ/mol"$ . Pick out the correct statement? (Assume that $DeltaH_(vap)$ to be independent of pressure)

Question

A) $DeltaS_(vap) > "100 J/K"cdot"mol"$ at 350 K and 0.5 atm
B) $DeltaS_(vap) < "100 J/K"cdot"mol"$ at 350 K and 0.5 atm
C) $DeltaS_(vap) < "100 J/K"cdot"mol"$ at 350 K and 2 atm
D) $DeltaS_(vap) = "100 J/K"cdot"mol"$ at 350 K and 2 atm

Specify your reason.

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Answer 1 · 2018-01-02T22:14:43

Both A) and C) are correct.

The equilibrium at the boiling point is

$"A(l) ⇌ A(g)"; Δ_text(vap)H = "35 kJ·mol"^"-1"$

$Δ_text(vap)S = (Δ_text(vap)H)/T_text(vap) = "35 000 J/mol"/("350 K") = "100 J·K"^"-1""mol"^"-1"$

At 350 K and 0.5 atm

The boiling point ( $T_text(vap)$ ) will be lower at the decreased pressure.

Since $T_text(vap) < "350 K", Δ_text(vap)S > "100 J·K"^"-1""mol"^"-1"$

The correct answer is A).

At 350 K and 2 atm

The boiling point ( $T_text(vap)$ ) will be higher at the increased pressure.

Since $T_text(vap) > "350 K", Δ_text(vap)S < "100 J·K"^"-1""mol"^"-1"$

Answer C) is also correct.