Set 2

Transcript

0:00Introduction

Hello everyone! Let us begin with a strip of ten iron tablets. Each tablet contains ferrous sulphate, and here is the question: how many milligrams of iron are there in one tablet? You cannot see the iron, and you cannot pick it out and weigh it. So how will you find it? It looks tricky, right? By the end of this video, we will find it to the milligram, with just a burette and some potassium permanganate solution. This problem is from Set two of our mixed problems, and titration questions like this come up often in JEE Main. So, let's begin.

0:36How to use Set 2

First, what is Set two? It has fifteen JEE Main style problems, from all the chapters of this unit, and you give yourself twenty five minutes for the whole set. Okay? Time yourself first, and only then check the solutions. That way you practise against the clock, as in the real exam. Right? Today, we will solve two titration problems from this set together, an easier one and then a harder one. But first, the tools we need. With me?

1:04Tool 1: count in millimoles

So, tool one: we count in millimoles. Capital M is the molarity, in moles per litre, and capital V is the volume in millilitres, the m L in brackets. Molarity times volume gives millimoles, written m mol; the cross means times. Okay? Now the balanced equation. Small a moles of substance capital A react with small b moles of substance capital B, and the arrow means gives the products. Small a and small b are just the numbers written in front, in the balanced equation. Right? Now let me ask you this. At the equivalence point, are the millimoles of the two substances always equal? No, only when small a equals small b. In general, divide the millimoles of each substance by its own number. So, molarity times volume of capital A, over small a, equals molarity times volume of capital B, over small b. The capital A and capital B written low say which solution, and each long line is a fraction bar. Clear? And one more line for later. Millimoles times molar mass gives milligrams, written m g. Keep this ready for our iron tablets. Got it?

2:21Tool 2: choose the right indicator

Tool two is the indicator, and the course gives us two of them. Methyl orange is red in acid and yellow in alkali. Okay? And phenolphthalein is colourless in acid and pink in alkali. With me? Now, oxalic acid is a weak acid, and sodium hydroxide, N a O H, is a strong base. So which of these two indicators will you choose? Yes, exactly, we choose phenolphthalein. For a weak acid with a strong base, the equivalence point lies above p H seven, in the alkaline region, and that is where phenolphthalein changes colour. Clear?

2:58Tool 3: permanganate needs no indicator

Tool three is potassium permanganate, K Mn O four, with ferrous ions. On screen, one permanganate ion, M n O four minus, five ferrous ions, F e two plus, and eight hydrogen ions, H plus, give one manganese two plus ion, five ferric ions, F e three plus, and four water molecules. The raised signs are the charges, and the small numbers written low count the atoms. Okay? So tell me, one millimole of permanganate oxidises how many millimoles of ferrous ion? Very good, it is five millimoles. Read it straight from the equation: one permanganate, five ferrous ions. Keep this five in mind, because it is the trap in our second example. Right? Now look at the course figure: the burette above, and the conical flask below, on a white tile. The flask label says no indicator with K Mn O four, because permanganate is a self-indicator. At the end point, the solution in the flask turns a permanent pale pink. Clear? For oxalic acid with permanganate, the flask is warmed. So with ferrous ions also, should we warm it? No, there is no need. We titrate in dilute sulphuric acid, H two S O four, at room temperature. Got it? The reason is that heating would let the air oxidise some of the ferrous ions, and then our answer would come out wrong. With me?

4:28Example 1: molarity of NaOH from oxalic acid titration

Now Example one. Twenty five point zero millilitres of zero point zero five molar oxalic acid needs twenty point zero millilitres of a sodium hydroxide solution, with phenolphthalein. Find the molarity of the sodium hydroxide. Here, capital M after a number means molar. Pause the video and try it first. Done? Okay, let's check. Volume times molarity: twenty five point zero times zero point zero five gives one point two five millimoles of oxalic acid. Okay? Twenty millilitres of alkali neutralise this. So the molarity is one point two five over twenty point zero, which gives zero point zero six two five molar, right? Oh no, careful, that is exactly the trap, because each oxalic acid molecule gives two H plus ions. So two times one point two five gives two point five millimoles of H plus, and the alkali must neutralise all of them. Got it? So the sodium hydroxide supplies two point five millimoles in twenty point zero millilitres. Two point five over twenty point zero gives zero point one two five molar; the small N a O H written low says whose molarity it is. And see, phenolphthalein was the right choice here: weak acid, strong base. Very good if you got that!

5:53Example 2: mg of iron in one tablet by KMnO4 titration

And now Example two, our puzzle from the start. Ten iron tablets containing ferrous sulphate are dissolved in dilute sulphuric acid and made up to two hundred and fifty millilitres. Twenty five point zero millilitres of this needs twelve point five millilitres of zero point zero one molar K Mn O four. Find the mass of iron in one tablet. Pause the video and try it first. Here is a hint: watch the volumes. Done? Okay, let's check. Permanganate used: twelve point five times zero point zero one gives zero point one two five millimoles. Okay? Iron has molar mass fifty six. So the iron is zero point one two five times fifty six, which gives seven milligrams, right? No, wait, that misses the five, because one permanganate oxidises five ferrous ions. So five times zero point one two five gives zero point six two five millimoles of ferrous ion, in the twenty five millilitres we titrated. Clear? Now look at the volumes. We titrated only twenty five millilitres, out of two hundred and fifty. So what comes next? That was just one tenth of the solution, so all two hundred and fifty millilitres hold ten times as much, which is six point two five millimoles. And that came from ten tablets, so one tablet has six point two five over ten, which gives zero point six two five millimoles. See, the two factors of ten cancel here, but write both steps, so you never miss one. With me? Finally, millimoles times molar mass gives milligrams, and the bracket reminds you that iron, F e, has molar mass fifty six. Zero point six two five times fifty six gives thirty five milligrams. So each tablet holds thirty five milligrams of iron. Puzzle solved, with one burette of permanganate! Got it?

7:53Quick revision: millimoles, indicators, factor of five

So, quick revision. Millimoles equal molarity times volume in millilitres, and then use the ratio from the balanced equation. Oxalic acid gives two H plus ions, and one permanganate oxidises five ferrous ions. For a weak acid with a strong base, use phenolphthalein, and permanganate is its own indicator. Now set your timer for twenty five minutes, and try the problems of Set two on this page. All the best!