The boiling point of a liquid increases as the pressure above the liquid increas

Question

The boiling point of a liquid increases as the pressure above the liquid increases. In contrast to boiling food in an open pot, pressure cookers trap the steam, which raises the internal pressure, which raises the boiling point of water, causing a higher cooking temperature. A higher cooking temperature means that the food will cook faster. The instruction booklet for your pressure cooker indicates that its highest setting is 12.3 psi . You know that standard atmospheric pressure is 14.7 psi, so the booklet must mean 12.3 psi ABOVE atmospheric pressure. At what temperature in degrees Celsius will your food cook in this pressure cooker set on "high"?

Explanation / Answer

At atmospheric pressure of 14.7 psi, water boils at 100 C or 373K.
At boiling point, the vapor pressure equals the atmosphericpressure.
So Vapor Pressure, P1 = 14.7 psi , T1 = 373 K
Vapor pressure , P2 = 14.7 + 12.3 =27 psi
T2 = ?
Hvap for water = 40.65 kJ/mol = 40.65 * 10^3 J/mol
.
ln (P1/P2) = Hvap / R * (1/T2 - 1/T1)
ln (14.7 psi /27 psi) = 40.65 * 10^3 / 8.314 * ( 1/ T2 - 1/373K)


-0.60798=4889.343 X (1/T2-0.00268)

-1.243 X 10 ^-4 = (1/T2-0.00268)

1/T2= .00255

T2= 392.3 K or 119.3c

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