Part B: Concept Checks

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Put a slice of cucumber in salt water, and after a while, it shrinks. Now take a raw egg, dissolve its shell in acid, and put it in pure water. It swells up! Water in both cases, but opposite results. Why? Keep that thought; we will settle it today. This is a concept check class on the whole unit of solutions: two statements, assertion and reason, true or false. In JEE, these short statements come up often. They look easy, but one word can flip the answer. So, let's begin.

0:40Two statements: three checks, in order

So, first, two statements. Read them with me. Statement one: the boiling point of a solution of a non-volatile solute, one that does not evaporate, is higher than that of the solvent. Statement two: the solution has a lower vapour pressure at every temperature. The method is on screen: is one true? Is two true? Does two explain one? The graph will help. Okay? Look at the graph: p, the vapour pressure, up the side; T, the temperature, along the bottom. Dark blue, labelled solvent, is the pure solvent; green, below it, is the solution. Statement two first. Is the solution's vapour pressure really lower? Think of Raoult's law. Yes, it is! The solute adds nothing to the vapour, so p equals x A times p A nought. Here x A, with a small A below, is the solvent's mole fraction; p A nought, with the small circle, is the pure solvent's vapour pressure. Since x A is less than one, p is lower at every temperature: statement two is true. Right? Now statement one. Any liquid boils when its vapour pressure reaches one atmosphere: the dashed line marked one atm. So, which curve gets there first? The dark blue one, the solvent, at T b nought, with a small b below. The green curve is lower, so it gets there later, at T b. So T b is greater than T b nought; the orange arrow between them is delta T b; delta, the little triangle, means change, so this is the boiling point elevation. On the left, by the grey curve labelled solid, delta T f, from T f to T f nought, is the freezing point version. Clear? So statement one is true too, and the lower vapour pressure is exactly why. Both true, and two is the reason for one. That's it!

2:38Isotonic: compare i times C, not just C

Next, an assertion and reason. Same three checks. Assertion: zero point one molar glucose and zero point one molar urea are isotonic, that is, equal in osmotic pressure; capital M means molar. Reason: both are non-electrolytes, so they dissolve as whole molecules, not as ions. Okay? Osmotic pressure is pi, the Greek letter. For a dilute solution, pi equals i CRT, where i is the van 't Hoff factor, C the molar concentration, R the gas constant, and T the temperature. With me? Same temperature, so R T is common, and isotonic just means equal i times C, okay? Now tell me, what is i for glucose and urea? One, for both! No dissociation, no association. So i C is zero point one for each, and the assertion is true. And being non-electrolytes is exactly what makes i equal to one, so the reason explains the assertion. Got it? Now, a trap. Zero point one molar sodium chloride has the same molarity. So it is isotonic with the glucose too, right? No, no, careful! Sodium chloride splits completely into two ions, so i is two, and i C is zero point two: double the osmotic pressure. Not isotonic! Always compare i times C, not just the molarity. Clear?

4:07Back to the cucumber and the egg

Now, back to our opening puzzle. First, osmosis, in the figure. The dotted orange wall is a semipermeable membrane: solvent can pass, solute cannot. The arrow shows solvent flowing into the solution, the more concentrated side. The solution rises by a height h, the double arrow, until h rho g equals pi, the osmotic pressure; rho, the Greek letter, is the density, and g is the acceleration due to gravity. Okay? Two words. Hypertonic means the higher osmotic pressure, pi; hypotonic, the lower. Water moves from the hypotonic side into the hypertonic side. Right? So, a cucumber in salt water, called brine. Compared with its cells, is brine hypertonic or hypotonic? Which way does the water flow? Hypertonic, exactly! So water leaves the cucumber's cells, and the cucumber shrinks. With me? And the egg? With the shell dissolved, only its membrane is left. Pure water is hypotonic compared with the egg contents, so water enters through the membrane, and the egg swells. Puzzle solved! Got it?

5:17Example 1: molality vs molarity, assertion and reason

Now, example one. Assertion: molality is preferred to molarity in colligative property measurements. Reason: molality does not change with temperature. Pause the video, and try it first. Done? Okay, let's check. Molarity, capital M, is n with a small B, the moles of solute, over V, the solution's volume in litres, the L in brackets. Heat the solution, it expands, and the molarity changes. Okay? Now, molality. Does it change when you heat the solution? No, it does not! Molality, small m, is n B over the mass of the solvent in kilograms, the k g in brackets. Masses do not change with temperature, so the reason is true. And colligative measurements, like boiling and freezing points, involve heating or cooling, so we want a concentration that stays fixed: molality. Both true, and the reason explains the assertion. Very good!

6:20Example 2: true or false on gases, KH, entropy, azeotropes

Now a harder one: true or false, four parts. Part A: gases become more soluble in water on heating. Part B: a larger Henry's constant means lower solubility. Part C: an ideal solution has delta mix S equal to zero, with mix written small below the delta: the entropy change on mixing. Part D: negative deviation gives a maximum boiling azeotrope. Pause and try. Hint: one idea for each part. Done? Alright, part A first. Many solids dissolve more in hot water. So gases do too, right? No, no, careful! Dissolving a gas is exothermic, it gives out heat, so heating drives the gas back out. Solubility falls, so part A is false. Clear? Now part B, Henry's law, in the badge: p equals K H times x. Here p is the gas's partial pressure; K H, with a small H below, is Henry's law constant; and x is the gas's mole fraction in the solution. At the same pressure, does a larger K H mean more gas, or less? Less, exactly! Fixed p, larger K H, smaller x: lower solubility, so part B is true. This holds at constant temperature, for a gas that does not react with the solvent. Okay? Part C, the ideal solution. Mixing gives out no heat and causes no volume change. So no entropy change either, right? No! Delta mix H and delta mix V are both zero, but delta mix S is greater than zero. Mixed molecules have more ways of arranging themselves, so the entropy goes up; that drives the mixing. So part C is false. Part D, negative deviation: the A dash B attractions, between the two different molecules, are stronger than in the pure liquids. So molecules escape less easily: lower vapour pressure, higher boiling point, with a maximum at the azeotrope. So part D is true. So the answer: false, true, false, true. One idea for each part. With me?

8:33Quick revision: two-statement checks, isotonic, osmosis

So, quick revision. One: for two statements, or assertion and reason, check: is the first true, is the second true, and does the second explain the first? Two: isotonic means equal i times C, not just equal concentration. Three: water flows towards the more concentrated side, the hypertonic one. Now go and try the practice problems on this page. You can do them. All the best!