Part C: Concept Checks

Transcript

0:00Introduction

Hello everyone! Okay, one quick question before we start. One mole of any gas occupies twenty two point four litres. You have written this a hundred times, right? Now, a normal day: twenty five degrees Celsius, one atmosphere pressure. Does one mole of gas still fill twenty two point four litres? True or false? Most students say true, without thinking. Hold your answer; example one will settle it. Today is a quick concept check class. And look, in JEE, these short questions are where good students lose easy marks. So, let's begin.

0:40Assertion–Reason: ask three questions

So, first, assertion and reason. Here is the rule: for every such question, ask three things, in order. One: is the assertion true? Two: is the reason true? And three: does the reason explain the assertion? Clear? Let's try one. Assertion: one mole of hydrogen and one mole of oxygen, at the same temperature and pressure, occupy equal volumes. The capital A on screen stands for assertion, and T and P are temperature and pressure, okay? Reason, the R on screen: equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules. Now you tell me: are both true, and does the reason explain it? Think, think. Both true, yes! One mole of each gas means the same number of molecules, so the same volume. And the reason is Avogadro's law itself, so it explains the assertion. Absolutely correct.

1:38Assertion–Reason: one mole of electrons

Now a harder one. Assertion: one mole of electrons weighs about zero point five five milligram. Reason: the mass of one electron, m e on screen, is nine point one times ten to the power minus thirty one kilogram. So how do you go from one electron to one mole? Multiply by Avogadro's number, exactly! Nine point one zero nine times ten to the minus thirty one, times six point zero two two times ten to the twenty three. That is five point four nine times ten to the minus seven kilogram, which is zero point five four nine milligram. About zero point five five. See? The assertion is correct! So both are true. And the reason, together with Avogadro's number, the N A on screen, gives the assertion. So the answer: both true, and the reason explains. Got it?

2:30Molar volume: why 22.7 L, not 22.4 L?

Next, the molar volume. For years, standard pressure was one atmosphere, that is one hundred and one point three two five kilopascal, at zero degrees Celsius. There, one mole of an ideal gas occupies twenty two point four litres. But IUPAC changed the standard pressure to one bar, which is one hundred kilopascal. Slightly lower than before, right? So tell me: will one mole now take a slightly bigger volume, or a smaller one? Bigger, exactly! At slightly lower pressure, a mole occupies a slightly larger volume. Check it: the volume of one mole is R T by P, where R is the gas constant and T is the temperature in kelvin, okay? Eight point three one four, times two seventy three point one five, divided by ten to the power five pascal. Zero point zero two two seven cubic metre, which is twenty two point seven litres. Perfect match! So, topper's tip: read the molar volume from the question. Some papers use twenty two point four, some twenty two point seven. And notice, both are at zero degrees. What about twenty five degrees? Example one will tell us.

3:39Empirical formula: reduce, then compare

Now, empirical formulas. Five compounds on screen: glucose, acetic acid, formaldehyde, ethanol and methyl formate. Which of these have the empirical formula C H two O? Let's go one by one. The empirical formula is the simplest whole number ratio of atoms. Glucose is C six H twelve O six. Divide everything by six: C H two O, done. Acetic acid is C two H four O two. Divide by two: C H two O again. And methyl formate? Also C two H four O two, so also C H two O, okay? Two different compounds, same formula. Interesting, right? Formaldehyde is C H two O itself, nothing to divide. So here the empirical and the molecular formula are the same. Remember this; it comes back in example one. And ethanol, C two H six O, okay? Careful: only one oxygen atom, so you cannot divide by two. Not C H two O! So the answer: all except ethanol.

4:43Which concentrations change with temperature?

One more check. Molarity, molality, mole fraction, normality, mass percent, and PPM, parts per million, by mass. Which of these change when the temperature changes? Look, the trick is in the definition. Heat a solution, and its volume changes, but its mass does not. So only the terms defined with the volume of the solution can change. The double arrow on screen simply means: so, it changes with temperature. Molarity and normality both use litres of solution, so these two change. Molality, mole fraction, mass percent and PPM use only masses or moles, so these four stay the same. Clear?

5:26Example 1: true or false on 22.4 L and limiting reagent

Now, example one, and our opening question is right here. True or false. One: one mole of any gas occupies twenty two point four litres at twenty five degrees Celsius and one atmosphere. Two: a compound's empirical and molecular formulas can be the same. Three: the limiting reagent is always the reactant present in smaller mass. Pause the video, and try it first. Done? Okay, let's check. Statement one. Twenty two point four litres is the volume at zero degrees, remember? At twenty five degrees, one mole occupies about twenty four point five litres. So statement one is false. Our opening puzzle, solved! Statement two. Can the empirical and molecular formulas be the same? Of course! Formaldehyde already showed us that. Water, H two O, and carbon dioxide, C O two, are two more. They cannot be reduced further. So, true. Now statement three, the real trap. Take ten grams of hydrogen and sixty four grams of oxygen. Hydrogen is the smaller mass, so hydrogen limits, right? No, no, careful! Always think in moles. Ten grams of hydrogen is five moles, and sixty four grams of oxygen is two moles. Now, two H two plus O two gives two H two O, so divide the moles by the coefficients. Hydrogen: five by two, that is two point five. Oxygen: two by one, just two. The smaller number limits. So oxygen, the heavier one, runs out first! So statement three is false, and the answer is: false, true, false. Never compare masses; always divide moles by the coefficients. Got it?

7:18Example 2: fractional n-factor of Fe0.95O

Now a harder one. Can an n-factor be a fraction? Find the n-factor of F E zero point nine five O, when it is oxidised to F E two O three. Pause the video and try it. Hint: find the total charge on all the iron first. Done? Okay, let's check. Many students say: iron goes from plus two to plus three, that is one electron, so the n-factor is one. No, no, careful! This formula has only zero point nine five iron atoms for each oxygen, okay? So, start with the charges. Oxygen is minus two, so all the iron together carries plus two: the total iron charge. In F E two O three, each iron is plus three. So our zero point nine five iron atoms end up with zero point nine five times three: two point eight five. So, electrons lost per formula unit: two point eight five, minus two. Zero point eight five! Right? So the n-factor is zero point eight five. Yes, an n-factor can be a fraction. Always work with the total charge. With me?

8:30Quick revision: assertion-reason, molar volume, n-factor

So, quick revision. One: for assertion and reason, ask: is the assertion true, is the reason true, and does the reason explain it? Two: twenty two point four litres at one atmosphere, twenty two point seven at one bar, both at zero degrees; and about twenty four point five at twenty five degrees. Three: molarity and normality change with temperature, and an n-factor can be a fraction. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!