[fosfolipasa_A]

Hi everyone.

I'm stucked with a problem of analytics since two days ago.
I've never seen a problem like this and I've tried to solve it with many formulas and reasonable thinking but it always fail apparently.

I need to prepare a 2.5 N HCl from two already prepared solutions which are 3N HCl and 0.5N HCl.

I gave it a shot with the NiVi=NfVf but since it's an unusual dilution it didn't work.

So i figured if i assume that the half of the water in the 0.5 N HCl solutions is available I just have to apply the formula NiVi=NfVf this way:
Ni: 3N
Vi: ?
Nf: 2.5 N
Vf: 1 L

And the result was 833.33 mL so my approach was: If I could use pure water I'd have to use 166.67 mL. But in this case water is forbidden, so I'll use the 0.5 N HCl solution as the diluent, but it only have 0.5 of H2O molecules usable. So I reduce the 833.33 mL 3N HCl solution to half and decided to dilute to the flask mark with 583.4 mL of 0.5 N HCl. But of course it didn't work.

My first questions is: Is it possible to prepare this kind of solutions?
Yes: what kind of special or modifications formula works.
No: why?

Thanks for your reading and response.

[Borek]

http://www.chembuddy.com/?left=concentration&right=dilution-mixing

[fosfolipasa_A]

http://www.chembuddy.com/?left=concentration&right=dilution-mixing

Thanks but I'm still confused.
Should I need to use the C1V1+C2V2=CfVf?
'cause I don't have any volumes, only the Vf = 1 L

[Kofi]

Do you have an answer to the question already?

[fosfolipasa_A]

Do you have an answer to the question already?

Not an answer at all.

a) 200 mL HCl 3 N and 800 mL HCl 0.5 N
b) 500 mL each
c) It's not possible prepare it
d) 800 mL HCl 3 N and 200 mL HCl 0.5 N
e) None

[Borek]

Should I need to use the C1V1+C2V2=CfVf?

Yes.

'cause I don't have any volumes, only the Vf = 1 L

Hardly surprising - you are asked to FIND volumes, so you can't have them.

What are your knowns? What are your unknowns? So far you wrote one equation, is there another one that you can write? It is discussed on the page you have visited.

[fosfolipasa_A]

It's so confusing.
C1 V1 + C2 V2 = Cf Vf
So:
3 N V1 + 0.5 N V2 = 3.5 N 1L

So the answer could be.

V1 = 0.5 L
v2 = 0.5 L

But of course the Concentration is not what I'm looking for.

[fosfolipasa_A]

Should I need to use the C1V1+C2V2=CfVf?

Yes.

'cause I don't have any volumes, only the Vf = 1 L

Hardly surprising - you are asked to FIND volumes, so you can't have them.

What are your knowns? What are your unknowns? So far you wrote one equation, is there another one that you can write? It is discussed on the page you have visited.

applying that formula I guess the answer is: 800 mL HCl 3 N and 200 mL HCl 0.5 N.

Am I right?

[Borek]

Why do you guess instead of calculating?

What is sum of V₁ and V₂ equal to?

[fosfolipasa_A]

Why do you guess instead of calculating?

What is sum of V₁ and V₂ equal to?

I did I calculated and the V is 1L and the N is 2.5. But I'm a falure, 'cause I couldn't calculate the values by myself.

[Borek]

What is sum of V₁ and V₂ equal to?

I did I calculated and the V is 1L

OK, so you have two equations:

C₁V₁+C₂V₂=C_fV_f
V₁+V₂=1

and two unknowns (V₁ & V₂) - everything else is given in the problem (and V_f=1, just like in the second equation.

Solve for V₁ & V₂ and you are done.

[fosfolipasa_A]

I made an equation's system.
And got the result.
It match with the comprobation's answer so I assume it's OK.

But theres no other way to simplify the process?

Thanks a lot for your guides.

[Borek]

But theres no other way to simplify the process?

Short answer: no.

I believe there are some mnemotechnic devices that help you learn the final formulas, but IMHO learning them doesn't make sense - you already know the general method that solves this types of questions, so learning several separate methods is a waste of time.

Plus, I don't remember these separate methods - I never needed them.

[curiouscat]

But theres no other way to simplify the process?

This is the simplest way. Perhaps you didn't understand what the equation means?

[fosfolipasa_A]

No, I got it. Thanks