Okay, let’s begin today’s discussion. First of all, we have to discuss these questions which
yesterday I had given as homework. I will show you how to tackle these two questions. In
the last lecture we had discussed neutralization of HCl and NaOH. So what we will do, for
first question, we will mark the reaction HCl plus NaOH gives you what, NaCl plus water. Is
this reaction balanced? Yes, okay, now we will do one thing, you see here, 2.5ml of solution
of HCl, we will mark it here, it is neutralizing what – 4.5 ml of NaOH solution, additionally
given is the molarity of NaCl is how much – 3 molars, so we have to find the molarity of this.
So, we will take it as x molars. Okay, now see how we will solve it. We will say, we will find
their milli moles, molarity into volume with millilitre is what? Millimoles. We will say how
many milli moles it has. 2.5x milli moles and milli moles of NaOH will be 3 into 4.5 millimoles
and we know that 1 mole HCl reacts with 1 mole NaOH, so 1 millimole will react with, so
these many millimoles will react with these many millimoles for complete neutralisation. So,
you have a mark here 2.5 into x is equal to what, 3 into 4.5, now you can cancel this, now
forget the decimal, 5 into 5 and 5 into 9, so this is what x into 27 by 5 which is equal to what
5.4 m, everybody understood how to do this.
Second question is very easy. You had to produce 34 grams H2SO4. What is the mole of
H2SO4 that you have to produce? See, why are you taking moles, you will say in data there is
gram why have we got moles? Because the molarity is co-related to what, volume and
moles. If we see number of moles, molar mass of H2SO4 is 34 grams per mole. So, number
of H2SO4 mole is how much? So, 1 mole of H2SO4 is produced by how many moles of
H2SO4, so therefore I can say that, I want 5 mole H2SO4, n number of moles of H2SO4
required will be equal to 5, which should be equal to the molarity of H2SO4 into the volume.
So what is the molarity given? 0.20 into V, what is 0.2, 1 by 5 into V, this implies V is equal to
what? So, we can write down 5 into 5, which is how much, 25Litres. Now everybody
understood, is there any problem. There is no problem, right. Can we do one more question
like this? All of us will do based on stoichiometry. If you want to take down this solution, you
can take it down and we will discuss the next question. Everyone has written down, okay.
See, the next one.
This question is from your NCERT book. The question is calcium carbonate plus 2HCl which
give CaCl2 plus H2O plus CO2. What mass of calcium carbonate is required to react with
25ml of 0.75 molar HCl. That means to react fully, how much calcium carbonate mass you
will need? Okay, everyone, I will demonstrate and then I will give you another question to
try. See this once more, I hope that you should be able to handle this question. 0.9375
grams is the answer, you are not getting it. We will write the reaction, calcium carbonate
plus 2HCl gives you calcium chloride plus H2O plus CO2, given things are 25ml HCl and 0.75
molar and we have to find the mass. Right, this is what is required. So, what will you do?
First of all find out molar mass of calcium carbonate. 100, you must have memorised it. So,
this is how much, W by 100 moles, okay, and how many moles this has and whose moles
should be double? HCl, so here how many moles should be there, 2W by 100. And this data
should be equal to the molarity in volume we have. Shall I make it equal? We will say 25, we
have to convert it into litre because we want moles. 25 divided by 1000 into what, 0.75. So
from here cancel two zeros. Now, if you solve, what will be W? W is equal to 25 into 0.75
divided by 20. This will be 5, 4 you may solve this will be 3.75 divided by 4 and the answer is
0.9375 g, okay. Everyone understood how we did it. You wrote this correctly, this should be
right and after this calculation we had. Everyone understood, completed this.
Let’s see one more question. Solve this question. See, if we talk about the question given
here, according to that we will write it. Na2SO4 reacts with what, what, very good, I am sure
if you have found out BaSO4 molar mass, it must be 233 grams. You can write down 2.33
grams divided by 233 gram per mole. From here you strike off.