Home/Archive/Archive - 2015-16/Chemistry 02, Stoichiometry and Limiting Reagent

## Chemistry 02, Stoichiometry and Limiting Reagent

We will start, friends in the previous lecture we have discussed what are atoms, what are
molecules, how to calculate total number of atoms and how to calculate total number of
molecules. If one sample is given we can find out many entities are present in that sample
let it be molecules, or individual atoms or we can also take mass, if we are told the number
of atoms, so we can calculate, the mass of the sample itself. We have to use the same
problem again and again. The number of entities or the number of atoms or number of
molecules is equal to the mass of the sample divided by the, yes, atomic mass or molecular
mass or mass of one entity. That could be any entity, it could be ion, atom or a molecule. We
can take anything. Till now you studied that how we interpret things by one mole atom. One
mole atom implies 0.622 into 10 raise par 23 atoms. Now, see you will be able hearing one
more language. When we are talking about the sodium case, let’s say we are talking about
sodium, or silicon. We don’t prefer sodium because when we write Na it gets confused with
Avogadro’s number. Let us say, we will talk about 28 grams of silicon. What does this imply?
What does this imply, how many atoms, do I have? One, one mole atom, or it is, it is correct.
1 mole atom or it is equal to Avogadro’s number of atom of silicon. Okay, if I give you a 28
gram atoms of silicon. Understand, that gram atom also means mole. Gram means normal
weight but gram atom means mole atom, gram molecules means mole molecules, gram ion
means mole ion. Generally what we do is, when we define things in grams with that
particular term, we refer to the mole of that quantity. For example, 28 gram atoms of silicon
implies 28 mole atoms of silicon and that implies 28 times Avogadro’s numbers of silicon.
There is huge difference. 28 gram of silicon is 1 mole atom but 28 gram atom of silicon again
implies 1 mole atom. Similarly, if I say, 44 gram of carbon dioxide implies how many mole
molecules, 1 mole molecule. Okay, what about, if you say, 44 grams molecules carbon
dioxide. This is definition. Yes, this is definition, this is how you define atom or atom gram
molecule. So, you say 44 gram molecules of carbon dioxide implies 44 mole molecules, okay.
Okay, similarly, if I say, here you correlate with this, let’s say 23 gram sodium ion implies 1
mole sodium ion or I can say that here, 23 gram ions of sodium 23 moles of sodium ion.
Everybody understood this. So, in this type of interpretation you will be asked data about
gram atoms, gram molecule and gram ions. And that is also interpreted as mole atoms,
mole molecules or mole ions.
If I ask you one simple question, what is the difference between 1 gram of oxygen and 1
gram of atom of oxygen? Now, what will you say? Correct, 1 gram atom. Repeat once more,
1 gram of oxygen implies mass of sample of oxygen is equal to 1 gram whereas here, mass of
sample of oxygen will imply, sample of oxygen, it is 1 mole and what is mass of 1 mole
oxygen is 16 gram. Everybody understood this. This differentiation we will make, this you
can keep in mind easily. Mark this, mark this entire thing. This is important, this is another
way of interpretation of mole. Okay, mole is also defined as gram entity of something. Mole
is also defined as gram entity of something. Everybody wrote it down, mark it. Then we will
start study of chemical reactions. Everybody understood the meaning now. What is the
meaning of gram of something, gram atom, gram molecule or gram iron. Everybody wrote it.
Okay, now let’s start the study of chemical reaction. First of all define what are chemical
reactions? Is preserved and what actually occurs, yeah tell. Products, reactant combined to
give products or like you said rearrangement of atoms. So, let us write, write rearrangement
of atoms between various molecules of reactants. Rearrangement of atoms, rearrangement
of atoms between various molecules of reactants, rearrangement of atoms between various
molecules of reactants to form new molecules of products, between various molecules of
reactants to give new molecules of products is called chemical reaction, is called chemical
reaction. Rearrangements of atoms between various molecules of reactants to form a new
molecules of products is called chemical reaction, okay. Write ahead, since atoms are
rearranged since atoms are rearranged they remain conserved. Since atoms are rearranged
they remain conserved and therefore the mass is also conserved. You already know these
two things, conservation of atoms and conservation of mass, okay. Since, atoms are only
rearranged, the atoms are conserved and their mass is also conserved. So, therefore we say
in a chemical reaction mass is conserved.
Okay, now let us define the term Stoichiometry. What is Stoichiometry? Okay, so you write
here, it is a study of chemical reactions, it is a study of chemical reactions, it is a study of
chemical reactions and calculation related to it. Study of chemical reactions and calculations
related to it, okay. Now, I will write a chemical reaction here which we will be repeating
again and again in the next few examples, okay. And through that you have to explain to me
some interpretations, okay. SO2 plus O2 combines to give you SO2. This we know as a
reaction, correct. There is no balance. So, we say currently the equation is.

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