Topscore Question

[drpj]

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I have a question on a question I had from Topscore #2... I just don't understand the explanation that Topscore gives. Here's the question:

"What volume of HCl was added if 20mL of 1 M NaOH is titrated with 1 M HCl to produce pH=2?

Answer: 20.4 mL"

--- If someone can please explain this to me I would greatly appreciate it!

 

[marymatthews]

bump
and
If you dissolved 20 grams NaOH in 1000 grams H2O, then what is the molality of the solution?
Answer: 0.5
I understand you divide 20 g/40 g to get 0.5 moles NaOH.... then divide 0.5 moles/1 kg H2O to get 0.5.
But dont you also have to multiply by the van hoft factor of 2 in order to get 1.0? NaOH has a van hoft factor of 2? which becomes (0.5)(2) = 1?
Thanks

 

[drpj]

No you don't use the van hoft factor when calculating molality. Molality is defined as moles/kg of solvent... So when calculating that problem you stop right there and don't calculate the van hoft factor into it.

You only use the van hoft factor in calculations for the colligative properties such as freezing point depression, boiling point elevation, osmotic pressure.

Remember:
Freezing point depression: Tf=(Kf)(m)(i)
Boiling point elevation: Tb=(Kb)(m)(i)
Osmotic Pressure: II=MRTi

i = the van hoft factor.

Hope this helps.

 

[marymatthews]

thanks
so please clear something up for me...

what is the van hoff factor for H2SO4?
and what is the Normality for H2SO4? (lets say the molarity is 2 M)

 

[drpj]

Okay so for the van hoft vactor of H2SO4, I would say i=3 (2 from the H and 1 from the SO4). Correct me if I'm wrong...

For Normality, if I was given H2SO4 with 2M ... I would multiply it by 2 for the 2 H's for it to be 4M.

 

[MSUstudent]

Agreed with the post above.

i = 3 for H2SO4

for normality N=M * number of equivalents
(equivalents = the number of H's or OH's in a molecule)
so.. here we have 2 H's

N= 2*2 = 4

 

[marymatthews]

So van hoff factor is based on the number of ions produced in solution... which is used for colligative properties only
and normality is based on the number of hydrogens produced in solution.... which is used for titrations
??

 

[drpj]

Yes that is correct, but also remember that Normality applies to OH's too.

For instance, 2M of Ba(OH)2 = 4M for Normality.

 

[marymatthews]

ok cool thanks
i have a side note question please...
how would you rate the following compounds in terms of strength and BP/MP?
Ionic, molecular (covalent), and network solid (covalent)
which of the three has the strongest bonds and the highest BP/MP? Which has the lowest?
thanks

 

[MSUstudent]

Can someone answer the original question? I am having a problem with that as well.

 

[drpj]

Well I know that ionic bonds have extremely high BP (even higher than hydrogen bonding), so I would think that ionic bonds will have the highest BP, then network solid and finally molecular.

From Chad's chem video, he mentioned that network solids have higher BP/MP than molecular.

 

[MSUstudent]

"What volume of HCl was added if 20mL of 1 M NaOH is titrated with 1 M HCl to produce pH=2?

Answer: 20.4 mL"

I understand the solution.....

 

[drpj]

MSUstudent, can you please explain the solution in detail? Thanks.

 

[MSUstudent]

I'll try my best...

"What volume of HCl was added if 20mL of 1 M NaOH is titrated with 1 M HCl to produce pH=2?

Answer: 20.4 mL"

Lets break it down to two parts:
1) neutralizing the NaOH
2) getting the neutralized solution to a pH of 2.....

1) It takes 20 ml of HCl to neutralize 20 ml of 1M Nacl... which we found out by using M1V1=M2V2... The total solution thus far is 40 ml of the neutralized solution (20ml of NaOH and 20ml of HCl.)

2) To create a pH of 2 of the final solution, we will need to find out how much HCl to add to the total neutralized solution. Which will be...

(1M HCl)(x) = (.01M)(40ml+x)

(40ml +x) = the total volume of the overall solution (40ml of the neutralized solution + the volume of addition HCl needed to get the final solution to pH of 2)

.01M concentration of the final solution. Which comes from pH=[H+] which is 2 = -log[1 x 10^-2]).
x= volume of HCl needed.
1M of HCl given in the question


Then you solve for x, which x=0.4

Then you find out how much of the total volume will contain HCl only, so
First we added 20 ml of HCl to neutralize 20ml of NaOH. Then we need and additional 0.4 ml of HCl to get the neutralized solution to a pH of 2.

Therefore we have 20ml of HCl + 0.4 ml of Hcl = 20.4ml

 

[MSUstudent]

I'll try my best...

"What volume of HCl was added if 20mL of 1 M NaOH is titrated with 1 M HCl to produce pH=2?

Answer: 20.4 mL"

Lets break it down to two parts:
1) neutralizing the NaOH
2) getting the neutralized solution to a pH of 2.....

1) It takes 20 ml of HCl to neutralize 20 ml of 1M Nacl... which we found out by using M1V1=M2V2... The total solution thus far is 40 ml of the neutralized solution (20ml of NaOH and 20ml of HCl.)

2) To create a pH of 2 of the final solution, we will need to find out how much HCl to add to the total neutralized solution. Which will be...

(1M HCl)(x) = (.01M)(40ml+x)

(40ml +x) = the total volume of the overall solution (40ml of the neutralized solution + the volume of addition HCl needed to get the final solution to pH of 2)

.01M concentration of the final solution. Which comes from pH=[H+] which is 2 = -log[1 x 10^-2]).
x= volume of HCl needed.
1M of HCl given in the question


Then you solve for x, which x=0.4

Then you find out how much of the total volume will contain HCl only, so
First we added 20 ml of HCl to neutralize 20ml of NaOH. Then we need and additional 0.4 ml of HCl to get the neutralized solution to a pH of 2.

Therefore we have 20ml of HCl + 0.4 ml of Hcl = 20.4ml

 

[drpj]

Thank you for clearing things up! I was always wondering why they used 0.01 M but now I get it lol

 

[Ibraiz]

Thanks for the clear explanation. This requires so much calculation and it seems time consuming. Do you guys think or has any body experienced these kind of questions in the real DAT?

I have always heard the calculations are very easy in real DAT General Chemistry section and it is mostly conceptual question.

 

[MSUstudent]

Thanks for the clear explanation. This requires so much calculation and it seems time consuming. Do you guys think or has any body experienced these kind of questions in the real DAT?

I have always heard the calculations are very easy in real DAT General Chemistry section and it is mostly conceptual question.

This problem can happen...its just anybody's luck.. haha

Question: Does anyone have the 2009 ordered ADA sample test? I would like to have a copy and are willing to pay. Or is this illegal by the ADA?