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Topic: Miscibility and partial pressure concepts  (Read 1720 times)

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Offline Darren

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Miscibility and partial pressure concepts
« on: April 27, 2015, 12:16:43 AM »
Hi everyone, i was thinking about a simple concept, miscibility. But what confused me is what determines 2 liquids are miscible or not. What property does miscibility rely on?
1) densities of the 2 liquids are close to each other
2) molecules of one liquid can fit into spaces between the molecules of the other liquid

What examples are there of miscible liquids that have densities not close to each other?

Thank you!

Edit: I have one more question and i feel maybe i should combine them into one post to save space on the forum.

In a fixed volume container with 2 gases, is the mole fraction of a gas equal to the partial pressure ratio of it? Considering ratio values only and not the actual values. I am only aware that mole fraction equals to volume ratio, but since volume and pressure are linked, can mole fraction be related to partial pressure?
« Last Edit: April 27, 2015, 01:12:46 AM by Darren »

Offline Borek

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Re: Miscibility and partial pressure concepts
« Reply #1 on: April 27, 2015, 03:26:16 AM »
1) densities of the 2 liquids are close to each other
2) molecules of one liquid can fit into spaces between the molecules of the other liquid

I am afraid neither is a correct condition. What matters are the interactions between molecules of both substances - how strongly they attract each other. If molecules of each substance attract strongly their own kind, but very weakly molecules of the other kind, they will not mix.

Quote
What examples are there of miscible liquids that have densities not close to each other?

I guess dichloromethane and diiodomethane should be miscible, despite the latter having the density more than twice as large as the former.

In general there is a saying "like dissolves like" - which is a pointer to what to look for when it comes to miscibility.

Quote
In a fixed volume container with 2 gases, is the mole fraction of a gas equal to the partial pressure ratio of it? Considering ratio values only and not the actual values. I am only aware that mole fraction equals to volume ratio, but since volume and pressure are linked, can mole fraction be related to partial pressure?

Yes.
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Offline Ranowa

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Re: Miscibility and partial pressure concepts
« Reply #2 on: April 27, 2015, 12:31:37 PM »
In a fixed volume container with 2 gases, is the mole fraction of a gas equal to the partial pressure ratio of it? Considering ratio values only and not the actual values. I am only aware that mole fraction equals to volume ratio, but since volume and pressure are linked, can mole fraction be related to partial pressure?

To elaborate on Borek's response, there's actually a commonly used rule for this, Raoult's Law: Total Pressure = Partial Pressure times Mole Fraction. So, if you have only two gases, you would have two raoult's law expressions which could be set equal to each other, and then some fiddling gets you X1/X2 = p1/p2.

Offline Darren

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Re: Miscibility and partial pressure concepts
« Reply #3 on: April 27, 2015, 12:39:09 PM »
Thank you for the responses! That cleared some fuzzy doubts i had haha

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