IIT-JEE Questions: Preparations

Transcript

0:00Introduction

Hello everyone! Let us start with a small lab puzzle. You are handed some light bluish green crystals with no label. You run three tests and see three colours: brown, then blue, then white. From those three colours alone, can you write the full formula of the salt? Sounds like detective work, right? Today we solve two past IITJEE questions on the compounds of this chapter, the first with colour tests and the second with grams and moles. Questions like these come up often in practical chemistry, so let's begin.

0:35Double salts: one crystal, many ions

So, first, here is one idea that both questions need: the double salt. It is a crystalline compound that contains two simple salts in a fixed mole ratio. Okay? And it exists only in the solid state, as the crystal. Right? So let me ask you this. What happens to a double salt when you dissolve it in water? Yes, exactly, it breaks up completely into the ions of both salts. Clear? So it gives the tests of every ion present. This line is the key to our first question: each ion gives its own test, and together the tests name the salt. Got it?

1:16Two double salts from this chapter

Now, two double salts prepared in this chapter. First, Mohr's salt, ferrous ammonium sulphate: ferrous sulphate and ammonium sulphate joined by raised dots, with six water molecules. The small numbers written low count the atoms and groups, and the brackets hold the ammonium group. It forms pale green crystals. With me? In water it gives three ions: iron two plus, ammonium, N H four plus, and sulphate, S O four two minus. The small signs written high are the charges. Okay? The second family is the alums. The general formula is the sulphate of a metal M, the sulphate of a second metal M prime, and twenty four water molecules. The little dash after the second M is read as prime. Right? Now, M plus means that M is univalent, and M prime three plus means that M prime is trivalent. Clear? So, a quick one for you. In potash alum, which metal is M, and which one is M prime? Yes, exactly, potassium is M and aluminium is M prime. Potash alum forms colourless crystals. Remember this general formula, because our second question needs it. Got it?

2:28Three tests that name the ions

Next, the three tests. First, Nessler's reagent, which is alkaline K two H g I four: two potassium ions outside the square brackets, and mercury with four iodides inside them. With ammonium, it gives a brown precipitate. With me? Now, suppose potassium ferricyanide gives a blue colour. Ferri sounds like iron three, so the blue must mean iron three plus, right? That is the trap, so be careful here. Potassium ferricyanide, K three with iron and six cyanide groups, C N, inside the brackets, gives deep blue with iron two plus. Okay? And its partner, potassium ferrocyanide, gives Prussian blue with iron three plus. So remember the pair: ferricyanide with iron two plus, ferrocyanide with iron three plus. Right? The last test is for sulphate. Barium chloride, B a C l two, in a solution made in hydrochloric acid, H C l, gives white barium sulphate, B a S O four, which does not dissolve in the acid.

3:29Example 1: identify Mohr's salt from three ion tests

Clear? Now, Example one, from nineteen ninety two, which is our opening puzzle. Some light bluish green crystals are tested, and the small Roman numerals on screen number three tests: a brown precipitate or colour with alkaline K two H g I four, a blue colour with potassium ferricyanide, and a white precipitate with barium chloride, from the solution in hydrochloric acid. Identify the ions, and suggest the formula. Pause the video and try it first. Done? Okay, let's check. Alkaline K two H g I four is Nessler's reagent, so the brown precipitate shows ammonium, N H four plus. Okay? Blue with ferricyanide means iron two plus, just as we saw in the trap. Right? The white precipitate with barium chloride is barium sulphate, so the anion is sulphate. With me? So, three ions, all from one crystal. Which compound from this chapter gives exactly these three? Yes, exactly, it is the double salt, Mohr's salt. In water it gives exactly these three ions, and its pale green crystals fit the colour in the question. Clear? So the formula is ferrous sulphate and ammonium sulphate, with six water molecules. Three colours, and our puzzle is solved. Got it?

4:48Example 2: find KO2, K2SO4 and potash alum from masses

Now a harder one, Example two, from nineteen ninety four. Compound A is a binary compound of a univalent metal. One point four two two grams of it reacts completely with zero point three two one grams of sulphur in an evacuated sealed tube, giving one point seven four three grams of a white solid B, which forms a hydrated double salt C with aluminium sulphate. Identify A, B and C from these masses. Pause the video and try it first. Here is a hint: start with the sulphur. Done? Okay, let's check. At thirty two point one grams per mole, zero point three two one over thirty two point one gives zero point zero one moles of sulphur, S, and the slash means over. Solid B is the only product, so all this sulphur is in B, okay? Now the big clue: B forms a double salt with aluminium sulphate. Which family of double salts does that remind you of? Yes, exactly, it is the alum family. So B is M two S O four, with one sulphur in each formula unit, which makes zero point zero one moles of B in its one point seven four three grams. One point seven four three over zero point zero one gives one hundred and seventy four point three grams per mole, which is what g slash mol means. With me? Now find the metal. Sulphate is thirty two point one plus sixty four, which is ninety six point one, and the cross means times. Two times thirty nine point one, the molar mass of potassium, plus ninety six point one, gives one hundred and seventy four point three. So the metal is potassium, and B is K two S O four. Clear? Each K two S O four holds two potassiums and four oxygens, so B holds zero point zero two moles of potassium and zero point zero four moles of oxygen. Sulphur brought no oxygen, and the evacuated tube let in no air, so all of it came from compound A, okay? So compound A has only potassium and oxygen, and potassium is univalent. So it must be K two O, potassium oxide, right? That is the trap, so be careful here. The moles say two oxygens for every potassium. And one point four two two over zero point zero two gives seventy one point one, which is thirty nine point one plus two times sixteen. So compound A is K O two, potassium superoxide. Got it? The reaction is two K O two plus sulphur giving K two S O four, and the arrow means gives. And C, the double salt of potassium sulphate with aluminium sulphate, is potash alum. See how the moles did all the work?

7:30Quick revision: double salt tests, ferricyanide trap, moles

So, quick revision. First, a double salt gives the test of every ion it contains, and that is how three tests named Mohr's salt. Second, ferricyanide gives blue with iron two plus, and ferrocyanide gives Prussian blue with iron three plus. Third, in mass problems, find the moles of the element you know first, then use the mole ratio. Now go and try the practice problems on the Mohr's salt and potash alum pages of this chapter. All the best!