Nitrogen: Dumas' Method

Transcript

0:00Introduction

Hello everyone! Let me give you one small puzzle before we start. Two students each take zero point three grams of an organic compound, and each one collects fifty millilitres of nitrogen gas, okay? Same mass, same fifty millilitres. But one student reports twenty point eight percent nitrogen, and the other reports eighteen point seven percent. So, who made the mistake? Hmm, maybe nobody, right? By the end of this video you will see why fifty millilitres is not always fifty millilitres. And trust me, Dumas' method numericals come in JEE again and again, and they are easy marks once you know the two corrections. So, let's begin.

0:45Dumas' method: catching the nitrogen

So, here is the idea of Dumas' method. We take a weighed amount of the organic compound, and heat it with copper oxide, the CUO on screen, okay? And all this happens in an atmosphere of carbon dioxide, C O two. When it is heated, the nitrogen in the compound is set free as nitrogen gas, N two. Our whole job is to catch this N two and measure it. Clear? But wait. What if some of the nitrogen comes out as oxides of nitrogen, and not as N two? Then we would lose it, right? So how do we fix that? Yes, with a hot copper gauze! It reduces any oxides of nitrogen, so all that nitrogen reaches us as N two. Nothing is lost. Now look, the gas coming out is not pure nitrogen. There is the carbon dioxide we heated it in, and there is carbon dioxide from burning the compound itself. So how do we get rid of all that C O two? Exactly! We pass the gases into concentrated potassium hydroxide, KOH, okay? KOH absorbs all the C O two, so the gas left over is pure nitrogen. And here is a favourite one line question in exams: why concentrated KOH, and not plain water? Because KOH absorbs all the C O two, and only pure nitrogen is left. Got it? Finally, this nitrogen is collected, and its volume is measured. So in Dumas' method, the answer comes from a volume of gas, not from a mass. Remember that.

2:20From volume of N₂ to percentage of nitrogen

Alright, now the calculation. Let V nought be the volume of nitrogen reduced to STP, in millilitres, okay? That little zero below the V tells you: this is the STP volume. And let w be the mass of the compound in grams. But a percentage needs a mass, right? So how do we turn a volume of gas into a mass? Yes, through the mole! One mole of nitrogen gas occupies twenty two thousand four hundred millilitres at STP, that is twenty two point four litres, okay? And one mole of N two weighs twenty eight grams. Why twenty eight? Because each molecule is N two, two nitrogen atoms together. Clear? So, each millilitre of nitrogen at STP weighs twenty eight divided by twenty two thousand four hundred grams. Multiply by V nought, and you get the mass of N two. And all this nitrogen came from our compound, so this is exactly the mass of nitrogen in the sample. Now just divide by w and multiply by one hundred. Percentage of nitrogen equals twenty eight by twenty two thousand four hundred, times V nought by w, times one hundred. Write this down. But remember, V nought must be the STP volume. That is where most students lose marks.

3:39Collected over water: correct the volume first

Now, in many JEE questions, the nitrogen is collected over water, at some temperature T and some pressure P, okay? The pressure of the gas is P, so just use P directly, right? No, no, careful! This is the JEE trap. Look, gas collected over water is not only nitrogen. Water vapour is mixed in it, and that vapour also pushes. Its pressure is called the aqueous tension, p water. So the pressure of the nitrogen alone is P minus p water. Always subtract the aqueous tension first. Got it? Next, correct the volume to STP conditions. Here V is the volume you actually measured. V nought equals V, times p of N two by seven hundred sixty, times two hundred seventy three by T, okay? Seven hundred sixty millimetres of mercury is the standard pressure, and two hundred seventy three kelvin is the standard temperature, so T must also be in kelvin. And here is a topper's trick to catch silly mistakes. If the pressure is below seven hundred sixty, and the temperature is above two hundred seventy three, both fractions are less than one. So V nought must come out smaller than V, that is what the arrow on screen says, okay? If your STP volume comes out bigger, check your fractions; you have flipped one. Simple and quick.

5:00Example 1: percentage of nitrogen from 50 mL at STP

Right, first example. In a Dumas estimation, zero point three grams of a compound gives fifty millilitres of nitrogen at STP, okay? Find the percentage of nitrogen. Pause the video, and try it first. Done? Okay, let's check. First question: is any correction needed? No! The volume is already at STP, so V nought is simply fifty. Mass of N two equals fifty by twenty two thousand four hundred, times twenty eight. That is zero point zero six two five grams. Now divide by the mass of the compound, zero point three, and multiply by one hundred. Twenty point eight percent nitrogen. So that was our first student from the puzzle. Their fifty millilitres was already at STP, so his answer is perfectly right. That's it, done.

5:53Example 2: nitrogen over water, corrected to STP

Now the harder one, the real JEE pattern. Zero point three zero grams of a compound gives fifty millilitres of nitrogen, collected over water at two hundred ninety kelvin and seven hundred forty millimetres of mercury. The aqueous tension at two hundred ninety kelvin is seventeen millimetres of mercury. Find the percentage of nitrogen. Pause the video, and try it. Hint: two corrections first. Okay, some students see fifty millilitres and the same zero point three grams, and they think: same as example one, so twenty point eight percent again, right? No, no! Look, this gas was collected over water, at two hundred ninety kelvin, okay? That fifty millilitres is not an STP volume. So, correction number one, the pressure. Seven hundred forty, minus the aqueous tension seventeen, that is seven hundred twenty three millimetres. That is the pressure of the nitrogen alone. Correction number two, the STP volume. Fifty, times seven hundred twenty three by seven hundred sixty, times two hundred seventy three by two hundred ninety. Forty four point eight millilitres. Quick check: smaller than fifty? Yes, exactly as our topper's trick said. Now it is just like example one. Mass of N two equals forty four point eight by twenty two thousand four hundred, times twenty eight. Zero point zero five six zero grams. Divide by zero point three zero, and multiply by one hundred. Eighteen point seven percent. And there, our puzzle is solved! Nobody made a mistake. The second student's fifty millilitres was nitrogen over water at two hundred ninety kelvin, and at STP it shrinks to forty four point eight millilitres. Same fifty millilitres on paper, but less nitrogen. With me?

7:52Quick revision: Dumas steps, corrections and %N formula

So, quick revision. Heat the compound with copper oxide in carbon dioxide, let KOH absorb the C O two, and measure the nitrogen. If it is collected over water, subtract the aqueous tension, then correct the volume to STP, okay? And percentage of nitrogen is twenty eight by twenty two thousand four hundred, times V nought by w, times one hundred. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!