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TBR Chem Ch. 5 P4 Q15

Discussion in 'MCAT Study Question Q&A' started by brood910, Jan 14, 2014.

  1. brood910

    brood910 2+ Year Member

    Jul 21, 2011
    Which of the following indicators would be BEST for the titration of p-nitrophenol (pka 7.2) by NaOH?
    A. Penolphthalein (pH range of color change is 8 and 9.6)
    B. Thymol blue - pH 1.2 ~ 2.8
    C. Methyl Red - pH 4.6 ~ 5.8
    D. Bromthymol Blue pH 6 ~ 7.6

    Their answer says "To predict the pH near equivalence point, which is what you are doing when you choose an indicator, you should use the Henderson-Hasselbalch equation. At 2 pH units beyond the Pka of the acid, the ratio of conjugate base is 100:1, which means that the rxn is almost at equivalence. This is the point at which the indicator should start to show some color change. In this question, the pka for the weak acid is 7.2. The best indicator is therefore around pH = 9.2, which is between 8 ~ 9.6"

    So, my reasoning is that since a weak acid is titrated by a strong base, we know that pH at eq will be larger than 7. Also, we know that pH = 7.2 at half point. That means pH at eq should be larger than 7.2 as well. So, we can eliminate B and C. it is pretty "easy" to assume that A is the best answer, but how can we tell if the pH at eq is lower/higher than 8?
    Last edited: Jan 14, 2014
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  3. brood910

    brood910 2+ Year Member

    Jul 21, 2011
    I kinda figured this out but not fully.
    So, they are just assuming that A-:HA ratio is 100:1 as there is almost solely A- at eq. So, if we use the equation pH = pka + log[A-]/[HA], we get pH = pka - 2. Then we get pH of 5.2... But they got 9.2 WHAT DID I DO WRONG?
  4. Jepstein30

    Jepstein30 2+ Year Member

    Jun 29, 2011
    Log[A-]/[HA] = log(100/1) = log 100 = 2

    so should be pH = pKa + 2 = 7.2 + 2 = 9.2

    You made that negative for some reason
    brood910 likes this.
  5. brood910

    brood910 2+ Year Member

    Jul 21, 2011
    OMG.... My brain must have been turned off by studying for 5983453458 hours straight.
    I feel so stupid. Thanks a lot.
  6. Odi


    Jan 7, 2014
    Why is the 100:1 ratio important?

    Is it arbitrarily chosen over let's say 10:1 or 2:1? Because the latter ratios are more indicative of the ratio at the EQ point which is 1:1.
  7. brood910

    brood910 2+ Year Member

    Jul 21, 2011
    Yes, it is arbitrarily chosen. Some other passages in TBR directly said "we can assume 1000:1...." too. So they just made an arbitrary ratio to show you that [A] predominates at eq.
    Odi likes this.

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