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Topic: 2 questions- vapor pressure and pH  (Read 2856 times)

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Offline orgo814

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2 questions- vapor pressure and pH
« on: November 06, 2013, 01:45:05 AM »
1) What is the pH of a solution containing 0.2 g/L sodium carbonate and 0.1 g/L sodium hydrogen carbonate? (Ka1 = 4.1 x 10^-7, Ka2 = 5.6 x 10^-11)

I know to use the henderson-hasselbach equation but I'm unsure of which Ka to use for my pKa. Any insight would be great. I would assume Ka1?

2) What is the standard enthalpy of a reaction for which the equilibrium constant increases by a factor of ten when the temperature is increased from 273 K to 298 K?

I know the equation is lnK2/K1 = -delta H/R (1/T2 - 1/T1). I'm just confused on the wording "increased by a factor of ten". Would I put something like 30 as my K2 and 20 as my K1? Or would I just do the natural log of 10?

Offline Borek

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Re: 2 questions- vapor pressure and pH
« Reply #1 on: November 06, 2013, 02:38:21 AM »
I know to use the henderson-hasselbach equation but I'm unsure of which Ka to use for my pKa. Any insight would be great. I would assume Ka1?

You have CO32- and HCO3- present in the solution - which Ka value described their equilibrium?

Quote
I know the equation is lnK2/K1 = -delta H/R (1/T2 - 1/T1). I'm just confused on the wording "increased by a factor of ten". Would I put something like 30 as my K2 and 20 as my K1? Or would I just do the natural log of 10?

It means K2=10K1.
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Offline orgo814

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Re: 2 questions- vapor pressure and pH
« Reply #2 on: November 06, 2013, 08:26:37 AM »
I would say ka1 because it only went through one step?

Offline Borek

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Re: 2 questions- vapor pressure and pH
« Reply #3 on: November 06, 2013, 09:01:15 AM »
Write both dissociation steps, write Ka for each one. Which contains concentrations of the substances present in the solution?
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Offline orgo814

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Re: 2 questions- vapor pressure and pH
« Reply #4 on: November 06, 2013, 11:00:23 AM »
CO3(2-) + H2O yields HCO3-. Is that not the first step?

Offline Borek

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Offline orgo814

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Re: 2 questions- vapor pressure and pH
« Reply #6 on: November 06, 2013, 12:04:09 PM »
Well then I'm confused as to what it is

Offline orgo814

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Re: 2 questions- vapor pressure and pH
« Reply #7 on: November 06, 2013, 12:05:37 PM »
Oh HCO3- yields CO32- so second ionization. First would be starting with h2co3

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Re: 2 questions- vapor pressure and pH
« Reply #8 on: November 06, 2013, 12:13:12 PM »
Exactly.
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