AAMC 9 PS #19 : Acids and Base calculations

[ihatebluescrubs]

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The question: 2mL of 0.1M NaOH added to 100mL of soln with 0.1M HCLO and 0.1M NaCLO. It asks for the qualitative change in pH of the solution (not a exact number). In the passage they tell us the Ka of HCLO is 3.2E-8

Choices:
slight increase
slight decrease
significant increase
significant decrease



What I don't get is how Kaplan explains why the answer is a slight increase in pH. (I thought it was a significant increase in pH.) They said that "a buffer works best when the acid and conj base are at equal concentrations and when the added base is small in comparison".

How can I assume that HClO is a buffer? Did they get this from the Ka? Also, what is considered a "small amount of base"?

How can I solve this using math?

 

[ihatebluescrubs]

sorry, forgot to add why I thought this was a significant increase in pH:

Since HA = A- (0.1=0.1M) and that the Ka of HClO is 3.2E-8, I assumed that H+ is 3.2E-8 M since Ka = ( A- * H+)/ (HA). A- and HA cancels out so the Ka = H+ conc. Is that even right?

The added NaOH has a total of 2E-3M OH- and since thats larger than the H+ of 3.2E-8 M, there will be a drastic shift in pH.

What part of that thought process is wrong?

Thanks!

 

[gwjib04]

What I don't get is how Kaplan explains why the answer is a slight increase in pH. (I thought it was a significant increase in pH.)

How can I assume that HClO is a buffer? Did they get this from the Ka? Also, what is considered a "small amount of base"?

How can I solve this using math?

You really don't have to go as far as using math.

As you read the question I would recommend imagining that you are setting this up in real life.

Adding 2 mL into 100 mL is considered a small amount of base because its only 2% of the total volume. The pH will increase because NaOH is a strong base therefore it raises the pH. "Slight" because 1. it is a really small volume 2. being added to a buffer which absorbs H⁺

We know its a buffer by inspecting HClO and NaClO. Whenever I see Na, I immediately pretend it isn't there. So we have HClO and ClO^- in the same solution. Written like that, they should stand out as a conjugate acid/base pair because their chemical formulas only vary by 1 proton.

Since we have a buffer we use Henderson-Hasselbalch

pH = pKa + log (A^-/HA)

pH = 3.28x10^-8 + log₁₀1

pH = 7.7 + 0

 

[dsoz]

You don't even need the Henderson-Hasselbalch. Know weak acid and the salt of a weak acid combo (HClO + NaClO) form a buffer. It will resist changes in pH. Adding a base will increase the pH, but adding it to a buffer will make the change small. Also, as noted above, adding 2 mL to 100 mL will not significantly change the ion concentrations.

All this can be done conceptually, without the use of any formulas. On the real test, you need to know HOW doing something to a substance will affect it. You don't have time for complex calculations.

dsoz

 

[postbacpremed87]

The question: 2mL of 0.1M NaOH added to 100mL of soln with 0.1M HCLO and 0.1M NaCLO. It asks for the qualitative change in pH of the solution (not a exact number). In the passage they tell us the Ka of HCLO is 3.2E-8

Choices:
slight increase
slight decrease
significant increase
significant decrease



What I don't get is how Kaplan explains why the answer is a slight increase in pH. (I thought it was a significant increase in pH.) They said that "a buffer works best when the acid and conj base are at equal concentrations and when the added base is small in comparison".

How can I assume that HClO is a buffer? Did they get this from the Ka? Also, what is considered a "small amount of base"?

How can I solve this using math?

You made this question harder than it had to be. You quickly find out that this is a buffer system of hypochlorous acid and it's base...weak acid and weak base.....thus if a strong base is added then it will resist this and increase only slightly. If a strong acid was added it would decrease slightly. 15 seconds and move on.