# General Chem trick question

#### Kami

##### Member
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The Osmotic pressure at STP of a solution made from 1 L of NaCl (aq) containing 117 g of NaCl is:

a) 44.77 atm
b) 44.87 atm
c) 89.54 atm
d) 117 atm

Hint: the tricky part has to do with the moles of NaCl

#### sgdorfl90

##### Senior Member
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Kami said:
The Osmotic pressure at STP of a solution made from 1 L of NaCl (aq) containing 117 g of NaCl is:

a) 44.77 atm
b) 44.87 atm
c) 89.54 atm
d) 117 atm

Hint: the tricky part has to do with the moles of NaCl
aqueous, so you get 2 moles out of 1 NaCl, so prolly C.

#### thehipster

##### Senior Member
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Yeah, pretty straight forward, gotta be C

#### mlle

##### Senior Member
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what?! can someone please explain this in more detail... how do you approach this? thanks.

#### R.L.H

##### LoVe mY liFE
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sgdorfl90 said:
aqueous, so you get 2 moles out of 1 NaCl, so prolly C.

Why 1 mole is 44.77 atm?

#### Orthodox Bear

##### Columbia Class of 2011
10+ Year Member
People, the solution is simple....
Osmotic Pressure=MRTi
THis can be manipulated to Osmotic Pressure=nRTi/V
n= 117/58.5= 2 moles NaCl
R*T at STP= 22.4
i= 2, because NaCl breaks up into two ions
Therefore Osmotic pressure= (2)(22.4)(2)= 89.6

OP
K

#### Kami

##### Member
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Thats what I thought as 2 moles but it is actually 4 moles of NaCl and I am trying to figure it out why it is 4 moles. The rest is just plugging in the numbers to get the answer.

Kami said:
The Osmotic pressure at STP of a solution made from 1 L of NaCl (aq) containing 117 g of NaCl is:

a) 44.77 atm
b) 44.87 atm
c) 89.54 atm
d) 117 atm

Hint: the tricky part has to do with the moles of NaCl

#### tom_servo_dds

##### Senior Member
10+ Year Member
Kami said:
Thats what I thought as 2 moles but it is actually 4 moles of NaCl and I am trying to figure it out why it is 4 moles. The rest is just plugging in the numbers to get the answer.
It is 2 moles of NaCl, but NaCl completely dissociates in water so you get two moles of Na+ and 2 moles of Cl- total. If it didn't dissociate you would have:

(2)(22.4)(1) = 44.8

According to the equation OP = nRTi/V, but instead you get:

(2)(22.4)(2) = 89.6

#### thehipster

##### Senior Member
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7+ Year Member
Osmotic pressure, Boiling-Point Elevation, and Freezing-Point Depression all depend on Colligative Properties. So for this example, NaCl, each mole dissociates into 2 ions (Na+ and Cl-). If you got something that doesn't dissociate, like an organic compound (i.e. ethanol, C2H5OH) you only have the one mole. If you get a problem that deals with Colligative properties, you'll have to use this approach.