AAMC Chem Qpack #49 explanation.

[Antiseptic11]

When 2.0 mL of 0.1 M NaOH(aq) is added to 100 mL of a solution containing 0.1 M HClO(aq) and 0.1 M NaClO(aq), what type of change in the pH of the solution takes place?

Hey ya'll, so the answer here is A. I do understand the "intuitive" way of answering this (which is the basis of the AAMC explanation) but I've been working on this for hours and cannot figure out how to do it the long way. I know it's unlikely for us to come across long calculations but I really just want to make sure that I at least know the foundation. I saw this post on reddit, but I couldn't follow what they did after they've calculated the moles of the acid & the base. I would really appreciate if someone could walk me through this.

Thank you!!

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[deleted872555]

in order to do it the long way, we would need the pKa value of HClO.

since the concentration of the weak acid and conjugate base are equal, the initial pH of the buffer soln = the pKa of HClO.

steps for the "long way":
1. figure out the amount of moles of NaOH, HClO, and NaClO after NaOH is added (so total volume is 102 mL)
2. use the equation NaOH + HClO -> NaClO + H2O for your ice table
3. use the number of moles from step 1 for the ice table
4. you will be left w/ zero moles of NaOH, a lower than initial amount of HClO, and a higher than initial amount of NaClO
5. use these numbers, along w/ the pKa, for the Henderson-Hasselbalch equation to figure out the new pH
6. compare this new pH to the initial pH (which was the pKa)

 

[Antiseptic11]

in order to do it the long way, we would need the pKa value of HClO.

since the concentration of the weak acid and conjugate base are equal, the initial pH of the buffer soln = the pKa of HClO.

steps for the "long way":
1. figure out the amount of moles of NaOH, HClO, and NaClO after NaOH is added (so total volume is 102 mL)
2. use the equation NaOH + HClO -> NaClO + H2O for your ice table
3. use the number of moles from step 1 for the ice table
4. you will be left w/ zero moles of NaOH, a lower than initial amount of HClO, and a higher than initial amount of NaClO
5. use these numbers, along w/ the pKa, for the Henderson-Hasselbalch equation to figure out the new pH
6. compare this new pH to the initial pH (which was the pKa)

Thank you!!!