Dilution Problem on Destroyer

[Jongho Yoon]

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20ml of 0.12M solution Ca(OH)2 is added to 48ml of HBr

What is the concentration of HBr?

Since 20ml is being added to 48ml,
(20)(0.24) = (68)(x)

is what I did to get x as an answer

but the solution did it this way (20)(0.24) = (48)(x)

Which way is right?

 

[potbelly]

solution is right.. you don't add up two volumes when you dilute...

But when you find molarity of the mixed solution, you have to find the difference in mole and then divide by the total volume. (In this case, you have to add up two volumes..)

 

[Jongho Yoon]

Can you elaborate? I don't understand why because the final diluted volume will be 68ml.

If the question said "20ml of 0.12M solution Ca(OH)2 is added to "make" 48ml of HBr, then it makes sense in my understanding.

But the question says 20ml was added to 48ml, which means to make 68ml

 

[hham]

Can someone please elaborate this question?
I thought dilution always follows V1C1 = V2C2
therefore, it should be
V1 = 20mL
C1 = .12M
V2 = 48mL
C2 = x

and when plug in the numbers, it should be:
(20mL)(.12M) = (48mL)(x)
x = .05M = C2

no????

 

[Jongho Yoon]

She is right. And I think it's not a dilution problem, more like a titration problem.

 

[potbelly]

Can someone please elaborate this question?
I thought dilution always follows V1C1 = V2C2
therefore, it should be
V1 = 20mL
C1 = .12M
V2 = 48mL
C2 = x

and when plug in the numbers, it should be:
(20mL)(.12M) = (48mL)(x)
x = .05M = C2

no????

In this question, you have to deal with Normality...

 

[purplepanda]

All you need to know is the moles of HBr so that you can put them over the mL--->L of HBr to equal the M (mol/L) HBr


So the eqn is: Ca(OH)2 + 2HBr --> CaBr2 +2H2O

.012 M x .02 L x (2 mol HBr)/(1 mol CaOH2) x 1/(.048 L) = .01 M HBr

So basically, my professor never taught us Normality and instead I visualize these problems as strictly mole/mole ratios.

 

[ethanyoo726]

Jongho Yoon, this is not a dilution problem. This is a neutralization problem which is why you don't add the volumes together. Destroyer is correct. Potbelly is wrong, you have to add the volumes when it is a dilution problem.

the problem is solved as follows...

(20)(0.24) = 48(N2)
---> N2 = [(20)(0.24)] / (48) = 0.1

Since HBr has only 1N, N2=M2, so M2= 0.1M