Problem Solving Strategies

4:31 AM

@Sarabsrimt I think the question only asks for the upward acceleration to be 4 regardless of the horizontal acceleration

@RishiNandhaVanchi did you give the test yesterday? I saw in internet some qns were repeated from previous year with different values this year. did you get that feeling? I feel uneasy since there was only on person pointing this out...

Sarabsri mt

12:10 PM

@napstablook k

Sarabsri mt

4:15 PM

@JohnRennie Hello sir

I have confusion regarding mole. Are you free sir

John Rennie

@Sarabsrimt hi, I'm about to start lunch. Is it quick?

Sarabsri mt

The most used definition of 1 mole is this:
“A mole is defined as $6.02214076 × 10^{23}$( I let to call this number as X here)of some chemical unit, be it atoms, molecules, ions, electrons or others”.

Also , why is it that many examples write that one mole of glucose molecules
is X and one mole of metal is X metal particles or atoms. Not molecules here. Now , one molecule of h20 has 3 atoms. How can we in the end even say that for every 1 mole of water. It has X atoms and molecules both. It has to be one way. Either X molecules or 3*X particles.

This is my confusion sir @JohnRennie

You can answer anytime

John Rennie

One mole of a compound is Na molecules of that compound.

So a mole of water is 6.022 x 10^23 molecules of water

One mole of glucose is 6.022 x 10^23 molecules of glucose

Most metals are elements not compounds, so for example a mole of iron is 6.022 x 10^23 Fe atoms

Sarabsri mt

Yes sir

cant we say mole of iron molecules

Sir , I would suggest you to have lunch first. Enjoy it. It is 10pm here. I will be here till 11pm. If not today , hen Tom also I will come.

I write the points of my difficulties .

Why can’t we a mole of $N_A$ molecules , electrons , atoms of water ?

And the second Q ,I mean does it mean a glass of water contain $N_A$ electrons ,atoms , molecules all together ?

Sarabsri mt

4:38 PM

@JohnRennie Sir , this is my confusion in a nutshell

John Rennie

4:57 PM

@Sarabsrimt I don't understand what you are asking.

You can have a mole of electrons.

Sarabsri mt

Ok

1 min

John Rennie

A mole of electrons is one Faraday. It's used a lot in electrochemistry.

Sarabsri mt

@JohnRennie

Here. Red and blue are atoms of H and O

Molecule as one is H20

Ok

Then , by definition of a mole. My question is

One mole of H20 molecules = $N_A$ molecules . ( Yellow colour) is this correct ?

John Rennie

@Sarabsrimt yes, correct.

Sarabsri mt

Here. Yellow is H20 molecules whose quantity is $N_A$ inside a blue space

OK sir

Here, blue correct is different thing.

Then ,

If I say No of mole of H2O atoms , then H20 has a total of 3 atoms. 2H and 1 O

So , total become 3 * $N_A$

Where am I wrong here ?

John Rennie

5:06 PM

Yes, a mole of water contains 2Na hydrogen atoms and Na oxygen atoms.

I don't see why there is anything surprising about this.

Sarabsri mt

It should not like this.

John Rennie

I don't see the problem.

Sarabsri mt

What should I say as 1 mole of H20 atoms is

$N_A$ atoms

John Rennie

There is no such thing as an H2O atom

In water the two hydrogens and one oxygen are joined together. Yes?

Sarabsri mt

Yes sir

John Rennie

5:09 PM

So the three atoms cannot act independently. They are part of a single unit i.e. part of a water molecule.

That's why when we talk about a mole of water we mean Na water molecules.

Sarabsri mt

Ok. So a molecule of water has H20 molecules

inside the H20

2 NA of O atoms and 1 Na of H atoms

John Rennie

1 min ago, by Sarabsri mt

Ok. So a molecule of water has H20 molecules

Did you mean "a mole of water" ?

Otherwise I don't understand what that sentence means.

Sarabsri mt

A mole is just a unit

By a mole of water , I don’t mean anything . But by a “mole of water molecule “ I mean to say sth

Yellow circles which I drew

John Rennie

So "a mole of water molecules" means 6.022 x 10²³ water molecules?

Sarabsri mt

Yes sir

Clrrect

Then , inside yellow molecule

we have 3 atoms

with 2 NA of H atoms

and 1 NA of O atom

correct sir ?

John Rennie

5:15 PM

I think when we say "a mole of water" it means the same as "a mole of water molecules" i.e. 18g of water in both cases.

@Sarabsrimt yes

Sarabsri mt

@JohnRennie Ok. I will keep that in my ntbk.

-1

Q: Confusion in solution for this Question regarding mole concept

48gms of graphite and 16gmd of $O_2$ is given. The question is to count the no of molecules of $CO_2$ produced ? Now , the solution in book is this No of C atoms = 48gm/12gm = 4mole. No of O atoms = 16/16 = 1 mole , and no of O2 atoms = 0.5 mole. The reaction given is C + 2O gives $CO_2$ . CO2 pr...

Sir , is it possible for one last doubt

please

If you’re free for just one last Q

John Rennie

Yes ... ?

Sarabsri mt

Sir , this one that I posted

John Rennie

I'd guess it seems a very basic question.

