Methylamine has a vapor pressure of 344 torr at -25 °C and a boil...

Question

Methylamine has a vapor pressure of 344 torr at -25 °C and a boiling point of -6.4 °C. What is the ΔHvap for methylamine?

Accepted Answer

Identify the Clausius-Clapeyron equation: $ \ln \left( \frac{P_1}{P_2} \right) = \frac{\Delta H_{\text{vap}}}{R} \left( \frac{1}{T_2} - \frac{1}{T_1} \right) $, where $ P_1 $ and $ P_2 $ are the vapor pressures at temperatures $ T_1 $ and $ T_2 $, respectively, and $ R $ is the ideal gas constant.. Convert the temperatures from Celsius to Kelvin by adding 273.15 to each temperature. Thus, $ T_1 = -25 + 273.15 $ K and $ T_2 = -6.4 + 273.15 $ K.. Recognize that at the boiling point, the vapor pressure $ P_2 $ is equal to 1 atm (or 760 torr). Therefore, $ P_1 = 344 $ torr and $ P_2 = 760 $ torr.. Substitute the known values into the Clausius-Clapeyron equation: $ \ln \left( \frac{344}{760} \right) = \frac{\Delta H_{\text{vap}}}{R} \left( \frac{1}{T_2} - \frac{1}{T_1} \right) $.. Solve for $ \Delta H_{\text{vap}} $ by rearranging the equation: $ \Delta H_{\text{vap}} = R \cdot \ln \left( \frac{344}{760} \right) \cdot \left( \frac{1}{T_2} - \frac{1}{T_1} \right)^{-1} $.