Raoult's Law - How To Calculate The Vapor Pressure of a Solution

This chemistry video tutorial provides a basic introduction into Raoult's law which says that the vapor pressure of a solution is the product of the mole fraction of the solvent and the partial pressure of the pure solvent at a given temperature. This video explains how to calculate the vapor pressure of a solution with a nonvolatile solute. It also explains how to calculate the mass of solute / glucose needed to reduce the vapor pressure of the solution to a certain value.
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[@TheOrganicChemistryTutor]

Colligative Properties - Free Formula Sheet: bit.ly/3BBulk6 Direct Link to The Full 45-minute Video: bit.ly/3VY3X8p Chapter 11 - Video Lessons: www.video-tutor.net/solutions.html

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Maybe your professor was also learning from here and then teaching you

[@SyenPie]

FANTASTIC concise explanation and the practice questions you chose were perfect at covering this topic. Thank you!

[@ronielyncuison7244]

I would like to correct or point out a slight mistake in the last problem. Both sides should be divided by 0.9765. The answer would be 0.3338 moles.

[@sachinbhandari3506]

Yeah that confused me for a bit

[@JayAgustin-ic5cl]

he converted it into gram. that 0.3338 is a mol

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[@georgesadler7830]

Professor Organic Chemistry Tutor, thank you for a basic Introduction and explanation of Raoult's Law and how it's used to calculate Vapor Pressure of a Solution in AP/General Chemistry. Th example problems are awesome when it comes to calculating Vapor Pressure, however, there is an error in problem number three at the 13:01 minute mark. Professor Organic Chemistry Tutor, there is a division error in your calculation. Please correct this error in the video.

[@jia-xuan-shuo]

You’re my life savor. Yet at 13’10”, I believe the denominator is supposed to be 0.9765. I made lots of similar mistakes.

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yep.

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Why u post this the day after the exam that I needed this ;-; jk, great video!

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[@shivanireddy3993]

Why moles of ions? Can't we take no Of moles of cacl2 directly?

[@Addiwasabi]

can someone explain why at 5:38 he calculates the molarity, but not of the compound. Instead the # of ions??

[@toperaheem]

From what I could discern it's because CaCl2 is an ionic compound and after calculating the number of moles of the entire CaCl2 compound as a whole, we need to take into consideration the no of ions in the compound and multiply the number of moles by that number as opposed to the previous question where C6H12O6 is a molecule which doesn't break down into ions so we just calculate the number of moles directly

[@OmniversalInsect]

@@toperaheem I think your explanation is correct. When dissolved, the lattice structure of ionic compounds break apart and the ions act as individual entities in solution.

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I wish that some of the questions weren't on patreon and the video too. I don't have any money lol

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[@corneliusmathebula9832]

how can you tell if an ionic solute is given in a question? like the difference between the 1st and 2nd question?

[@serpentsage]

When there are metal and nonmetal atoms bonded together, it forms an ionic compound.

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thanks alot man, you're a life saver

[@Gentleman139]

If you can please answer. How do you know what to find when the question asks what the vapor pressure?

[@meniscusmedical2471]

Slight error with the numbers sir. just 0.9765 and not 0.9675 but other than that thank you.

[@pete4043]

JG is an absolute monster

[@Wn_.468]

Why you added 3 inos in the second example ?

[@vaibhavimarathe9189]

thank you so much u have no idea how much this helped

[@value2887]

Why did you use the mols ions on the second one and not the first

[@toperaheem]

From what I could discern it's because CaCl2 is an ionic compound and after calculating the number of moles of the entire CaCl2 compound as a whole, we need to take into consideration the no of ions in the compound and multiply the number of moles by that number as opposed to the previous question where C6H12O6 is a molecule which doesn't break down into ions so we just calculate the number of moles directly

[@iqbalmahmud5072]

you got everything i need

[@estiiizy]

why did we have to convert the moles of cacl2 to ions for the second problem but not the first problem?

