CHRIS25 - 12-3-2014 at 01:52
I have been away from chemistry for almost 18 months and am embarrassed to say that I have forgotten some basics. My notes have confused me on this
one thing:
Which is the correct procedure?
Cu+2HCl=CuCl2+H2 (for etching)
63g copper + 1/11 (M concentration of HCl)) =90mLs
therefore approx 180 mLs of 11M conc HCl needed.
OR
63g copper needs 72g/mol/1.2 (density of approx 36%HCl)
therefore 60 mLs of 11M soln HCl needed.
I am embarrassed but after 18 months I am trying to revise through all my notes and have looked at my previous posts but could not find the answer.
Thankyou and sorry for the bother here.
HeYBrO - 12-3-2014 at 02:36
I don't think the equation is right. You need an oxidant. hence the equation is Cu(s) + H2O2(aq) + 2HCL(aq) ---->
CuCl2 (aq) + 2H2O(l).
I won't do it for you but remember:
n=m/M (solids) and n=cV (for liquids)
Where n= no.of moles. m=mass of sample. M=Molar mass
n=no.of moles. c= concentration. V=volume.
So:
M(Cu)= 63.546 g
M(HCL)= 36.46094 g
M(H2O2)=34.0147 g
From The equation: n(HCL)/n(Cu)= 2:1 ratio
Hence per one mole of copper we need two moles of HCL.
If you have the % concentration you don't even need the density.
I have to go, but i hope this helps; i'm sure you can manage.
[Edited on 12-3-2014 by HeYBrO]
CHRIS25 - 12-3-2014 at 02:48
Yes I know, I use air, or H2O2. I solved it, 72g HCl are needed This is 2moles (11M is my concentration). 2/11 = 0,189 L = 190mLs of 11M HCl.
Thanks for the reminder, I have no idea what I am doing dividing g/mol by the density.
MrHomeScientist - 12-3-2014 at 05:54
A great thing I learned in physics that helps me out all the time is dimensional analysis. Just look at the units of the quantities you know, and the
units of the quantity you want. Then arrange things so that the correct final units fall out. You're usually only off by a constant, which you can
easily figure out on a multiple choice test 
Dividing molecular weight by density gives you (g/mol) * (mL/g) = mL/mol , which is a good clue that something is amiss!
CHRIS25 - 12-3-2014 at 10:00
Is this a better way: What volume of 36% HCl needed to contain 72g?
100mLs * 72g / 36% = 200mLs of 36%HCl contains 72g?
I am so rusty, desperately trying to re-learn everything.
vmelkon - 12-3-2014 at 10:39
36% usually means 36% by weight.
100 mL would have 36 g of HCl, so yes, 200 mL is going to contain double.
CHRIS25 - 12-3-2014 at 12:14
Thankyou vmelkon