OK, this has to be a simple question with a simple answer, but hey guess what? Mmm not for me, as usual.
I titrated my CuCl etchant before and after. It now has a free HCL molarity of 1.38,
I wish to make it 2.38mol. So I cleverly? made a 200mL solution of a 1 mole HCl concentration and was about to add it to my 2 Litres of CUCL. Until
I suddenly hit myself (Softly) and realised that if I do this then I am actually only adding 0.0005mol to my solution because it is 2 litres.
So I need to make 1mol as if I was preparing a 1mol HCl solution for 2 litres of water? In other words I need to add 186mL of concentrated HCl to my
CUCl etchant in order to bring it up from 1.38 to 2.38mol approx.
Ok I think this is a dumb question, but still shaky on the old foundations, but definately getting there.
Am I right? thanks
[Edited on 19-7-2012 by CHRIS25]Magpie - 19-7-2012 at 14:34
Edit: You need to tell us the molarity of your HCl. I assumed 12M for the calculation below.
Instead of answering your questions directly I'll show how I would have done it:
Given: 2 liters of etchant 1.38M in HCl
assume 12M HCl available
Desired: bring etchant to 2.38M in HCl
I'm sorry Chris, but algebra really is useful in such calculations, so I'm going to use it.
Let V = the volume of 12M HCl to be added
then (V + 2000 mL) (2.38M) = (2000mL)(1.38M) + V (12M)
2.38V + 2000(2.38) = 2000(1.38) + 12V
2.38V + 4760 = 2760 + 12V
9.62V = 2000
V = 2000/9.62 = 208 mL
If you look at the 1st equation above you will see that it is nothing more than an HCl material balance.
[Edited on 19-7-2012 by Magpie]CHRIS25 - 19-7-2012 at 14:59
Hi magpie, well it is certainly not something I would have thought of, but everything was fine until this:
2.38V + 4760 = 2760 + 12V I juggled around for 6 minutes and still this bit makes no sense to me: 2.38 x v + 4760 ??How do you arrive at 9.62?
Like wise the 2760 + 12 x v = 2000 ?? Adding and multiplication is really a basic, but this defies all of my understanding.
Secondly why then is my figure of 186mL wrong, being, as I understand things thus far, this is a 2mol solution of HCl if added and topped up to 2000mL
of water.. Perhaps if I understand where I am wrong I might be able to better comprehend that algebra.
EDIT: I have a 12mol solution of HCl. or 11.65 as some have it, don't know who is correct here. Also it is not the etchant that I measured for
molarity it is the amount of free H+ ions from the HCl in the etchant that is being measured, Just making sure because you said:...2litres of etchant
in HCL but I might be misunderstanding your wording..
[Edited on 19-7-2012 by CHRIS25]
[Edited on 19-7-2012 by CHRIS25]Magpie - 19-7-2012 at 15:35
Secondly why then is my figure of 186mL wrong, being, as I understand things thus far, this is a 2mol solution of HCl if added and topped up to 2000mL
of water.. Perhaps if I understand where I am wrong I might be able to better comprehend that algebra.
If you topped off 186mL of 12M HCl to 2000 mL the molarity would be = (0.186 liter)(12M)/2 liter = 1.116M
EDIT: I have a 12mol solution of HCl. or 11.65 as some have it, don't know who is correct here. Also it is not the etchant that I measured for
molarity it is the amount of free H+ ions from the HCl in the etchant that is being measured, Just making sure because you said:...2litres of etchant
in HCL but I might be misunderstanding your wording..
A 12M solution of HCl is 12M in H+ for your purposes.
I can't teach you algebra in one lesson. You'll have to take my word for it and use it as a template if you wish. CHRIS25 - 19-7-2012 at 15:50
Thanks magpie, that was what I wanted to do - use as a template, but it helps to understand things,
When I see something like this: 2v or 3y it always means 2 x V or 3 x Y hence the confusion with your 12v, which I still don't get,
2.38V + 4760 = 2760 + 12V
9.62V = 2000 ??? Means to me 9.62 x V = 2000, so how when you say 12 - 2.38 = 9.62 I had already noticed that but that answer made no sense...Have no
clue what is going on here then, and yes I sympathize with you, you don't have to teach me algebra.
It makes no sense so I guess I will have to leave it. I guess after titration I will do it the 12th century way - add a superfluous and subjective
amount of concentrated HCl and then take another titration before and after and calculate differences over weeks and then I will have a good idea of
what to add.
There is only a certain amount of maths that I can cope with and no more, there is too much learning here. Thankyou for your help though, thought it
was an uncomplicated issue that I was asking....Gulp!
[Edited on 19-7-2012 by CHRIS25]
I got it, after much head scratching. You crossed over. Ie + changes to - in 4760-2760. ok makes sense now. As for the difference between your 208
mL and my 186 mL, really it is only a miniscule figure of 0.1 mole thereabouts. But nevertheless I will use the algebra to calculate.
