Example 5.14 BR Buffers

[BeatMCAT]

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Question is
Given the following indicators on a pH stick, what is the pH of a solution that yields: X red, Y blue and Z red
Indicator X: pka is 4.96, when deprotonated, it goes from yellow to red
indicator y: pka is 7.01; when deportonated it goes from yellow to blue
Indicator Z: pka = 8.98; when deprotonated, it goes from red to blue.

A)5
B)6
C)7
D)8

Answer is D

However, I thought it should be C. cuz indicator X would have ph higher than 1 pka value. so around 6. Indicator Y would give pH of more than 8, and indicator Z would give pH of aroun 8. So average would be close to 7. So why is the answer 8? Thanks!!!

 

[yingao]

Question is
Given the following indicators on a pH stick, what is the pH of a solution that yields: X red, Y blue and Z red
Indicator X: pka is 4.96, when deprotonated, it goes from yellow to red
indicator y: pka is 7.01; when deportonated it goes from yellow to blue
Indicator Z: pka = 8.98; when deprotonated, it goes from red to blue.

A)5
B)6
C)7
D)8

Answer is D

However, I thought it should be C. cuz indicator X would have ph higher than 1 pka value. so around 6. Indicator Y would give pH of more than 8, and indicator Z would give pH of aroun 8. So average would be close to 7. So why is the answer 8? Thanks!!!

Because when pH is higher than the pKa, it would deprotonated the molecule, if pH is lower than the pKa it would not deprotonate.
so for the question, X turns red mean it is in its deprotonated form and therefore pH must be higher than its pka 4.95, and so does Y. But not Z. Z is not in its deprotonated form, so the pKa of Z is higher than pH of solution so the pH is greater than 7.01 but less than 8.98, so the best answer is D

 

[BerkReviewTeach]

• Indicator X: pKa is 4.96, when deprotonated, it goes from yellow to red

This tells us that pH is greater than 5.96, meaning it could be as low as 6 and as high as 14+.

• Indicator y: pKa is 7.01; when deportonated it goes from yellow to blue

This tells us that pH is greater than 8.01, meaning it could be as low as 8 and as high as 14+.

• Indicator Z: pKa = 8.98; when deprotonated, it goes from red to blue.

This tells us that pH is less than 7.98, meaning it could be as high as 8 and as low as 0-.

The three facts are that 1) pH >or= 6, 2) pH >or= 8, and 3) pH <or= 8.

Only pH of 8 (from the choices) fits the three restrictions. A pH of 7 would make Indicator Y green (half blue and half yellow).

 

[BeatMCAT]

Ohh I see. thanks a lot!

 

[californiamed]

I'm still confused by this question... Anyone understand it? Why do you add 1 to pKa of X, but -1 to pkA of Z? PLEEEASE help me on my last titration question 🙂

Given the following indicators on a pH stick, what is the pH of a solution that yields: X red, Y blue and Z red
Indicator X: pka is 4.96, when deprotonated, it goes from yellow to red
indicator y: pka is 7.01; when deportonated it goes from yellow to blue
Indicator Z: pka = 8.98; when deprotonated, it goes from red to blue.

A)5
B)6
C)7
D)8

 

[LoLCareerGoals]

• Indicator X: pKa is 4.96, when deprotonated, it goes from yellow to red

This tells us that pH is greater than 5.96, meaning it could be as low as 6 and as high as 14+.

• Indicator y: pKa is 7.01; when deportonated it goes from yellow to blue

This tells us that pH is greater than 8.01, meaning it could be as low as 8 and as high as 14+.

• Indicator Z: pKa = 8.98; when deprotonated, it goes from red to blue.

This tells us that pH is less than 7.98, meaning it could be as high as 8 and as low as 0-.

The three facts are that 1) pH >or= 6, 2) pH >or= 8, and 3) pH <or= 8.

