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Problem 147 Hard Difficulty

The boiling point of methanol is $65.0^{\circ} \mathrm{C}$ and the standard enthalpy of formation of methanol vapor is $-201.2 \mathrm{kJ} / \mathrm{mol} .$ Calculate the vapor pressure of methanol (in $\mathrm{mmHg}$ ) at $25^{\circ} \mathrm{C}$. (Hint: See Appendix 3 for other thermodynamic data of methanol.)

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Chemistry 102

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Chapter 11

Intermolecular Forces and Liquids and Solids

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Video Transcript

Okay, So for this problem, we need to figure out what the vapor pressure of methanol is at 25 degrees Celsius, and they made us feel like doing a little bit of extra work. So instead of just giving us the heat of vaporization, they gave us the heat of formation of methanol vapor, which is native 201 point to kill. Jewels promote. Remember, that's the heat that it takes to form methanol vapor just from the basic elements alone. So I need to use Hess's law to figure out what the heat of vaporization will be. According to has his law. Heat of vaporization will be the heat of formation of the vapor minus the heat, the formation of the liquid. So that means that our heat of vaporization will be, let's see negative 201.2 killed. Jules promote minus negative. 276 0.98 Killer jewels promote, and that will give us an answer of Let's see, 37 0.5 killer jewels for more their heat of vaporization. So now we can go and take that over to this equation, which is the natural log of the vapour pressure is the negative heat of vaporization, divided by R Times T plus in constant. They've given us the pressure and the temperature for one set of conditions. That means that we can find this constant, and then we can find the pressure for the other temperature that they gave us. So the constant will be the natural log of the vapour pressure at the boiling point. So the boiling point, the vapour pressure, will be 760 millimeters mercury, because that's the measurement, because that's what boiling means. Um, should be that, plus our heat of vaporization divided by rt. So that's going to equal natural log of 760 millimeters of mercury plus 307,500 Jules per mole, divided by 8.3145 patty jewels from old times Calvin Times the boiling point, which is 700 between you and Calvin. Sorry, it is going to be 338 Calvins. It's important that these Calvin's cancel out, so that's going to give us a see value equal to you equal to 20. Okay, so now that we know what see is we just need to plug in the other temperature that they gave us to find out what the vapour pressure will be at 25 degrees Celsius. So natural log of that vapor pressure. We're being 30,000 500 jewels from all divided by it went 3145 times this time 298 Calvin plus 20. Now I just need to take the exponents both sides to get rid of the natural log, and that gives us a value of 127 millimeters of mercury.

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Problem

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Watch More Solved Questions in Chapter 11

Video Transcript

Raymond Chang, Kenneth A. Goldsby

Chemistry

Lizabeth Tumminello

Stephen Pulliam

Allea Cauilan

Jake Rempel

This textbook answer is only visible when subscribed! Please subscribe to view the answer

Chemistry

Lizabeth Tumminello

Stephen Pulliam

Allea Cauilan

Jake Rempel

Raymond Chang, Kenneth A. GoldsbyChemistry

Lizabeth Tumminello

Stephen Pulliam

Allea Cauilan

Jake Rempel

Raymond Chang, Kenneth A. Goldsby

Chemistry