HW 12.77 (Calculating pH of alanine)

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704564167
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Joined: Fri Sep 25, 2015 3:00 am

HW 12.77 (Calculating pH of alanine)

Postby 704564167 » Tue Dec 01, 2015 1:26 pm

12.77 A 3.38 sample of the sodium salt of alanine, NaCH3CH(NH2)CO2 is dissolved in water and then the solution is diluted to 50.0mL. For alanine, Ka1=4.57 x 10-3, Ka2=1.30 x 10-10. What is the pH of the resulting solution?

The answer key uses the equation pH = 1/2(pKa1 + pKa2) but I don't recall learning this before. Where does this equation come from and when is it appropriate to use it?

Jessica Manzano 1B
Posts: 23
Joined: Fri Sep 25, 2015 3:00 am

Re: HW 12.77 (Calculating pH of alanine)

Postby Jessica Manzano 1B » Tue Dec 01, 2015 2:30 pm

This equation was discussed by my TA in discussion section and this is how I understood it:
On the titration curve of a polyprotic acid (an acid that can donate more than one H+ proton and has different Ka values for each proton lost), there are midpoints on the curve where pH=pKa. These midpoints are located about the same distance above or below the stoichiometric point on the curve where moles of base added equals moles of acid sample. Because these midpoints where pH=pKa are located about the same distance away from the stoichiometric point, adding together pKa1 and pKa2 and dividing the sum by 2 should result in the pH of the resulting solution.

Natalie Yakobian
Posts: 45
Joined: Fri Sep 25, 2015 3:00 am

Re: HW 12.77 (Calculating pH of alanine)

Postby Natalie Yakobian » Tue Dec 01, 2015 5:17 pm

HI,
I was wondering how do you know that it should be thought of as a titration problem?

Silvia Huang 3D
Posts: 30
Joined: Fri Sep 25, 2015 3:00 am

Re: HW 12.77 (Calculating pH of alanine)

Postby Silvia Huang 3D » Tue Dec 01, 2015 9:39 pm

Hi, I was wondering if this problem can be done by Ka(overall) = Ka1*Ka2, calculating the initial concentration first then used ka=x^2/ [initial] ?? I got 6.22 :( 0.11 off from the correct answer


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