November 30, 2021, 10:52:38 AM
Forum Rules: Read This Before Posting

### Topic: calculation P total in a mixture  (Read 375 times)

0 Members and 1 Guest are viewing this topic.

#### mana

• Full Member
• Posts: 174
• Mole Snacks: +3/-3
##### calculation P total in a mixture
« on: May 11, 2021, 11:55:09 AM »
hi all
in Dalton rule P total is as follow:
P total = PA + PB
but some times I found the formula for calculation of PA and PB is shown :
PB= XBP0B                     PA= XAP0A
and sometimes the formula is:
PB= XBPtotal           PA= XAPtotal

I would be grateful if you tell me what is the difference between this two classification

#### Meter

• Full Member
• Posts: 240
• Mole Snacks: +14/-3
• Take what I say with a grain of salt
##### Re: calculation P total in a mixture
« Reply #1 on: May 11, 2021, 12:27:08 PM »
P0  would be the total pressure of the system. I'm not a fan of that notation since it is evidently ambiguous.

#### mjc123

• Chemist
• Sr. Member
• Posts: 1975
• Mole Snacks: +288/-12
##### Re: calculation P total in a mixture
« Reply #2 on: May 11, 2021, 05:09:28 PM »
Dalton's law is Pi = xiPtot, where xi is the mole fraction of component i in the gas phase.
Your other formula is Raoult's law for ideal mixtures, describing the composition of the vapour in equilibrium with a liquid mixture. In this case Pi = xii, where P°i is the vapour pressure of pure liquid i, and xi is the mole fraction of i in the liquid phase.
The two are related by xi(g) = Pi/Ptot = xi(l)P°i/Σxi(l)P°i

It is pretty unobservant to say "sometimes I find this formula, sometimes that one". In each case, try to observe the context, what is the situation being described, and what the symbols mean (the same symbol may mean different things in different contexts).