[dsmyth]

Hi There,

I am a mech engineer who needs a bit of help with his chemistry! I need to make up a solution of (this is as described in literature studied..) 20% hydrogen peroxide/0.1 M cobalt chloride solution. (The solution is also referred to, in the same paper, as an oxidative solution of 20% hydrogen peroxide in 0.1 M cobalt chloride)

From what I have read thus far I am taking this to mean:

20% Hydrogen Peroxide/0.1 M Cobalt Chloride = 20%H2O2 with 13g CoCl2 Per litre of H2O2

from:
0.1M = 0.1 Molar Mass solute per Litre of Solution

Molar mass of CoCl2 –
Co – 59
Cl – 35.5
Cl2 – 71   
CoCl2 – 130g = M
0.1M = 13g


So my first question is am I on the right track?

My second involves getting the above solution from the chemicals available to me.

I have 30% W/W H2O2 and CoCl2 6H2O (Cobalt(II) Chloride Hexahydrate)

So I need to dilute the Hydrogen Peroxide (how much water H20?)
and do so accounting for the Hexahydrate?

I have spent a bit of time reading around this and have yet to arrive at a solution, probably as my grasp on chemistry is limited at best. Thanks for your help,

Daniel

[Arkcon]

I'm going to start you off with an easy formula, one that everyone should have in their toolbox:

C₁ * V₁ = C₂ * V₂

The first concentration times it's volume equals the second concentration times its volume.  It works for molarity, ppm, percent, oz/gal, you name it, it can handle it.  You know what you've got and you know what you want, so just plug it in.

And yes, you will have to account for the 6 water molecules tagging along with each CoCl₂ in your jar of purple crystals.  That's 6 * 18 added to the molecular mass you computed.

[AWK]

Calculate 0.1 mole of CoCl₂.6H₂O
it will be close to 24 grams
Density of 30% H2O2 is about 1.1 and 20% H2O2 about 1.07
From this data you can find volume of 30% H2O2 that after dilution with water gives 1 liter of 20 % H2O2.
Or aproximately you can take just 2/3 liter of H2O2 and 1/3 liter of water

[dsmyth]

Thanks for the help folks,

I have used the concentration formula and obtained what I hope is the correct amounts to dilute etc... could you please confirm (confidence in my chemistry!!!)? Thanks again

Have
CoCl2 6H2O
30% H2O2

Need
20% H2O2 / 0.1M CoCl2

Molar Mass of CoCl2 6H2O is

Co – 59g
Cl – 35.5g
Cl2 – 71g
H – 1g
H2 – 2g
O – 16g

H2O – 18g
6 H2O – 108g

CoCl2 – 130g

1M CoCl2 6H2O = 238g
0.1M  CoCl2 6H2O = 23.8g

20% H2O2 from 30% H2O2 (+0.6M H2O)
Concentration C1V1 = C2V2
0.3(1000ml) = 0.2 (X)
X = 1500ml

Therefore we dilute to 20% H2O2 by adding 500ml, less the 0.1M hexahydrate…


Which is 10.8ml from:
1M H2O = 18g, 1M 6H2O = 108g
0.1M 6 H2O = 10.8g ≈ 10.8ml

Therefore add 489.2ml of H2O and 23.8g CoCl2 6H2O to the 30% w/w H2O2

Thanks again for your help.

[Borek]

CoCl2 – 130g

OK

1M CoCl2 6H2O = 238g
0.1M  CoCl2 6H2O = 23.8g

OK

20% H2O2 from 30% H2O2 (+0.6M H2O)
Concentration C1V1 = C2V2

No. CV = const works for molar concentrations but not for % w/w. That's because density of solution depends on its concentration. When dealing with % w/w concentrations you have to set up mass balance - separately for solvent and solute - and work with solution mass, then convert it to volume after checking densities in tables.

Therefore we dilute to 20% H2O2 by adding 500ml, less the 0.1M hexahydrate…

Which is 10.8ml from:
1M H2O = 18g, 1M 6H2O = 108g
0.1M 6 H2O = 10.8g ≈ 10.8ml

Therefore add 489.2ml of H2O and 23.8g CoCl2 6H2O to the 30% w/w H2O2

When preparing 1L of solution you don't add measured amount of water, but you fill up to 1L. Once again, that's because of solution densities changing. Then your calculation of amount of water in hexahydrate seems OK, it is just not necessary.

Ready recipe follows:

[dsmyth]

Hi Borek, and thanks for that, it is much appreciated.

I had come up with something a little different following what I thought was logic. Maybe someone could point out the flaws , bearing in mind it could be quite fundamental!

Working on the following info from http://www.unc.edu/~rowlett/units/dictW.html

an abbreviation for "by weight," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. Properly speaking, 2% w/w means that the mass of the substance is 2% of the total mass of the solution or mixture. The metric symbol g/g has the same meaning as w/w.

I went ahead and calculated the total mass of H2O2 required for 1l -

m1 = (0.2)(density)(vol)    -    0.2(1070kg/m3)(0.001m3) = 0.214kg

So how much 30% H2O2 do I need to have this mass and then top up to 1l using H2O

0.214 = (0.3)(1.1)(V)       -     V = 0.713m3 or 713ml

Thanks again for the help folks.
Daniel


 

[Borek]

0.214 = (0.3)(1.1)(V)       -     V = 0.713m3 or 713ml

You forgot to divide by d.

Still, it is much easier to fill up to 1L then to add 642.87 mL

See http://www.chembuddy.com/?left=concentration&right=toc for detailed lectures on different types of concentrations.

[dsmyth]

Still, it is much easier to fill up to 1L then to add 642.87 mL

I guess it is
Thanks for all the help.

Daniel