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### Topic: Prepare dilutions without H2O  (Read 6514 times)

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#### fosfolipasa_A

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##### Prepare dilutions without H2O
« on: January 09, 2013, 01:35:59 PM »
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?

#### Borek

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##### Re: Prepare dilutions without H2O
« Reply #1 on: January 09, 2013, 01:51:58 PM »
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#### fosfolipasa_A

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##### Re: Prepare dilutions without H2O
« Reply #2 on: January 09, 2013, 02:14:29 PM »
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

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##### Re: Prepare dilutions without H2O
« Reply #3 on: January 09, 2013, 02:17:31 PM »

#### fosfolipasa_A

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##### Re: Prepare dilutions without H2O
« Reply #4 on: January 09, 2013, 02:24:53 PM »

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

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##### Re: Prepare dilutions without H2O
« Reply #5 on: January 09, 2013, 02:54:34 PM »
Should I need to use the C1V1+C2V2=CfVf?

Yes.

Quote
'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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### fosfolipasa_A

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##### Re: Prepare dilutions without H2O
« Reply #6 on: January 09, 2013, 03:10:33 PM »
It's so confusing.
C1 V1 + C2 V2 = Cf Vf
So:
3 N V1 + 0.5 N V2 = 3.5 N 1L

V1 = 0.5 L
v2 = 0.5 L

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

#### fosfolipasa_A

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##### Re: Prepare dilutions without H2O
« Reply #7 on: January 09, 2013, 03:24:20 PM »
Should I need to use the C1V1+C2V2=CfVf?

Yes.

Quote
'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

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##### Re: Prepare dilutions without H2O
« Reply #8 on: January 09, 2013, 03:41:51 PM »
Why do you guess instead of calculating?

What is sum of V1 and V2 equal to?
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#### fosfolipasa_A

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##### Re: Prepare dilutions without H2O
« Reply #9 on: January 09, 2013, 03:46:17 PM »
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

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##### Re: Prepare dilutions without H2O
« Reply #10 on: January 09, 2013, 03:50:15 PM »
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=CfVf
V1+V2=1

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

Solve for V1 & V2 and you are done.
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#### fosfolipasa_A

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##### Re: Prepare dilutions without H2O
« Reply #11 on: January 09, 2013, 07:08:57 PM »
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

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##### Re: Prepare dilutions without H2O
« Reply #12 on: January 10, 2013, 04:44:14 AM »
But theres no other way to simplify the process?

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.
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#### curiouscat

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##### Re: Prepare dilutions without H2O
« Reply #13 on: January 10, 2013, 07:27:59 AM »
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

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##### Re: Prepare dilutions without H2O
« Reply #14 on: January 10, 2013, 11:24:54 AM »
No, I got it. Thanks