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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mana on May 11, 2021, 11:55:09 AM

Title: calculation P total in a mixture
Post by: mana on May 11, 2021, 11:55:09 AM

hi all
in Dalton rule P _total is as follow:
 P _total = P_A + P_B
but some times I found the formula for calculation of P_A and P_B is shown :
P_B= X_BP⁰_B                     P_A= X_AP⁰_A
and sometimes the formula is:
P_B= X_BP_total           P_A= X_AP_total

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

Title: Re: calculation P total in a mixture
Post by: Meter 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.

Title: Re: calculation P total in a mixture
Post by: mjc123 on May 11, 2021, 05:09:28 PM

Dalton's law is P_i = x_iP_tot, where x_i 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 P_i = x_iP°_i, where P°_i is the vapour pressure of pure liquid i, and x_i is the mole fraction of i in the liquid phase.
The two are related by x_i(g) = P_i/P_tot = x_i(l)P°_i/Σx_i(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).