Phase Changes and DiagramsMedium

Boiling point and pressure

Water boils at 100 °C at 1 atm. At 2000 m altitude (P ≈ 0.80 atm), ΔH_vap = 40.7 kJ/mol. Use Clausius-Clapeyron: ln(P₂/P₁) = -(ΔH_vap/R)(1/T₂ - 1/T₁). R = 8.314 J/(mol·K).
Estimate T₂.

Given data

P₁ = 1.00 atm, T₁ = 373 KP₂ = 0.80 atmΔH_vap = 40.7 kJ/mol = 40700 J/molR = 8.314 J/(mol·K)

Review the theory: Equilibri tra Fasi

Steps

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Calculate ln(P₂/P₁).
Write your working — the numerical result must appear in the text
Calculate 1/T₁ = 1/373.
Solve C-C for 1/T₂.
Calculate T₂.

Full worked solution

Calculate ln(P₂/P₁).
$\ln(0.80/1.00)$
ln(0.80) = -0.2231.
Calculate 1/T₁ = 1/373.
$1/373$
1/373 = 0.002681 K⁻¹.
Solve C-C for 1/T₂.
$0.002681 - \frac{8.314 \cdot (-0.2231)}{40700}$
1/T₂ = 0.002681 + 0.0000456 = 0.002727 K⁻¹.
Calculate T₂.
$1/0.002727$
T₂ ≈ 367 K = 94 °C.

Result:T₂ ≈ 367 K (≈ 94 °C).

Boiling point and pressure

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