Only pH of 8 (from the choices) fits the three restrictions. A pH of 7 would make Indicator Y green (half blue and half yellow).

I don't understand this either. Why do you keep adding/subbing 1pH sized margins from everything? I just constructed 3 inequalities right at the boundaries of indicator colors and only D fit the intersection of all 3. I just assumed deprotonated meant [H+] is reduced (pH increased). Pretty obv they made one of the boundaries 7.01 to allow elimination of C without any margin assumption.
Does deprotonated mean have its solution increased by 1pH? Seams so arbitrary.

 

[Jepstein30]

Question is
Given the following indicators on a pH stick, what is the pH of a solution that yields: X red, Y blue and Z red
Indicator X: pka is 4.96, when deprotonated, it goes from yellow to red
indicator y: pka is 7.01; when deportonated it goes from yellow to blue
Indicator Z: pka = 8.98; when deprotonated, it goes from red to blue.

A)5
B)6
C)7
D)8

Answer is D

However, I thought it should be C. cuz indicator X would have ph higher than 1 pka value. so around 6. Indicator Y would give pH of more than 8, and indicator Z would give pH of aroun 8. So average would be close to 7. So why is the answer 8? Thanks!!!

I don't understand this either. Why do you keep adding/subbing 1pH sized margins from everything? I just constructed 3 inequalities right at the boundaries of indicator colors and only D fit the intersection of all 3. I just assumed deprotonated meant [H+] is reduced (pH increased). Pretty obv they made one of the boundaries 7.01 to allow elimination of C without any margin assumption.
Does deprotonated mean have its solution increased by 1pH? Seams so arbitrary.

Not sure why either because well, you can solve just by looking at the pKa values but I think it has to do with the indicator being good for 1 ph unit above and below its pKa.

Also, the indicator (or anything) would be deprotonated if its pKa is LOWER than the pH of the solution.

As in, if the pKa of substance X is 3. Any pH above 3 would mean it is in a relative basic environment and will be deprotonated.

Likewise, any pH below a pKa would mean it is protonated.

 

[AreaE]

I think we should all forget this add one minus one stuff. There are pH strips that exist that have a pKa of 7.0 and will turn a certain color, let's say blue, if the solution is above 7.0 (basic) or turn red if the solution is below 7.0 (acidic). There is not a pH at which the indicator won't work at. In this case, there are three indicators so that the user can better estimate the pH of the solution. Everyone who has used the inequalities is doing the exact right thing, but don't get hung up on this plus one minus one, it is not the case with pH strips.

Hope this helps

 

[ilovemedi]

Sorry, also confused by this. so X: since it's deprotonated, pH>Pka (4.96). For X, same thing - ph>Pka (7.01) since it's deprotonated.. HOWEVER for Z, it also says deprotonated, so shouldn't pH>8.98? which isn't even an answer choice!

 

[Sandstorm]

Sorry, also confused by this. so X: since it's deprotonated, pH>Pka (4.96). For X, same thing - ph>Pka (7.01) since it's deprotonated.. HOWEVER for Z, it also says deprotonated, so shouldn't pH>8.98? which isn't even an answer choice!

Z is not deprotonated. It is red which means protonated

 

[ilovemedi]

Z is not deprotonated. It is red which means protonated

this might seem dumb for me, but the question clearly says "when protonated it goes from red to BLUE".. so it is deprotonated, and it is blue? how are you getting that it isn't deprotonated and it's red?

 

[Sandstorm]

this might seem dumb for me, but the question clearly says "when protonated it goes from red to BLUE".. so it is deprotonated, and it is blue? how are you getting that it isn't deprotonated and it's red?

When deprotonated... So when it loses the hydrogen.... It turns blue.

When protonated.. So when it still has the hydrogen... It is red.

The question stem says z is red. Therefore it is protonated. Therefore the ph has not gone high enough(become basic enough) to deprotonate it. So the ph must be less than the pka for this indicator.