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Topic: stumped on a homework problem  (Read 9115 times)

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Offline bioguy2

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stumped on a homework problem
« on: November 15, 2009, 01:57:26 AM »
i was wondering if any one could help me out on this problem, i cannot figure it out. 
determine the [NH3] when HH4HCOO- (s) is dissolved in water.  the Ksp is 1.00. the pH is fixed at 10.00. 

thanks,
Taylor
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Offline Borek

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Re: stumped on a homework problem
« Reply #1 on: November 15, 2009, 04:31:56 AM »
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #2 on: November 15, 2009, 11:56:14 AM »
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Offline Borek

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Re: stumped on a homework problem
« Reply #3 on: November 15, 2009, 01:33:27 PM »
Still wrong - still no such compound (charged solid?)
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #4 on: November 15, 2009, 03:04:37 PM »
yah it is NH4(+)HCOO(-) together as one solid compound so it would become NH4+ + HCOO-
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Offline Borek

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Re: stumped on a homework problem
« Reply #5 on: November 15, 2009, 03:10:01 PM »
So, ammonium formate it is.

Are you sure about the Ksp part? Ksp is a solubility product, using it in this context - while possible - is rather unusual.
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #6 on: November 15, 2009, 03:24:50 PM »
yah that's the exact question, i thought it was unusual too, so i don't really no where to go with it
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Offline Borek

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Re: stumped on a homework problem
« Reply #7 on: November 15, 2009, 03:39:35 PM »
OK, that is not going to be very easy, but not very hard either.

Can you write Ksp equation?

Do you know how to calculate [NH3]/[NH4+] ratio from pH (and ammonia pKa or pKb)?

How is sum of concentrations [NH3]+[NH4+] related to concentration of formate anion?
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #8 on: November 15, 2009, 03:48:37 PM »
yah ksp would be 1.00 =[NH4+][HCOO-]
i am not sure how to calculate the NH3/NH4 ratio from the pH
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Offline Borek

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Re: stumped on a homework problem
« Reply #9 on: November 15, 2009, 03:53:15 PM »
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #10 on: November 15, 2009, 04:10:58 PM »
pka of NH4=9.245
so i did 10.00 = 9.245 + log (NH3/NH4+)
= .755= log (NH3/NH4)
= 5.69, idk it seems high
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Offline Borek

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Re: stumped on a homework problem
« Reply #11 on: November 15, 2009, 04:27:15 PM »
5.69 is OK.
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #12 on: November 15, 2009, 05:13:04 PM »
okay so i got that, what would you do from there?
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Offline Borek

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Re: stumped on a homework problem
« Reply #13 on: November 15, 2009, 05:23:02 PM »
Quote from: Borek on November 15, 2009, 03:39:35 PM
How is sum of concentrations [NH3]+[NH4+] related to concentration of formate anion?
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Offline bioguy2

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Re: stumped on a homework problem
« Reply #14 on: November 15, 2009, 05:28:57 PM »
not sure, this is where i am lost
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