Potash Alum

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Take a glass of muddy water and stir in a little of a colourless crystal. The mud settles, and the water clears. That crystal is potash alum. Now a second puzzle. One book writes its molar mass as nine hundred and forty eight, and another book writes four hundred and seventy four. So which book is wrong? We will settle it today. The preparation of potash alum is part of the JEE practical chemistry syllabus. So, let's begin.

0:34What potash alum is

So, what exactly is potash alum? It is potassium sulphate, K two S O four, joined to aluminium sulphate and to twenty four molecules of water. The small numbers written low count the atoms, the brackets around S O four with a three outside mean three sulphate groups, and the raised dot means these parts are held together in one crystal. Okay? Now, potash alum is one member of a whole family, and every alum follows this general formula. The letter M takes the place of potassium, and M prime, the M with the little dash, takes the place of aluminium. The twenty four waters stay. Right? Each sulphate ion carries a charge of two minus. So what charge must the ion of M carry, and what charge must the ion of M prime carry? Yes, exactly, two ions of M balance one sulphate, so M plus is univalent, with a charge of one plus. Two ions of M prime balance three sulphates, so M prime three plus is trivalent. The small signs written high are the charges. Clear?

1:38Preparing potash alum in the lab

Now, how do we make it in the lab? Here is the equation, and the arrow means gives. Potassium sulphate, plus aluminium sulphate with eighteen waters, which we call the octadecahydrate, plus six more waters, gives potash alum. Count the waters: eighteen plus six gives twenty four. With me? We take equimolar amounts, the same number of moles of each salt. Capital M on screen is the molar mass: one hundred and seventy four for potassium sulphate, and six hundred and sixty six for aluminium sulphate octadecahydrate. Okay? Each salt is dissolved separately, in the minimum of hot water. Remember both words, separately and minimum. Got it? But look at the aluminium sulphate. It is dissolved with a little dilute sulphuric acid, H two S O four. Now, why would we add acid here? The acid prevents hydrolysis of the aluminium sulphate. Without it, as the bracket says, the solution turns turbid with aluminium hydroxide. The acid suppresses that reaction. Right? Then the two solutions are mixed, concentrated and cooled, and crystals of potash alum separate out. Clear?

2:49Two ways to write one formula

Now the crystals themselves. Potash alum forms colourless, octahedral crystals. And the full formula we just prepared has molar mass nine hundred and forty eight. Okay? But many books write it like the next line: one potassium, one aluminium, two sulphate groups and twelve waters, with molar mass four hundred and seventy four. So, our puzzle from the start: which book is wrong? Both books are right, and here is why. Halve every count in the first formula: two potassiums become one, two aluminiums become one, four sulphates become two, and twenty four waters become twelve. And nine hundred and forty eight over two gives four hundred and seventy four. Same compound, half the formula unit. With me?

3:37Why an alum solution is acidic

Now, dissolve some alum in water. Alum is just a salt, so the solution should be neutral, right? That is the trap, so be careful here. The solution is acidic, because the aluminium ion is hydrolysed. On screen it is written A l with a small three plus, the charge, written high. Okay? Here is the hydrolysis. The aluminium ion reacts with three molecules of water to give aluminium hydroxide and three hydrogen ions, H plus. The pair of half arrows means the reaction is reversible. Those hydrogen ions make it acidic, and this is the hydrolysis the dilute acid suppressed in the preparation. Got it?

4:20Uses, and burnt alum

Now the uses. Alum coagulates suspended mud in water, so the mud settles, and that was our muddy glass from the start. It also stops bleeding, and it is used as a mordant in dyeing. Right? But why does the mud settle at all? Think about the charge on the aluminium ion. Yes, exactly, it is all about charge. Clay particles in muddy water are negatively charged colloid particles. The highly charged aluminium ions neutralise that charge, so the particles coagulate and settle. Clear? And finally, heat the crystals. Alum melts in its own water of crystallisation, the twenty four waters in its formula. With me? Then it swells into a porous mass, called burnt alum. Okay?

5:09Example 1: percentage of water of crystallisation in alum

Time for Example one. Find the percentage of water of crystallisation in potash alum. Pause the video and try it first. Done? Okay, let's check. Now, the aluminium sulphate we started with had eighteen waters. So we take eighteen waters for the alum, right? That would be wrong, and it is a common trap. The alum has twenty four waters: eighteen from the aluminium sulphate, and six more from the equation. Each water has molar mass eighteen, and the cross on screen means times. So twenty four times eighteen gives four hundred and thirty two. Okay? So the percentage of water is four hundred and thirty two over nine hundred and forty eight, then times a hundred. That gives forty five point six percent. So nearly half the mass of alum is water. Right? And what if you use the half formula? Twelve times eighteen gives two hundred and sixteen, and two hundred and sixteen over four hundred and seventy four, times a hundred, gives forty five point six percent again. Same compound, same answer. Clear?

6:18Example 2: mass of alum from 2.7 g of aluminium foil

Now a harder one. Alum is also made from aluminium foil. The foil is dissolved in hot KOH solution, potassium hydroxide, and then sulphuric acid is added. What mass of the half formula alum, with twelve waters, can two point seven grams of aluminium give? Pause the video and try it first. Here is a hint: count the aluminium atoms. Done? Okay, let's check. Two aluminium atoms, with two potassium hydroxide and six waters, give two units of the compound in square brackets, where each aluminium holds four O H groups, and hydrogen gas, H two, is formed. So every aluminium atom ends up in one unit of that compound. Okay? Then the sulphuric acid turns it into the alum, and this alum formula has one aluminium in each formula unit. So one mole of aluminium gives one mole of this alum. Right? Now the moles of aluminium, and the small n on screen means the number of moles. Twenty seven is the molar mass of aluminium, so two point seven over twenty seven gives zero point one mole. With me? Now, alum has molar mass nine hundred and forty eight, so zero point one times nine hundred and forty eight gives ninety four point eight grams, right? That is the trap, so careful here. The nine hundred and forty eight formula has two aluminium atoms, but the question asks for the half formula, with one aluminium, of molar mass four hundred and seventy four. So the mass is zero point one times four hundred and seventy four, which gives forty seven point four grams. Got it?

7:55Quick revision: formula, preparation, acidity, burnt alum

So, quick revision. Potash alum, with its twenty four waters, has molar mass nine hundred and forty eight, or four hundred and seventy four for the half formula. To prepare it, dissolve equimolar salts in hot water, with dilute sulphuric acid for the aluminium sulphate, then mix, concentrate and cool. Its solution is acidic because the aluminium ion is hydrolysed, and heating gives burnt alum. Now go and try the practice problems on this page. All the best!