So, let’s resume with the discussion of strength terms, in the previous lecture we had
discussed parts (00:13), now we will start with the next term and it is one of the very
important terms. The first term that we are going to do is, Molality. Molality is defined with
the symbol small m, okay, so first all of you write the definition, it represents the moles of
solute, it represents the moles of solute, it represents the moles of solute present in 1
kilogram of solvent, present in 1 kilogram of solvent. Please note, and where you have
written the definition make a box around it and write solvent. The word ‘solvent’ you again
make a mistake, molality is not moles of solute upon mass of solution, it is the mass of
solvent. Highlight it, it is not solution it is solvent. What will be the unit? Moles per kg, we
have already written it down, moles per kg and this is also called as molal represented by
small m. Okay, so the symbol also is this and what do we call it, molal represented by small
m. okay, now, if I take an example here, let’s say I have got a 2.5 molal solution of a solute,
so what is the meaning. What is the meaning? So can I say that 1 kg of solvent contains
what, if that solvent happens to be water, in case we assume it to be, so would we get to
know the moles of water? Tell me. Children, please participate. If we take the solvent to be
water, we know the mass of water. 18 gram per mole. Can we find out the moles of water?
If we know the moles of water, and if we know the mole of solute then we can get the mole
of fraction. Okay, assume I have the moles of solute and I know the molecular mass of
solute, then can I calculate here. What can I calculate here? Mass of solute. And if I know
the mass of solute, mass of solvent then I will also know the mass of solution. Can I remove
the mass percentage? Can you understand? You will get questions of inter-conversions.
You will have to understand from which information you can do the inter-conversions. Is
the matter clear? Okay, so molality is clear to all of you.
So let’s take the next term which is, this is molarity, represented by capital M. And write
down the definition, it represents the moles of solute, it represents the moles of solute, it
represents the moles of solute present in 1 litre of solution. It represents the moles of solute
present in 1 litre of solution. So as a formula we can represent it as, molarity is equal to
moles of solute, moles will be the unit of this, and volume of solution, litres. Okay, and if we
write it like this then we will get, what is the unit in this case. Unit is mole per litre, it is also
called as molar and it is symbolised as capital M again. Okay, even in the unit it will be
written as capital M. So just suppose that we have got for example 1.7 molar NaCl aqueous
solution, what does this imply. So in 1 litre of solution, what do you have? Correct, okay.
So what can you convert this into? Tell me, what data you will require to convert this into.
If we have the density then we can calculate the mass of solution, because we know the
volume of solution. If we know the mass of solution then if I may tell you we know the
molecules of NaCl mass. We will also get the NaCl mass. So what can you calculate? Mass
percent. Okay, if we remove the mass of solution and from there we remove the mass of
solute then what will be get, mass of solvent. Then we can find out molality also. And if
required we can convert this into mole fraction, for both of them, we know the moles for
the two of them. We have found out the mass of solvent and also got the moles. Are you
understanding how the inter-conversion is done. What should be kept in mind and which is
the term which will be the central term for us for inter-conversion – density. What does
density inter-relate? Mass of solution with the, you have to relate this and then the rest of
the inter-conversion will be done. We will shortly see the question, first write a comment
on molarity. Write down, molarity is generally taken to be synonymous to concentration. If
I do not have anything mentioned, that this is a concentrated solution, or it is a highly
concentrated, and there is a less concentrated solution, the indication is by the unit of
molarity, if it is highly concentrated solution then the molarity will be more, and if it is less
concentrated solution then the molarity will be less. So generally when we say a highly
concentrated solution and a low concentrated solution or a less concentrated solution we
are referring to which term. That is whichever terms we have studied till now, all of them
are concentration terms. But if no specific concentration is defined then you will assume it
to be Molarity. Did all of you understand this? So that is Molality and Molarity.
Now I am going to put up three questions in front of you, which you will have to do. Please
no one give the answer in between. Once you are done raise your hand, I will come and
check your answer. Okay. Do it yourself and then we will discuss. Okay. Solve these three
questions.