Permanganate against Mohr's Salt

Transcript

0:00Introduction

Hello everyone! Let us begin with a small story from the lab. Yesterday, a student titrated permanganate against oxalic acid, and warmed the flask to sixty to seventy degrees Celsius, as the method says. Today it is Mohr's salt, and out of habit, the student warms the flask again. This time the titre, the volume of permanganate used, comes out too low. How can a little heat spoil a titration? We will answer that today. Redox titrations come up often in JEE practical chemistry, so let's begin.

0:34Mohr's salt and its reaction with permanganate

First, what sits in the flask? It is Mohr's salt, which is ferrous sulphate and ammonium sulphate together, with six water molecules. The raised dots join the parts, the small numbers written low count the atoms, and the bracket with a small two outside means two ammonium groups. It is pale green, with molar mass three hundred and ninety two. Okay? Permanganate oxidises its ferrous ion, F e two plus; F e is iron, and the two plus written high is the charge. So, one permanganate ion, M n O four minus, five ferrous ions and eight hydrogen ions, H plus, give one manganese two plus ion, five ferric ions, F e three plus, and four water molecules; the arrow means gives. Right? Now count the electrons. Each ferrous ion becomes a ferric ion. So tell me, how many electrons does each ferrous ion lose? Just one electron each, because iron goes from plus two to plus three; the small arrow means goes to. And manganese, M n, goes from plus seven to plus two, so it gains five electrons. With me? So one permanganate needs five ferrous ions to supply its five electrons. Potassium permanganate, K Mn O four, and the ferrous ion react in the mole ratio one to five; the colon is read as to. Clear?

2:02The formula, and a trap with oxalic acid

Now the titration formula, which links molarity and volume, okay? Permanganate gains five electrons, so many students divide the permanganate side by five. Where should the five really go? It goes under the iron side; dividing permanganate by five is the trap. One permanganate handles five ferrous ions, so its millimoles are one fifth of the iron's. So capital M, the molarity, times capital V, the volume, of permanganate equals molarity times volume of the iron solution, over five. The small labels name each solution, and F e is our Mohr's salt solution. Got it? The same electron count gives the equivalent mass. Each unit of Mohr's salt holds one ferrous ion, which loses one electron. So the equivalent mass is three hundred and ninety two over one, just the molar mass itself. Okay? Now a quick test. Take equal volumes of zero point one molar oxalic acid and zero point one molar Mohr's salt, and titrate both with the same permanganate. Same volume and same molarity, so the same titre, right? No, and that is exactly the trap: the oxalic acid needs twice as much permanganate. The reason is simple: oxalic acid loses two electrons per mole, while the ferrous ion loses only one. Clear?

3:28The set-up and the end point

Now the set-up, in the course figure: the burette above, and the conical flask below, on a white tile that shows the colour change clearly. The stop-cock lets us add drop by drop near the end point. Into the flask goes a pipetted volume of the Mohr's salt solution, made up in dilute sulphuric acid, H two S O four. With me? Then more dilute sulphuric acid is added to the flask. We will see why in a moment. Right? Permanganate, the titrant, runs in from the burette; for dark K Mn O four, the label says read the upper meniscus. The flask label says no indicator with permanganate, because permanganate is self-indicating. Clear? While ferrous ions remain, each drop loses its colour at once, and the solution stays colourless, really a very pale green. The first extra drop stays, and the solution turns a permanent pale pink; the arrow means changes to. That is the end point. Got it?

4:27Acid twice, and never warm the flask

Now, the acid goes in twice: once while making the Mohr's salt solution, and again in the flask. Why should it go in twice? While dissolving, the acid stops the hydrolysis of the ferrous ion, and also its oxidation. Okay? And in the flask, it supplies the hydrogen ions needed to reduce permanganate to manganese two plus. That is why our equation has eight H plus ions in it. Clear? Now back to our story from the start. For oxalic acid, we warmed the flask. So here also, should we warm it? No, there is no need, because with ferrous ions the reaction is already fast at room temperature. Right? And heating actually harms it. In a hot solution, air oxidises some ferrous ions, so less is left for the permanganate, and the titre comes out low. That is what happened to our student. Okay? Now just before the pink appears, the solution often looks yellowish, because the ferric ions formed are yellow. Adding a little phosphoric acid complexes them and sharpens the end point. With me?

5:35Example 1: KMnO4 volume for 25 mL of 0.1 M Mohr's salt

Alright, example one. What volume of zero point zero two molar K Mn O four is needed for twenty five point zero millilitres of zero point one molar Mohr's salt? Here m L is millilitres, and capital M means molar, moles per litre. Pause the video and try it first. Done? Okay, let's check. Millilitres times molarity gives millimoles, the m mol on screen; the cross means times. Each unit of Mohr's salt gives one ferrous ion, so twenty five point zero times zero point one gives two point five millimoles. Right? Now, how much permanganate do two point five millimoles of ferrous ions need? One fifth of it, or five times it? One fifth, because one permanganate deals with five ferrous ions. One fifth times two point five gives zero point five millimoles of permanganate. Clear? So the volume, capital V, is zero point five over zero point zero two, which gives twenty five point zero millilitres. Okay?

6:44Example 2: percentage purity of a Mohr's salt sample

Now a harder one. Four point zero grams of impure Mohr's salt is dissolved to make one hundred millilitres. Ten point zero millilitres of it needs nine point six millilitres of zero point zero two molar permanganate. Find the percentage purity. Pause the video and try it first. Here is a hint: watch the volumes. Done? Okay, let's check. Permanganate used: nine point six times zero point zero two, which gives zero point one nine two millimoles. Iron needs five times that, which gives zero point nine six millimoles of ferrous ions. Right? Now be careful. Is zero point nine six millimoles all the iron in our four point zero grams? No, and this is the trap. We titrated only ten millilitres out of one hundred, just a tenth. So the whole sample holds zero point nine six times ten, which is nine point six millimoles. Clear? Millimoles times molar mass gives milligrams, the m g on screen; the wavy equals sign means about. Nine point six times three hundred and ninety two gives about three thousand seven hundred and sixty three milligrams, or three point seven six grams. With me? So the purity is three point seven six over four point zero, times one hundred, which gives ninety four per cent; the last symbol is the per cent sign. Millimoles all the way, and it is done. Okay?

8:15Quick revision: 1 to 5 ratio, acid, no warming, end point

So, quick revision. Permanganate and the ferrous ion react one to five, so the five goes under the iron side, and Mohr's salt has equivalent mass three hundred and ninety two. Dilute sulphuric acid goes into the solution and the flask, and we titrate at room temperature, never warm. We need no indicator, and the colour goes from colourless, really very pale green, to a permanent pale pink. Now go and try the practice problems on this page. All the best!