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### Topic: buffer question I cant figured out  (Read 7250 times)

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#### ziccoh

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##### buffer question I cant figured out
« on: August 05, 2008, 01:25:54 PM »
1st problem
the solubility of PbCL(s) <--> Pb(ag) in water is 4.4 g/L at 25C. wgat is the value of Ksp for PbCL?.. molar mass of PbCL=278.1 g/mol..

secound question.
a formic acid buffer contains 0.50 M HCOOH and 0.50 M HCOONa is prepared before the titration (Ka HCOOH=1.8x10^-4) what will be the pH be after the addition of 10.0 M NaOH to the 100.00 buffer?
0 seconds ago - 4 days left to answer.

I missed  this classes and I dont know dwhat to do... bonus questions in a HW

#### tamim83

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##### Re: buffer question I cant figured out
« Reply #1 on: August 05, 2008, 01:40:39 PM »
Quote
the solubility of PbCL(s) <--> Pb(ag) in water is 4.4 g/L at 25C. wgat is the value of Ksp for PbCL?.. molar mass of PbCL=278.1 g/mol..

What is the "net ionic" equation and Ksp expression for the equilibrium?  (do you mean PbCl2?)

#### Borek

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##### Re: buffer question I cant figured out
« Reply #2 on: August 05, 2008, 03:34:33 PM »
For the second question you need Henderson-Hasselbalch equation.
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#### ziccoh

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##### Re: buffer question I cant figured out
« Reply #3 on: August 05, 2008, 03:58:34 PM »
i GOT THE ANSWER FOR THE 2 QUESTION .. I WANT SOMEONE TO DO IT SEE IF I GOT THE SAME ANSWER, FOR 1 ONE YES I MEAN  PbCl2

#### Borek

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##### Re: buffer question I cant figured out
« Reply #4 on: August 05, 2008, 05:07:17 PM »
Show your answer and I will tell you if it is right.

You really think people who know how to calculate buffer pH are that dumb?
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#### ziccoh

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##### Re: buffer question I cant figured out
« Reply #5 on: August 05, 2008, 05:30:47 PM »
I want to learn how to do this problems i just having a hard time, i always work my problems ..

for my first questions I had 2 answer the way i worked it

1.6x10^-5 and 4.0x10^-6

and for the second question i got

3.67, 3.81, 3.92  i dont know if im doing it right or not

#### Borek

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##### Re: buffer question I cant figured out
« Reply #6 on: August 05, 2008, 06:05:55 PM »
If you have several results, you must be doing it wrong, only one answer can be correct. Show what you did and how.
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#### ziccoh

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##### Re: buffer question I cant figured out
« Reply #7 on: August 05, 2008, 08:50:21 PM »
since the forma solubility is 4.4 grams/liter then that is 4.4 grams/278.1 grams/mole = 0.01577 moles/liter of PbCL2 which means
1.58X 10^-2 moles/liter for Pb+2 and 2 X 1.58 X 10^-2 moles /liter or Cl^-2 so the Ksp = [1.58X 10^-2 ][3.16 X 10^-2]^2 = 15.78 X 10^-6
= 1.58 X10^-5

I think this is my corrct answer..  1.6x10^-5

#### ziccoh

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##### Re: buffer question I cant figured out
« Reply #8 on: August 05, 2008, 08:57:44 PM »
im trying to use the HH equetion, but I dont think im doing it right for the second question, also im using the before and after table

#### ziccoh

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##### Re: buffer question I cant figured out
« Reply #9 on: August 05, 2008, 09:39:59 PM »
if i have MgCO3 Ksp 3.5x10^-8
and       NiCO3 Ksp  1.3x10^-7

what subtance has greatest solubility in water?

if the Ksp is larger  is more soluble right?  thank you for yur help

#### vhpk

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##### Re: buffer question I cant figured out
« Reply #10 on: August 05, 2008, 11:42:24 PM »
if i have MgCO3 Ksp 3.5x10^-8
and       NiCO3 Ksp  1.3x10^-7

what subtance has greatest solubility in water?

if the Ksp is larger  is more soluble right?  thank you for yur help
For comparing the solubilty of subtances, you need to calculate its solubility, not base on Ksp. Can you show me your calculations?
Furthermore, your statement doesn't seem to be right in all cases. If 1 subtance has a larger Ksp but its solubility is smaller ( I mean the molar concentration of its constituent) can't solube better in water
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#### ziccoh

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##### Re: buffer question I cant figured out
« Reply #11 on: August 06, 2008, 12:00:38 AM »
I have another question, if a chemical reaction has a AS(sys) > 0

the disorder of the system increses and the reaction is spontaneus, I know is the 2 law thermodinamics but im really confused about this...

#### Borek

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##### Re: buffer question I cant figured out
« Reply #12 on: August 06, 2008, 03:59:02 AM »
im trying to use the HH equetion, but I dont think im doing it right for the second question, also im using the before and after table

Usually you don't mix them in one question. Assume reaction (neutralization or protonation) went to completion, use simple stoichiometry to calculate concentration of acid and conjugated base, put into HH equation.
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#### vhpk

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##### Re: buffer question I cant figured out
« Reply #13 on: August 06, 2008, 05:46:29 AM »
I have another question, if a chemical reaction has a AS(sys) > 0

the disorder of the system increses and the reaction is spontaneus, I know is the 2 law thermodinamics but im really confused about this...
As for me, to predict whether the reaction can take place, we should base on deltaG, because it depends on dH and dS also. Hence, based on one aspect, we can't give the correct answer
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