Osmosis and Osmotic Pressure

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Drop a dry, wrinkled raisin in a glass of water, and leave it overnight. Next morning? It is fat and swollen. Water has gone inside, but nobody pushed it in. No pump, no squeezing. So what drove the water in? Hold that thought. Today we learn osmosis, and osmotic pressure. It comes up often in JEE, and it gives the best way to find the molar mass of a polymer. So, let's begin.

0:33Osmosis: the solvent moves in

So, look at the figure. It is a U-shaped tube. On the left, in blue, pure solvent. On the right, in green, a solution. And at the bottom, this orange dashed line is a semipermeable membrane. It lets the solvent pass through, but not the solute. Okay? Now think. The solute cannot cross the membrane, but the solvent can. Which way will the solvent move: from left to right, or from right to left? Left to right, into the solution, yes, exactly! See the arrow at the bottom: solvent flows in. On its own, the solvent moves from the pure solvent, or a dilute solution, into the more concentrated one; the arrow on screen means goes to. This flow is osmosis. Clear? Can we stop this flow? Yes: push on the solution side. The extra pressure needed on the solution, just to stop the flow, is the osmotic pressure, written with the Greek letter pi. Careful, here pi is a pressure, not three point one four! Got it? In the figure, nobody pushes. So the solution side rises above the grey dashed line, the level of the solvent, by a height h, shown by the double arrow. That extra column presses back, and when h times rho times g equals pi, the flow stops. Rho, the Greek letter like a curly p, is the density, and g is the acceleration due to gravity. With me?

1:57The van 't Hoff equation

Now, how big is pi? The van 't Hoff equation tells us: pi equals CRT, okay? And C is n B over V: n B, with the small B below, is the moles of solute, and V is the volume of the solution. Right? Now a question. For the boiling and freezing points, we used molality, small m. Is this capital C molality, or molarity? Molarity, correct! Moles of solute per litre of solution, that is n B over V, with V in litres, okay? R is the gas constant, and T is the temperature, always in kelvin. So osmotic pressure uses molarity. Got it? Now the really useful part. n B is w B over M B: w B grams of a solute of molar mass M B, okay? Put that in; the double arrow means so. M B equals w B times R T, over pi times V, right? Weigh the solute, measure pi, and you have the molar mass! And the conditions, as for every colligative property: a dilute solution of a non-electrolyte, so the solute does not dissociate into ions or associate into bigger units. Clear?

3:09Isotonic solutions and reverse osmosis

Alright, some words that come up often. Two solutions with equal osmotic pressure are isotonic: no net flow between them. Okay? The one with higher pi is hypertonic; the one with lower pi, hypotonic. Hyper means more, hypo means less. Simple, right? Now think. I put a red blood cell in pure water, which is hypotonic. Which way does the water move, and what happens to the cell? Water goes in, yes! The cell swells, and may even burst. And our raisin? Same story: water enters by osmosis, so it swells. Mystery solved! With me? Now the opposite. In a strong salt solution, the outside is hypertonic, so water leaves, and the cell shrinks. Clear? Now flip it around. pi is the pressure that just stops the flow. What if I push on the solution with even more than pi? The flow reverses, exactly! Solvent is pushed out of the solution. This is reverse osmosis; the sign on screen is the greater-than sign. It is used for desalination, getting fresh water from seawater. Got it? And last, the big use. For polymers and proteins, osmotic pressure is the best method for the molar mass, because pi gives large, measurable values at room temperature. Example two will show you.

4:31Example 1: osmotic pressure of 0.1 M glucose at 27 °C

Okay? Alright, example one. Find the osmotic pressure of zero point one molar glucose at twenty seven degrees Celsius. Capital M means molar, and the small circle means degrees. R is zero point zero eight two one litre atmosphere per kelvin per mole. Pause the video, and try it first. Done? Okay, let's check. pi equals CRT, and the temperature is twenty seven. So T equals twenty seven, right? No, no, careful! T must be in kelvin: twenty seven plus two hundred and seventy three, three hundred kelvin, the capital K on screen. With me? Glucose is a non-electrolyte, so it does not dissociate, and C is simply zero point one mole per litre. Okay? So, pi equals CRT: zero point one, times zero point zero eight two one, times three hundred. Right? That gives two point four six atmospheres, the a t m on screen. Remember: kelvin, always. Clear?

5:34Example 2: molar mass of a polymer from osmotic pressure

Now a harder one, example two. One point two six grams of a polymer, in two hundred millilitres of water, has an osmotic pressure of two point five seven times ten to the power minus three bar, at three hundred kelvin. Find its molar mass, with R as zero point zero eight three litre bar per kelvin per mole. Pause and try. Hint: check the units of the volume. Done? Okay, let's check. T is three hundred, already in kelvin this time. And the volume: just put two hundred, right? Oh no, careful! R has litres in it, so V must be in litres: two hundred millilitres, the m L on screen, is zero point two litres. w B is one point two six grams, and pi is in bar, matching the bar in R, okay? Now, M B equals w B times R T, over pi times V, okay? On top: one point two six, times zero point zero eight three, times three hundred. At the bottom: two point five seven times ten to the power minus three, times zero point two. Right? Work it out: M B is about six point one times ten to the power four grams per mole; the wavy equals sign means approximately. About sixty one thousand: really big, as you expect for a polymer. Got it? One more question. Why not find this molar mass from the depression of freezing point instead? Because that effect is tiny! For this same polymer, delta T f, the drop in freezing point, is only about one point nine times ten to the power minus four kelvin. The little triangle is the Greek letter delta, for change. Far too small to measure. With me? But pi is about two point six millibar, the m bar on screen: enough to hold up a column of water about two point six centimetres high. Easy to measure! That is why osmotic pressure is the best method for polymers. Clear?

7:34Quick revision: pi = CRT, isotonic, reverse osmosis

So, quick revision. Osmosis: the solvent crosses a semipermeable membrane into the more concentrated solution. pi equals CRT, with C in moles per litre and T in kelvin; and M B equals w BRT, over pi V, okay? Isotonic means equal pi; more than pi gives reverse osmosis; and osmotic pressure is the best method for polymers. Now go and try the practice problems on this page. All the best!