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Tips About Osmotic Pressure - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are some tips and tricks about osmotic pressure. As you know osmotic pressure is a colligative property. And so, a colligative property is based on the concentration about what your solute is and not based on the identity of the solute. Osmotic pressure is the amount of pressure that’s required to prevent water from crossing a membrane.

So like in your eye, you have a certain amount of liquid inside, and then there is a certain amount of pressure that pushes against the eye. And that’s what the optometrist or ophthalmologist will actually use an instrument to measure your osmotic pressure in your eye.

Other things that are like, in organisms, they have osmotic pressure. And basically, it allows them to not implode or to not shrivel up or anything like that. So osmotic pressure plays a large role especially in biology. But in chemistry, we use the equation. This is the symbol for osmotic pressure. So osmotic pressure equals iMRT all multiplied times each other. And if you remember the equation, then that’s half the battle because you can actually figure out the rest and I’ll show you how.

The i stands for the Van’t Hoff factor and that’s based on the number of particles that you have. If you dissociate an ionic compound, then that would be the Van’t Hoff factor. I’ll show you that in a little bit. Capital M is Molarity and that’s in moles per liter of course or molar. The R would be the universal gas constant. And you could probably figure out that this one would be the 0.08206 liters times atmosphere over moles times Kelvin. And then t, that should make sense just like in the gas laws, this would be for temperature. What would the units be? Well since the universal gas constant has Kelvins, then it's Kelvins.

So if you multiply all those, then what you’re left with, is you’re left with the units. And the units would be in atmospheres for osmotic pressure. Let’s give you an example.

So say if we have 0.3 molar Sodium Chloride and that’s aqueous, and say that we have a temperature of 7 degrees Celsius, and we want to figure out the osmotic pressure in say some area. Osmotic pressure equals blank. All we do is we plug it in. So osmotic pressure equals, now the i. How do we calculate the i in this case? Well for NaCl since NaCl is ionic, and it's soluble, it splits up into Na+ and Cl-. So from 1 mole or 1 set of the compound, I end up with 1 plus 1 based on the coefficients, 2 moles of ions. So that would be the Van’t Hoff factor which would be 2. So say if I had like say Na2SO4, then that would split up into Na+ plus SO4- and then I would balance off obviously to get the Na+. So then I would end up with a Van’t Hoff factor of 3 because, 2 plus 1 for the coefficients.

If I had sugar, well sugar is covalent, so the Van’t Hoff factor would be 1 because it does not ionize. It doesn’t change into ions or separate into ions. The molarity, pretty easy. 0.3 molar or moles per liter and then the R, 0.080206 liters times atmosphere over moles times Kelvin. And then temperature, well don’t get tricked up, 7 degrees Celsius is in the wrong units. So we have to add 273 so you get 280 Kelvin. Kelvins cancel out, moles per liter cancel out, with the molarity. You’re left with atmospheres. So you do the math, osmotic pressure, you get pretty close to 13.8 atmospheres.

So basically that tells you the osmotic pressure, or the pressure required to prevent that water from crossing that membrane. And so don’t forget with osmotic pressure that often times you do have a Van’t Hoff factor, because most of those solutions are made of ionic compounds as their solids. And then you have molarity, and then you have the universal gas constant and then you have the temperature of your solution that you have there.

Keep in mind osmotic pressure, the concentration is different because it’s in molarity, whereas with boiling point elevation and freezing point depression, it’s in molality which is different from molarity. Remember molality is moles per kilogram. So here we’re using molarity which is moles per liter.

Hopefully these tips and tricks about osmotic pressure will help you out in solving your calculations. Have a good one.