GChem Buffers Question 96

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leathersofa

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I get why the answer is A, but my question is actually on choice D. does this mean that the pH stick can't estimate well for values beyond 5.37? i feel that it still can estimate pH well for values beyond 5.37 because the color blend range still applies for pH from 6.5 to 9..?


(see image in url above)

96. Which of the following statements is INVALID?
A. A solution that turns bromthymol blue to blue would turn Phenolphthalein to magenta.
B. A solution cannot show two green marks on the pH stick.
C. A solution that turns methyl red to yellow would turn bromcresol green to blue.
D. The pH stick can estimate pH best when that value falls between 4.21 and 5.37.

Answer: A

Choice D is a valid statement. When the pH of the solution falls between 4.21 and 5.37, it falls into the color blend range of two separate indicators. As a result, the pH can be approximated with twice the accuracy.
 
wouldn't it be equally as good as a ph of, say, 6 (that could also be tested for by two separate indicators)?

.. the question says which one is invalid. I guess I'm confused could you clarify
 
okay, the answer is A and here is the full answer explanation.

A solution that turns blue with bromthymol blue has a pH greater than 7.78 (pKa = 6.68, so the pure blue color starts at 6.68 + 1). A pH greater than 7.78 does not guarantee that the pH is greater than 8.79, the pKa for phenolphthalein. This means that the solution may or may not turn magenta with phenolphtihatein. Choice A is invalid. If a solution tums green with bromcresol green, the pH is roughly 4.37. if a solution turns bromthymol blue green, the pH is roughly 6.68. The pH cannot simultaneously be 4.37 and 6.68, so the stick cannot simultaneouily hurre two green marks. Choice B is a valid statement. A solution that turns methyl red yellow has a pH greater than 6.21 (pKa = 5.21,so the pure yellow color starts at 5.21' + 1). A pH greater than 6.2a guarantees that the pH is greater than 4.31, the pKa for bromcresol green' This means that the solution must turn blue with bromcresol green. Choice C is a valid statement. When the pH of the solution falls between 4.27 and5.37, it falls into the color blend range of two separate indicators. As a result, the pH can be approximated with twice the accuracy. Choice D is a valid statement.

I just do not understand their reasoning for choice D. Does the pH stick not estimate well for values beyond 5.37?
 
okay, the answer is A and here is the full answer explanation.

A solution that turns blue with bromthymol blue has a pH greater than 7.78 (pKa = 6.68, so the pure blue color starts at 6.68 + 1). A pH greater than 7.78 does not guarantee that the pH is greater than 8.79, the pKa for phenolphthalein. This means that the solution may or may not turn magenta with phenolphtihatein. Choice A is invalid. If a solution tums green with bromcresol green, the pH is roughly 4.37. if a solution turns bromthymol blue green, the pH is roughly 6.68. The pH cannot simultaneously be 4.37 and 6.68, so the stick cannot simultaneouily hurre two green marks. Choice B is a valid statement. A solution that turns methyl red yellow has a pH greater than 6.21 (pKa = 5.21,so the pure yellow color starts at 5.21' + 1). A pH greater than 6.2a guarantees that the pH is greater than 4.31, the pKa for bromcresol green' This means that the solution must turn blue with bromcresol green. Choice C is a valid statement. When the pH of the solution falls between 4.27 and5.37, it falls into the color blend range of two separate indicators. As a result, the pH can be approximated with twice the accuracy. Choice D is a valid statement.

I just do not understand their reasoning for choice D. Does the pH stick not estimate well for values beyond 5.37?

Indicators typically have a color band change in the range of pKa +/- 1. Now the passage says:
A pH stick is a device that can approximate the pH of an aqueous solution by reference to a color blend band.
The pH range of 4.21 and 5.37 would mean that the color blend band of both bromcresol green and methyl red would be changing from their respective protonated color to deprotonated colors. The pH stick would have two indicators it could reference to provide a more accurate pH reading. The 4.21 comes from the methyl red pKa minus 1 and the 5.37 comes from the bromcresol green pKa plus 1.

This isn't to say that the pH can't estimate well for values beyond 5.37, it's talking about how two indicators will provide a more accurate depiction of the pH than just one. Of the four indicators in table 1, let's look at what the ranges would be for the other two combinations (using the +/- of the pka):

1) Bromcresol Green & Methyl Red: 4.21-5.37 [This is the one from the question]
2) Methyl Red & Bromthymol Blue: 5.78-6.21
3) Bromthymol Blue & Phenolphthalein: 7.78-7.79

Methyl Red's pka is 5.21 and Bromthymol Blue's is 6.78... notice how their pka's are outside of the range. Again for the third, we know Bromthymol's pKa, and Phenolphthalein is 8.79, which is also outside the ranges.

Now since the pH stick is referencing the color blend band, it'll give a more accurate representation of when the indicators are in the midst of their color change. In scenario 2, the methyl red is predominantly yellow (even at the lowest part of that range), while bromthymol blue would only begin changing colors. In scenario 3, it's even worse, as the bromothymol blue would be completely finished changing colors, while phenolphthalein would only be beginning.

So answer choice D is valid for the indicators being used by the pH stick. That range of pH would produce the most accurate pH because the pH stick could reference the color band blend of two indicators instead of one. Not to say that the pH stick couldn't calculate the pH of a solution of a different pH (outside of that range)... it would just be less accurate.

Hope that helps.
 
Indicators typically have a color band change in the range of pKa +/- 1. Now the passage says:

The pH range of 4.21 and 5.37 would mean that the color blend band of both bromcresol green and methyl red would be changing from their respective protonated color to deprotonated colors. The pH stick would have two indicators it could reference to provide a more accurate pH reading. The 4.21 comes from the methyl red pKa minus 1 and the 5.37 comes from the bromcresol green pKa plus 1.

This isn't to say that the pH can't estimate well for values beyond 5.37, it's talking about how two indicators will provide a more accurate depiction of the pH than just one. Of the four indicators in table 1, let's look at what the ranges would be for the other two combinations (using the +/- of the pka):

Thank you so much for your reply, it really makes sense but i have one more question

the pH range is 4.21 to 5.37. Why is it pka + 1 for bromcresol green and pka - 1 for methyl red. Why can't it just be pka +- 1 for both, making the pH range 3.37 to 6.21?
 
Thank you so much for your reply, it really makes sense but i have one more question

the pH range is 4.21 to 5.37. Why is it pka + 1 for bromcresol green and pka - 1 for methyl red. Why can't it just be pka +- 1 for both, making the pH range 3.37 to 6.21?

Well you could do a pH range from 3.37 to 6.21, but that's a pretty broad range. Doing the plus one of the lower and minus one of the upper gives you a tighter range that still overlaps the color band blend of the two indicators. Imagine if you had a solution with pH in the outer part of that range... like 5.5 or something, the color band blend of bromcresol green would be almost useless for the pH stick in helping calculate pH.
 
Okay, what do you mean by color band blend?
for instance, let's say a solution has ph = 5, then would the color band blend be purple since bromcresol green is deprotonated (blue) and methyl red is protonated (red)?

is color band blend different from color range?
 
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Okay, what do you mean by color band blend?
for instance, let's say a solution has ph = 5, then would the color band blend be purple since bromcresol green is deprotonated (blue) and methyl red is protonated (red)?

is color band blend different from color range?

FG15_13.JPG


Whenever there is a change and pH, the indicator doesn't all change from one color to the other. Some are protonated and some are deprotonated, creating the affect you see above. Now I'm not quite sure how the pH indicator works (in the sense that is it detecting color or relative concentrations of protonated and deprotonated indicator, which would be the same thing).
 
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