pH and pOH calculations

[deleted678432]

When reviewing Chad's GC, I have a very hard time calculating pOH and pH in my head from their [H+] or [OH-]... and vice versa. Does anybody have a good explanation for this? The biggest thing is if, for example, I was given an [OH-] of 7.7x10^-5. I know that the pOH has to be around 5, but then it would be lower due to the increase of OH- from 1.0, right? But I just dont know if it would be lower and closer to 4 or to 5. The flip-flop of higher decimals converting to lower pOH/pH confuses me. Any help?

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[MickeyMug]

Well here's how I think about it, you know that 7.7x10^-5 is between 1x10^-4 and 1x10^-5 ( pOH 4-5). 1x10^-4 is the same as saying 10x10^-5.
7.7x10^-5 is much closer to 10x10^-5 than 1x10^-5, which means it is closer to 1x10^-4.
Therefore your pOH is going to be much closer to 4 than 5, I would estimate it at 4.2. It doesn't have to be exact because most of the time the multiple choice answers are pretty spread apart. If your pOH is 4.2 then your pH = 14 - 4.2 => 9.8.
Hope this helps!

 

[deleted678432]

Well here's how I think about it, you know that 7.7x10^-5 is between 1x10^-4 and 1x10^-5 ( pOH 4-5). 1x10^-4 is the same as saying 10x10^-5.
7.7x10^-5 is much closer to 10x10^-5 than 1x10^-5, which means it is closer to 1x10^-4.
Therefore your pOH is going to be much closer to 4 than 5, I would estimate it at 4.2. It doesn't have to be exact because most of the time the multiple choice answers are pretty spread apart. If your pOH is 4.2 then your pH = 14 - 4.2 => 9.8.
Hope this helps!

Ah okay awesome! I'll start thinking about it like this!

 

[promethion]

Something mentioned in Chad's videos that has helped me TREMENDOUSLY is applying the number 3.16 to the above method - you're still estimating, but it's a little more accurate especially if the value you are given is close to 3.16. Basically, given your original [OH-] value of 7.7 x 10^-5, see where it falls between 1.0 x 10^-5, 3.16 x 10^-5, and 1.0 x 10^-4. A value of 3.16 x 10-5 correlates to a pOH of 4.5. Since 7.7 is greater than 3.16, you know it will be between 4 and 4.5. If you were given, say, 2.2 x 10^-5, that falls between 1.0 x 10^-5 and 3.16 x 10^-5 which means your pOH value would be between 4.5 and 5. 3.16 will always correlate with the 0.5 value between 2 pH/pOH values if that helps! Hope that makes sense.

 

[deleted678432]

Something mentioned in Chad's videos that has helped me TREMENDOUSLY is applying the number 3.16 to the above method - you're still estimating, but it's a little more accurate especially if the value you are given is close to 3.16. Basically, given your original [OH-] value of 7.7 x 10^-5, see where it falls between 1.0 x 10^-5, 3.16 x 10^-5, and 1.0 x 10^-4. A value of 3.16 x 10-5 correlates to a pOH of 4.5. Since 7.7 is greater than 3.16, you know it will be between 4 and 4.5. If you were given, say, 2.2 x 10^-5, that falls between 1.0 x 10^-5 and 3.16 x 10^-5 which means your pOH value would be between 4.5 and 5. 3.16 will always correlate with the 0.5 value between 2 pH/pOH values if that helps! Hope that makes sense.

Yeah I use this too! I think I may have copied down wrong in my notes that it would be 5.5 rather than 4.5 which kinda messed me up


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[uhds]

When reviewing Chad's GC, I have a very hard time calculating pOH and pH in my head from their [H+] or [OH-]... and vice versa. Does anybody have a good explanation for this? The biggest thing is if, for example, I was given an [OH-] of 7.7x10^-5. I know that the pOH has to be around 5, but then it would be lower due to the increase of OH- from 1.0, right? But I just dont know if it would be lower and closer to 4 or to 5. The flip-flop of higher decimals converting to lower pOH/pH confuses me. Any help?

If the pH is 4.9, what is the [OH-] of the solution? Has anyone seen this problem from Chad's videos, 5.6-pH Scale and pOH calculations?

The answer choices are:
6.9x10^-9M
8.6x10^-8M
1.6x10^-10M
7.9x10^-10M (correct answer)

The solution at the bottom of the page, makes me even more confused. Shouldn't the answer be 1.6x10^-10M?

 

[Cool Beans]

1) If the pH is 4.9, then the pOH is 9.1. This is close to 9.0 and if the pOH were 9.0 then the [OH-] would be exactly 1.0x10^-9.

2) There is an inverse relationship (not inversely proportional though) between pOH and [OH-]...as one increases the other decreases.
Therefore, since the pOH is slightly higher than 9.0, the [OH-] will be slightly lower than 1.0x10^-9.

So the question boils down to which of these answer choices is just slightly lower than 1.0x10^-9. 1.6x10^-10 is a little higher than 1.0x10^-9 rather than a little lower and can be eliminated.
But 7.9x10^-10 is indeed a little lower than 1.0x10^-9.

Note that the comparison is easier if you realize that 1.0x10^-9 is equal to 10.0x10^-10. This means you're looking for an answer choice with a power of 10^-10 but with a coefficient that is a a little lower than 10 (like 7.9).

Hope this helps!