osmotic pressure question:

[kozmik]

If you have a solution of 1L of H2O and 234g of NaCl, what is the osmotic pressure at STP (R=0.1L*atm/mol*K)?

I would've said that:
osmotic pressure = MRT

M = n/V = (234g/58.44g/mol)/1L = 4.00 mol/L

osmotic pressure = (4.00 mol/L)*(0.1L*atm/mol*K)*(273K) = 109.2 atm

but the answer is 218.4 atm (twice of what I calculate). why?

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[delsol7]

Osmotic pressure = MRTi

NaCl completely dissociates in water into Na+ & Cl- making that "i" value to be 2. That's why your calculation is half of answer.

If compound was Sugar, then they don't dissociate. Therefore i = 1.
If compound was MgCl2, they also completely dissocaite, i = 3.

 

[jigabodo]

If you have a solution of 1L of H2O and 234g of NaCl, what is the osmotic pressure at STP (R=0.1L*atm/mol*K)?

I would've said that:
osmotic pressure = MRT

M = n/V = (234g/58.44g/mol)/1L = 4.00 mol/L

osmotic pressure = (4.00 mol/L)*(0.1L*atm/mol*K)*(273K) = 109.2 atm

but the answer is 218.4 atm (twice of what I calculate). why?

Like delso has mentioned, its because you forgot to take consideration of i, aka the van hoff's factor. because NaCl will dissociate in solution into 2 different type of ions, the "i" would be 2 in that case, which is how they got 218.4 atm.

I am guessing that you got the original equation from Kaplan? They tend to have a nasty habit of leaving out i with a lot of their equations.

 

[kozmik]

thank you delsol7 and jigaboda, that really helped
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i did take the equation from kaplan - i guess i should look at other sources for information.

 

[crrees]

i think dat destroyer points out that it is Pi = MRTi

where i is equal to how many moles are present when dissolved, the other posters already explained this.

this was ON my dat so i think its a good idea to know it