Carbonates and Mixtures

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Two students titrate the same sodium carbonate solution with the same hydrochloric acid. The first uses phenolphthalein. The second uses methyl orange, and she needs much more acid. So who made the mistake? Maybe nobody, right? Today we see why carbonate gives two end points, and how they tell us what is inside a mixture. This comes up often in JEE practical chemistry, so let's begin.

0:30Carbonate is neutralised in two stages

So, here is our set-up: sodium carbonate, N a two C O three, in the flask, and hydrochloric acid, H C l, from the burette; the small numbers are subscripts. The carbonate ion has a charge of two minus, so it needs two H plus ions. So tell me, does it take both at once? No, it takes them one at a time, in two stages. In stage one, the carbonate ion, C O three with a two minus up top, picks up one H plus, and the arrow means it gives the bicarbonate ion, HCO three minus. This is over near p H eight point three; the brackets just hold the p H, and the wavy equals sign means approximately. With me? Then in stage two, the bicarbonate ion takes the second H plus, and gives water, H two O, and carbon dioxide, C O two. This is complete near p H four. Okay? Now, stage one ends near p H eight point three. So which of our two indicators, phenolphthalein or methyl orange, tells us that stage one is over? Yes, exactly, it is phenolphthalein. Its range, in the brackets, is eight point three to ten point zero; the dash means to. It is pink in the alkaline solution and turns colourless right at the end of stage one. Clear? And methyl orange changes between three point one and four point four, so only at the end of stage two, near p H four. That is the complete neutralisation. Got it?

2:04What each end point tells you

So what does each end point measure? Suppose the flask also has sodium hydroxide. By the end of stage one, all the hydroxide is neutralised, but each carbonate ion has taken only one H plus. So the phenolphthalein volume shows all the hydroxide plus half the carbonate. Right? And methyl orange waits until the very end, so its volume shows the complete neutralisation. Okay? Now here is a quick check. If the flask has only sodium hydroxide, and we titrate once with each indicator, how will the two titres, the volumes of acid, compare? They come out exactly equal, because hydroxide has no second stage, so both indicators change after the same volume of acid. Clear? And what about sodium carbonate alone? Each stage needs one H plus per carbonate ion, so the same amount of acid. So the phenolphthalein titre is half the methyl orange titre; the one over two means half. This is the key to our first example. With me?

3:07Mixtures: reading V1 and V2

Now let us look at mixtures. First we titrate with phenolphthalein, and V one, a capital V with a small one below it, is the volume of acid up to its end point. Okay? Then we carry on with methyl orange, and V two is the further volume, only the extra acid from the first end point to the second. Watch that word, further: taking V two as the total from zero is a classic trap. Right? Now take sodium hydroxide with sodium carbonate. In stage two, only the bicarbonate made from the carbonate is still reacting. So which volume measures the carbonate? Yes, exactly, carbonate corresponds to V two; that three-line equals sign means corresponds to. And V one held the hydroxide plus the carbonate's first stage, which is again V two. So hydroxide corresponds to V one minus V two. With me? Now sodium carbonate with sodium bicarbonate, N a HCO three. Phenolphthalein sees only the carbonate's first stage, so carbonate corresponds to V one. In stage two, the bicarbonate made from that carbonate uses V one again, and the original bicarbonate uses the rest. So bicarbonate corresponds to V two minus V one. Clear? Here is a topper's trick. Hydroxide is V one minus V two, so it needs V one greater than V two; that is the sign on screen. Bicarbonate is V two minus V one, so it needs V two greater. So the two volumes tell you which mixture you have. Got it?

4:38Example 1: methyl orange titre and g/L strength of Na2CO3

Alright, example one. Twenty five point zero millilitres of sodium carbonate solution needs twelve point zero millilitres of zero point one molar hydrochloric acid with phenolphthalein; capital M means molar. What volume would be needed with methyl orange, and what is the strength of the solution? Pause the video and try it first. Done? Okay, let's check. Same solution, same acid, so methyl orange also needs twelve point zero millilitres, right? No, that is exactly the trap from our puzzle. Phenolphthalein stops after stage one, but methyl orange waits for stage two as well, which needs the same acid again. So it is two times twelve point zero, which gives twenty four point zero millilitres. Twice as much, so nobody made a mistake. Okay? Millilitres times molarity gives millimoles, the m mol on screen. Twelve point zero times zero point one gives one point two millimoles of acid in stage one. Each carbonate ion took one H plus here, so, as the double arrow says, there are one point two millimoles of sodium carbonate. With me? These are in twenty five point zero millilitres of solution. One point two over twenty five gives zero point zero four eight molar. Right? Strength in grams per litre is molarity times molar mass, and the molar mass of sodium carbonate is one hundred and six. Zero point zero four eight times one hundred and six gives about five point one grams per litre; the litre to the power minus one on screen just means per litre. Got it?

6:21Example 2: molarity of NaOH and Na2CO3 in a mixture

Now a harder one, with a real mixture. Twenty five point zero millilitres of a mixture of sodium hydroxide and sodium carbonate needs twenty point zero millilitres of zero point one molar hydrochloric acid with phenolphthalein, and a further five point zero millilitres with methyl orange. Find the molarity of each. Pause the video and try it first. Hint: which volume is the carbonate? Done? Okay, let's check. Here V one is twenty point zero millilitres, and V two, the further volume, is five point zero. V one is greater, which fits hydroxide with carbonate. Right? Carbonate corresponds to V two. Five point zero times zero point one gives zero point five millimoles of acid in stage two, so there are zero point five millimoles of carbonate. Okay? Now the phenolphthalein volume. Twenty point zero times zero point one gives two point zero millimoles of acid. So the sodium hydroxide is two point zero millimoles, right? No, that would be the trap, so be careful here: phenolphthalein also counted half the carbonate. Stage one used zero point five millimoles of acid on the carbonate, so the hydroxide is two point zero minus zero point five, which gives one point five millimoles. Clear? Both are in twenty five point zero millilitres. Zero point five over twenty five gives zero point zero two molar sodium carbonate, and one point five over twenty five gives zero point zero six molar sodium hydroxide.

7:56Quick revision: two stages, V1 and V2 for each mixture

Got it? So, quick revision. Carbonate takes acid in two stages: phenolphthalein sees one, methyl orange both. V one is the volume to phenolphthalein, and V two is the further volume to methyl orange. For sodium hydroxide with carbonate, it is V two and V one minus V two; for carbonate with bicarbonate, V one and V two minus V one. Now go and try the practice problems on this page. All the best!