Chemistry Questions

[klpennin]

Can someone show me how to work this problem?

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

Also, what would be the molecular shape of NH3 according to the VESPR theory?

Thanks!

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[Contach]

NH3's molecular shape is trigonal planar
the electronic shape is tetrahedral.

 

[doc3232]

I think guess and check is the best...I like this question though.

 

[doc3232]

NH3's molecular shape is trigonal planar
the electronic shape is tetrahedral.

Did you just erase your answer for the first question???

 

[doc3232]

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

n = cv NaOH
n = 1*0.020 = 0.020 mols NaOH
pH = 2
pH = -log [H+]
therefore [H+] = 10^(-pH)
[H+] of HCl = 10^-2 = 0.01 M

if we added 20 ml of HCl, we would have [H+] of 10^-7 = 0.000001 M
because n of HCl = (0.02)(1)
and n mols of H+ - n mols of OH- = n mols of H20
if we have 0.03 mols of H+, and 0.02 mols of OH- then we have 0.01 mols of H+ left over. The other 0.02 mols --> H2O
thereforewe need the initial HCl to have 0.03 mols of H+
n = cv = 0.03 = v*1.0M
v = 0.030 L


Hrm.. this actually took me a while.. if someone has a more systematic way of doing it.. please share..

...

 

[Contach]

...

yeah I erased it, I realised it was wrong....

I assumed the final product was in 1L because i said 0.02 M. But really its not in 1L, not even close. I don't know how to factor in the variable Total litres the solution will be in... I was thinking a quadratic but it didn't work out...

 

[jjw822]

yeah I erased it, I realised it was wrong....

I assumed the final product was in 1L because i said 0.02 M. But really its not in 1L, not even close. I don't know how to factor in the variable Total litres the solution will be in... I was thinking a quadratic but it didn't work out...

so what doc quoted is wrong???

i just blacked out and want to see the answer....

i used to do these kind without any problems.........

 

[Tina324]

IF the answer is 2000 mL then here is how to do it:

(20mL -OH)(1M -OH)=V of HCl (1M HCl)
solve and volume of HCl needed to titrate -OH completely is 20mL.
this makes sense because at equilibrium, the mole (and molarity) of the acid and base are equal.

Now, we want to make the solution acidic, at a pH of 2..meaning we want the solution to have a H+ concentration of 1x10^-2. So, we just use c1v1=c2v2 again.

(20mL HCl)(1M HCl)=V of HCl (1x10^-2 M HCl)
V=2x10^3

let me know if that is correct!

 

[joonkimdds]

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

I got 20.04ml, am I right?

 

[jjw822]

I think what we have to see first is that when 20 ml of NaOH and 20 ml of HCl is mixed together, the ph will be 7. but question asks for the vol. of HCl when pH is 2. so i think we have to do (40ml)(1x10^-7)=V(1x10^-2)
and find V. and add 20ml to that.

no????

 

[joonkimdds]

I think what we have to see first is that when 20 ml of NaOH and 20 ml of HCl is mixed together, the ph will be 7. but question asks for the vol. of HCl when pH is 2. so i think we have to do (40ml)(1x10^-7)=V(1x10^-2)

I approched it in a different way but i think ur way is correct too

20ml + 20ml of same molarity = 40ml of neutral
since we want it to be acidic, we need to add more of acid than base.

1M (X) = 0.01M (40+X)
X = 0.04 + 0.01X
0.99X = 0.04
X = 0.0404

40 + 0.0404 - 20 = 20.0404

 

[jjw822]

i found this from other thread..

i dont know how to quote post from other thread so i am just copying and pasting it......


its from osimsDDS

this question is infamous...

pH of 2 = 1x10^-2

Neutralization between the HCl and the NaOH will give you 20ml, add that to 20ml you started out with and you get 40 ml...

Now you use this formula (many people do it differently but i find this the best way for me):

(HCl M)(x) = (pH)(total volume + x)

(1M)(x) = (1x10^-2)(40 + x)
1x= .4 + .01x
x= .4ml

Now add that to the starting volume of HCl which was 20 ml so your answer is 20.4ml

 

[jjw822]

I approched it in a different way but i think ur way is correct too

20ml + 20ml of same molarity = 40ml of neutral
since we want it to be acidic, we need to add more of acid than base.

1M (X) = 0.01M (40+X)
X = .4 + 0.01X
0.99X = 0.04
X = 0.0404

40 + 0.0404 - 20 = 20.0404

your calculation is off!!

i dont get that answer with the method i posted before... i don't know why...

 

[joonkimdds]

your calculation is off!!

i dont get that answer with the method i posted before... i don't know why...

oops
ok after fixing that, i get 20.4mL as my answer.

 

[jjw822]

I think what we have to see first is that when 20 ml of NaOH and 20 ml of HCl is mixed together, the ph will be 7. but question asks for the vol. of HCl when pH is 2. so i think we have to do (40ml)(1x10^-7)=V(1x10^-2)
and find V. and add 20ml to that.

no????

can anyone tell me why this is wrong?????

 

[klpennin]

i found this from other thread..

i dont know how to quote post from other thread so i am just copying and pasting it......


its from osimsDDS

this question is infamous...

pH of 2 = 1x10^-2

Neutralization between the HCl and the NaOH will give you 20ml, add that to 20ml you started out with and you get 40 ml...

Now you use this formula (many people do it differently but i find this the best way for me):

(HCl M)(x) = (pH)(total volume + x)

(1M)(x) = (1x10^-2)(40 + x)
1x= .4 + .01x
x= .4ml

Now add that to the starting volume of HCl which was 20 ml so your answer is 20.4ml

Yes, 20.4 is the correct answer.... thanks for that formula...that really helps, I guess i will just have to memorize that since i don't understang how to get it any other way

 

[joonkimdds]

can anyone tell me why this is wrong?????

I am working on it. Give me about 20 min...
I know that my way was the correct way of solving this problem but I know what you are thinking and not sure why it gives different answer.

I will figure it out...give me a sec

 

[jjw822]

I am working on it. Give me about 20 min...
I know that my way was the correct way of solving this problem but I know what you are thinking and not sure why it gives different answer.

I will figure it out...give me a sec

thanks !!

 

[jjw822]

Not done yet???

We probably cannot say pH is 7 when two solution is added with same vol...

I don't know...

 

[Chesnok]

lots of calculation for that problem --it's probably intended for spending more than 1 min per question and rushing thru the last questions

 

[xhamburgersamx]

NH3's molecular formula ie trigonal pryamidal because of the lone pair which creates an angle less than 120.