pH Question

[[DDS] Samer]

I am trying to do this question from Chad's quizzes and I am having a difficult time. He did not directly cover it in his video so I think this is why I am having a hard time. Can anyone explain this? I am super lost!

If the pH is 6.6, what is the [H+] of the solution?
a. 6.6x10-7M
b. 3.1x10-8M
c. 7.4x10-6M
d. 2.5x10-7M



[H+] = 10-pH = 10-6.6 which will be between 1.0x10-7M and 1.0x10-6M which still leaves us with 2 possible answer choices (2.5x10-7M and 6.6x10-7M) so we’ll have to be more precise.

You’ll have to remember not only the [H+] values when the pH is an integer but also for the halves (.5) too. The half pH occurs when [H+] = 3.16x10-whatever as demonstrated below:

[H+] = 1x10-6M pH = 6
[H+] = 3.16x10-7M pH = 6.5
[H+] = 1x10-7M pH = 7

Since a pH of 6.6 is between 6.5 and 7, the [H+] must be between 1x10-7M and 3.16x10-7M and therefore 2.5x10-7M must be the correct answer.

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[mkhan27]

This is the way I look at it. I don't know if I can explain it the best this way. So, if you look at the question it has a pH of 6.6 (so its more basic than pH of 6 because the pH is above 6). Since its a pH of 6.6, we would use 1x10^-7 as our base point here. A 1x10^-7 [H+] means that the pH is exactly 7. Now remember when the [H+] concentration gets larger the pH gets smaller (more acidic). In the example above the [H+] are the numbers in Molarity.

Since the pH is more acidic than pH 7 (i.e. less than 7) this means that the H+ concentration has to be a little higher than 1x10^-7. We know that a pH of 6.5 is more acidic than a pH of 6.6 (because 6.5 is lower than 6.6), therefore the [H+] of 6.5 would be greater than 6.6. We also know from Chads that a pH x.5 means that its H+ concentration will be 3.16x10^(-x-1). So for example, if a solution had a pH of 4.5, then the H+ concentration would be 3.16x10^-5. In this example the x=4 and in the H+ concentration we figured out that the -x-1 => -4-1=> -5. So the H+ concentration of the solution would be 3.16x10^-5.

Okay so back to the explanation.....So we know that a pH of 6.5 is more acidic than a pH of 6.6 because 6.5 is lower than 6.6. And since 6.5 is lower than 6.6, this would mean that the H+ concentration would be higher at pH of 6.5 than pH 6.6. We also know from the formula discussed above that a pH of x.5 has a H+ concentration of 3.16x10^-x-1. So, the H+ concentration of pH 6.5 would be 3.16x10^-7. Since pH 6.6 has a lower H+ concentration (i.e. its less acidic) than 6.5, this means that the H+ concentration value has to be less than 3.16x10^-7. Let's use our elimination methods from the answer choices

A is wrong because the H+ concentration is above 3.16x10^-7 (NOTE: 6.6 is greater than 3.16). We just stated that the H+ concentration has to be less than 3.16x10^-7 because the pH is greater than 6.5 (i.e. less acidic therefore smaller H+ concentration)

B is wrong because that is referring to a pH less than 8, but greater than 7. Since the H+ concentration is above 1x10^-8, it means that its lower than pH 8 but greater than 7. Remember the greater the H+ concentration the lower the pH.

C is wrong because that is referring to pH of something less than 6. Since the H+concentration is above 1x10^-6 it means that its going to be less than pH of 6 (more acidic).

D is the correct answer based on the explanations above and from process of elimination from A, B, and C.

Let me know if you are still confused. Hopefully someone else can explain it better than I can. If you still don't get it, then definitely watch Chad's video again. He explains the concept in there when he draws the table with [H+], pH, [OH-], and pOH. That's where its all explained. If you still don't get it then hopefully someone else can explain it better or definitely use Chad's forums to ask the question. Good luck!

 

[losinov]

Isn't a quick trick to a problem setup like this is to take the exponent (in this case 7) and then put the first number (2.5) into a decimal (.25) then subtract that from the initial number (7). So you get 7-.25 is approx. 6.75
You could work backwards form the answer choicese to realized that A and D are the only viable options since they have 7 as an exponent. Choice A would bring the pH down too low to about 6.3.

 

[[DDS] Samer]

This is the way I look at it. I don't know if I can explain it the best this way. So, if you look at the question it has a pH of 6.6 (so its more basic than pH of 6 because the pH is above 6). Since its a pH of 6.6, we would use 1x10^-7 as our base point here. A 1x10^-7 [H+] means that the pH is exactly 7. Now remember when the [H+] concentration gets larger the pH gets smaller (more acidic). In the example above the [H+] are the numbers in Molarity.

Since the pH is more acidic than pH 7 (i.e. less than 7) this means that the H+ concentration has to be a little higher than 1x10^-7. We know that a pH of 6.5 is more acidic than a pH of 6.6 (because 6.5 is lower than 6.6), therefore the [H+] of 6.5 would be greater than 6.6. We also know from Chads that a pH x.5 means that its H+ concentration will be 3.16x10^(-x-1). So for example, if a solution had a pH of 4.5, then the H+ concentration would be 3.16x10^-5. In this example the x=4 and in the H+ concentration we figured out that the -x-1 => -4-1=> -5. So the H+ concentration of the solution would be 3.16x10^-5.

Okay so back to the explanation.....So we know that a pH of 6.5 is more acidic than a pH of 6.6 because 6.5 is lower than 6.6. And since 6.5 is lower than 6.6, this would mean that the H+ concentration would be higher at pH of 6.5 than pH 6.6. We also know from the formula discussed above that a pH of x.5 has a H+ concentration of 3.16x10^-x-1. So, the H+ concentration of pH 6.5 would be 3.16x10^-7. Since pH 6.6 has a lower H+ concentration (i.e. its less acidic) than 6.5, this means that the H+ concentration value has to be less than 3.16x10^-7. Let's use our elimination methods from the answer choices

A is wrong because the H+ concentration is above 3.16x10^-7 (NOTE: 6.6 is greater than 3.16). We just stated that the H+ concentration has to be less than 3.16x10^-7 because the pH is greater than 6.5 (i.e. less acidic therefore smaller H+ concentration)

B is wrong because that is referring to a pH less than 8, but greater than 7. Since the H+ concentration is above 1x10^-8, it means that its lower than pH 8 but greater than 7. Remember the greater the H+ concentration the lower the pH.

C is wrong because that is referring to pH of something less than 6. Since the H+concentration is above 1x10^-6 it means that its going to be less than pH of 6 (more acidic).

D is the correct answer based on the explanations above and from process of elimination from A, B, and C.

Let me know if you are still confused. Hopefully someone else can explain it better than I can. If you still don't get it, then definitely watch Chad's video again. He explains the concept in there when he draws the table with [H+], pH, [OH-], and pOH. That's where its all explained. If you still don't get it then hopefully someone else can explain it better or definitely use Chad's forums to ask the question. Good luck!

You're the man!