Question

1) The vapor pressure of liquid cobalt is 400. mm Hg at 3.03×10³ K. Assuming that ΔH_vap for Co (450 kJ/mol) does not change significantly with temperature, calculate the vapor pressure of liquid Co at 3.07×10³ K.

2) The vapor pressure of methyl acetate (CH₃COOCH₃) has been measured over a range of temperatures.

(a) Which of the following should give a linear plot (temperatures in K)?

On the x-axis: _____T or lnT or 1/T   On the y-axis: _____P or lnP 1/P

(b) When the appropriate plot is made, the resulting straight line has a slope of -4.20×10³ K.

Use this information to determine ΔH_vap for CH₃COOCH₃. ______kJ/mol

Answer 1

Q1. vapor pressure of liquid Co at 3.7 x 10³ K = 504.8 mmHg

Q2. H_vap for CH₃COOCH₃ = 34.9 kJ/mol

Explanation

Q1. According to Clausius - Clapeyron equation,

ln(P2 / P1) = (H_vap / R) * (1/T1 - 1/T2)

ln(P2 / 400 mmHg) = (450 x 10³ J/mol / 8.314 J/mol-K) * (1/3030 K - 1/3070 K)

ln(P2 / 400 mmHg) = 0.233

P2 / 400 mmHg = e^0.233

P2 / 400 mmHg = 1.262

P2 = (400 mmHg) * (1.262)

P2 = 504.8 mmHg