Sarabsri mt

Ok. Can you please help in it

because my teacher only says that 1st reaction is correct

and 2nd reaction is wrong

John Rennie

5:19 PM

If you start with 4 moles of C and half a mole of O₂ then you are going to end up with 3½ moles of carbon and ½ a mole of CO₂

Isn't that obvious?

Sarabsri mt

Why will I end up with 3 and a half mole of carbon ?

John Rennie

So your teacher is saying that C + 2O -> CO₂ is correct ...

... but C + O₂ -> CO₂ is not?

Sarabsri mt

@JohnRennie Yes. And not the other one? I don’t understand the reason

John Rennie

I would have said both were correct.

Sarabsri mt

Why do you put a dash before greater than sign

John Rennie

5:23 PM

It's supposed to be an arrow

Sarabsri mt

This arrow right

John Rennie

Yes

Sarabsri mt

Also , both the reaction are different right. One has 2O and other one has O_2 . So , is there a difference in reaction but

John Rennie

Both ways of writing the reaction give the overall reaction i.e. the starting and ending points. The reaction is actually very complicated.

So both ways of writing it are equally good, or alternatively equally bad.

Sarabsri mt

Ok sir.

John Rennie

5:27 PM

The reaction actually takes place in two separate steps.

Sarabsri mt

Ok

What are the steps ? If I may know.

John Rennie

Carbon solid reacts with O₂ molecule to produce gaseous CO and an O atom.

Sarabsri mt

K

John Rennie

The the CO can either react with an O atom to give CO₂ or it can react with another oxygen molecule:

Sarabsri mt

Yes

ok

John Rennie

5:29 PM

CO + O₂ --> CO₂ + O

Sarabsri mt

K

John Rennie

So when we write C + O₂ --> CO₂ or C + 2O --> CO₂ this is just an overall description of a complicate multistage reaction.

Sarabsri mt

@JohnRennie C + O2 gives CO + O

@JohnRennie confused here a bit sir.

John Rennie

Typically carbon first reacts to produce carbon monoxide.

Then the carbon monoxide reacts to produce carbon dioxide.

It's a two step process.

Sarabsri mt

Ok

How did you say that you have a choice

to react with O2

O was present as reactant and you wrote also. From where does O2 come then

John Rennie

5:32 PM

Carbon monoxide is pretty reactive and will react with the first thing it bumps into. If it bumps into a free oxygen atom it will react with it, and if it bumps into an oxygen molecule it will react with that.

Sarabsri mt

Ok

Let me brief it

John Rennie

In a flame there are both free oxygen atoms and O₂ molecules around.

Sarabsri mt

@JohnRennie OHHK

C + O gives CO and CO + O gives CO2. This is two step but converted into one step by writing C + 2O gives CO2.

John Rennie

@Sarabsrimt yes

Sarabsri mt

Ahh thank god I got. All thanks to you

And the other reaction

John Rennie

5:34 PM

:-)

Sarabsri mt

C + O2

Why is that not 2 step

John Rennie

That's two step as well.

Sarabsri mt

Ohk. How then. We can’t separate O2

@JohnRennie This one here is two step that you did but is different from the 2nd reaction

John Rennie

You are reading too much into this. When we write C + 2O --> CO₂ that isn't supposed to mean the C reacts with an O atom then another O atom.

It's just an overall summary of how much carbon reacts with how much oxygen.

Sarabsri mt

Ohk. But that’s what I wrote earlier sir

John Rennie

5:37 PM

In that case I misunderstood what you were asking.

Sarabsri mt

C + O gives CO and CO + O gives CO2. This is two step but converted into one step by writing C + 2O gives CO2.

This statement

Is it wrong

John Rennie

Writing C + 2O --> CO₂ does not mean anything about the details of the reaction.

Sarabsri mt

Ohk.

John Rennie

It is just telling us how much carbon reacts with how much oxygen.

And writing C + O₂ --> CO₂ also does not mean anything about the details of the reaction.

It is also just telling us how much carbon reacts with how much oxygen.

Sarabsri mt

Ohk. No details are given of reaction like this

@JohnRennie but tells us the amount

John Rennie

5:41 PM

Correct.

That's why when we started this discussion I said I thought both equations were fine.

Sarabsri mt

@JohnRennie ohk. So , it is not necessary to have the reaction happened

John Rennie

Because both just tell you how much carbon reacts with how much oxygen.

Sarabsri mt

And then you find the amount

@JohnRennie hmm ok. So , it is like maths

but we know the right amount we need to use

John Rennie

Yes

Sarabsri mt

To make the reaction happen ?

If these are the ingredients

Ok. If some or the other ingredient is more. The reaction does happen but some part gets left out which is unreacted

Similarly with if the one or the other ingredient is less

Is this right ?

John Rennie

5:45 PM

Suppose you start with one C atom and two O atoms, then you'll get one molecule of CO₂. Yes?

Sarabsri mt

Yes

John Rennie

Now suppose you start with two C atoms and two O atoms. You still get one molecule of CO₂ and now there is one C atom left over that can't react because there is no oxygen left for it to react with.

So if you have extra C atoms they remain unchanged because there is no oxygen left for them to react with.

Sarabsri mt

Yes

Yes sir

I got it better now.

John Rennie

:-)

Sarabsri mt

So , C + 2O is a impossible reaction also because they are single oxygen atoms and 2 oxygen atoms need to react with each other and then react with C

But it tells us the amount of ingredients

@JohnRennie thank you sir

John Rennie