[@engertejada6930]

Because glucose is not an ionic compound, CaCl2 is

[@Cristi05_]

@@engertejada6930 how are we supposed to know CaCl2 is an ionic compound. No way you have to just memorize every single compound formula right? Is this something your teacher should give you?

[@weewuwuu]

@@Cristi05_An ionic compound is just a metal and a nonmetal

[@OmniversalInsect]

​@@Cristi05_ Generally all metal non-metal compounds will be ionic. Ammonium (NH4+) is probably the only non-metal positive ion you will come across, so compounds like NH4Cl and NH4OH are also ionic.

[@ghostveiwer4649]

Thank you man, you have saved the day 👍

[@mustafaaa772]

for the moles of water, why did he take it as 500g divided by 18 and not 0.5kg divided by 18?

[@mayaratner6996]

You are a godsend

[@Cristi05_]

Ok so you mentioned at 6:30 that you need to mutiply by 3 ions and you explained how CaCl2 forms one calcium ion and 2 chlorine ions. How were we supposed to know that? That shoudnt be just something we have to memorize Im sure right. Would that be something your teacher would have to give you kind of like the periodic table of elements. How are we supposed to know how many ions form from a solute dissolving? Please explain this

[@OmniversalInsect]

Your best bet is to check if the compound is made from a combination of a metal and non-metal, in which case you can assume it is ionic. The one exception you're likely to come across is compounds that contain NH4 (ammonium) like NH4Cl, these are also ionic despite containing only non-metals.

[@suuushitrash]

at 0:55 , mind if i ask why should the answer be less than 23.8 torr?

[@16_bit_Jitu]

dont know if you need it anymore or not but the bases of raoult's law says that the vapour pressure of solvent decreases if you mix a Solute

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perfect timing!

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[@Gentleman139]

Why do you find the mole fraction specifically for water?

[@mysciencecruise9708]

thank you so much. this is very helpful

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LITERALLY NEEDED THIS RN SHJSHSJKSH

[@Mister_F]

Thanks!

[@0101-o1q]

is the last example here correct? i computed for the mol of glucose and it turned out 576.64 mol (although i doubt this because the solvent should be greater). When i inserted this in the Raoult's equation it resulted to 54 Torr. Correct me if I'm wrong tho thank you

[@Micahiscoo]

2 months old but just here to tell you, he read his mol wrong, so I’m guessing what you got is the correct answer.

[@0101-o1q]

@@Micahiscoo lmao thanks! It's already vacation but I appreciate the reply man God bless on your studies

[@leandrang]

How would it be if we're to find the vapour pressure of only one substance and not the entire solution...??

[@L--QuachMinhThuy]

The vapor pressure of a solution is dependent on the total number of the solute particles and not really the identity of the solute particles.

[@awuahedward5435]

why did u not change mL to L

[@kimanhchung8372]

Thanks

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[@Who1]

how if the question dont have torr

[@aaronlee5362]

It does have torr

[@OmniversalInsect]

Just use the unit of pressure given in the question, probably atm or Pa.

[@zakajadi2744]

Can you explain when you're trying to find the mole of the compound you put the molar mass as 'g' instead of gmol^-1.Also why are you dividing the the number of moles by the molar mass instead of using the mole formula = mass/molar mass to find the mole of the compound. Thanks

[@n.9354]

at the last example, i didn't understand why he multiple the moles by 180.15.. from where did he get that number?

[@lenamathew5713]

180.15 is the molar mass of glucose. To convert moles to grams you have to multiply the number of moles( here he got 0.337 moles) with the molar mass. (in this case, multiply it with the molar mass of glucose)

[@n.9354]

@@lenamathew5713 Oh i see! Thank you so much!

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[@kyf6705]

1mL=1g you're welcome

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Not me watching this 1 houre before exam😂

[@pingpong3311]

You put 0.9675 instead of 0.9765

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