Having difficulty with pH log approximations

[StudentDentistsRock]

---

Members don't see this ad.

Hey everyone,

I'm doing fine on material so far, but the only math problem I am facing with gen chem is negative log approximations involved in pH calculations. I (almost) always get them wrong. I watched Chad's over and over again, tried tackling the problems but I spend too much time trying to figure them out (if I end up figuring them out that is). Any tricks or methods you guys know for this?

I have more difficulty with things like log (10^-3.2) as opposed to something like log (3.4x10^-4). But I still have difficulty with both.

Edit 5/22/2017:
thanks everyone! I believe I got the hang of it!

 

[rk1229]

How are you going about solving them? For the second one, how it made sense to me was that the pH for -log (1.0x10^anything) the answer will be exactly the positive exponent. But for something like -log (3.4x10^-4) anything above (1) in this case it's (3.4) the answer is the positive exponent (4) minus a little because the number is bigger than one. So it's like ~3.7 and the higher the number is away from 1, the lower the answer is. Does that make any sense? Lol. That's all assuming the calculation is directly for an acid. If it's the OH or Kb concentration or anything you have to go through the flow chart of turning pOH into pH. (14-pOH = pH).

Someone jump in if I totally butchered that.


Sent from my iPhone using SDN mobile app

 

[deleted738762]

Lets say [H+]= .00064 whats the pH?

You know it's 6.4 x 10^-4. So I write 1x10-3 above that number and 1x10-4 below that number. Which have a pH of 3 and 4. Also know that 3.14x10 is usally half. So 3.14x10-4=3.5 (Memorize that # it helps)

So like:
1x10-3 = 3
6.4x10-4 = ?
3.14x10-4= 3.5
1x10-4=4
Based off that I know 6.4x10-4 is greater than 3.14x10-4 so it would be between 3 and 3.5 so I'll guess like 3.3 for the pH. That's all there is to it.

That's how I did it and got all of the problems right on chad's quizzes. You start getting at faster at it and only takes a couple of seconds once you get the hang of it.

 

[711769]

Hey man,

use this guide. I used it and I loved it. Short and simple. If you use the log table long enough you'll memorize it and wont have to resort to looking the values up again. Enjoy!

 

[StudentDentistsRock]

Lets say [H+]= .00064 whats the pH?

You know it's 6.4 x 10^-4. So I write 1x10-3 above that number and 1x10-4 below that number. Which have a pH of 3 and 4. Also know that 3.14x10 is usally half. So 3.14x10-4=3.5 (Memorize that # it helps)

So like:
1x10-3 = 3
6.4x10-4 = ?
3.14x10-4= 3.5
1x10-4=4
Based off that I know 6.4x10-4 is greater than 3.14x10-4 so it would be between 3 and 3.5 so I'll guess like 3.3 for the pH. That's all there is to it.

That's how I did it and got all of the problems right on chad's quizzes. You start getting at faster at it and only takes a couple of seconds once you get the hang of it.

Right.

So this is what I have been doing:

1x10^-3 > 6.4x10^-4 > 1x10^-4

but since 1x10^-3 is equal to 10x10^-4 --> 10x10^-4 > 6.4x10^-4 > 1x10^-4
3 > x > 4

and since 6.4 is closer to 10 than it is to 1, I expect x to be closer to 3 (maybe ~3.3 like you suggested).

I think the 3.14x10^-x = half will definitely pull through though!
---------------------------------------------------------------------------------------------------------------------------------
However, my method does not work with all the time. See example below:

if [H+] = 4x10^-3 M, what is pH?

10x10^-3 > 4x10^-3 > 1x10^-3
2 3

in this instance, 4 is closer to 1 than it is to 10, so I'd say the pH is about 2.8-ish? but the answer is 2.4


I also don't know how to go the other way around (what is the H+ concentration if pH is 2.4 etc).

Hey man,

use this guide. I used it and I loved it. Short and simple. If you use the log table long enough you'll memorize it and wont have to resort to looking the values up again. Enjoy!

Holy guac n' tac! I think this solves most of my problems. thank you!! Rushing to try it out!

edit: the video really helps out. I'm gonna keep using it till I get the hang of it. Thanks everyone!

 

[deleted738762]

Right.

So this is what I have been doing:

1x10^-3 > 6.4x10^-4 > 1x10^-4

but since 1x10^-3 is equal to 10x10^-4 --> 10x10^-4 > 6.4x10^-4 > 1x10^-4
3 > x > 4

and since 6.4 is closer to 10 than it is to 1, I expect x to be closer to 3 (maybe ~3.3 like you suggested).

I think the 3.14x10^-x = half will definitely pull through though!
---------------------------------------------------------------------------------------------------------------------------------
However, my method does not work with all the time. See example below:

if [H+] = 4x10^-3 M, what is pH?

10x10^-3 > 4x10^-3 > 1x10^-3
2 3

You set it up wrong hence why you got it wrong lol. Also realize you aren't going by closer to 1 or 10 all the time because it isn't a linear scale but it's logarithmic scale. Hence use 3.14 as a reference because that's always half or 0.5 Since 4 is greater than 3.14 it's going to be closer to 2.
So
1x10-2 = 2
4x10-3 = 2.4
3.14 x 10-3 =2.5
1x10-3=3
4x10-3 is greater than 3.14 x 10-3 so it would like 2.4. You just set it up wrong before.

If pH is 2.4 then H+ = 1x10-2 - 1x10-3. It's closer to 2 so I would estimate like 4x10-2. You just need to get close!.

 

[tyjacobs]

Use this formula to estimate pH it worked very well for me


So with the general formula of:

A*10^-b

log (A*10^-b) = B-1.10-a

so for example
h+ = 3*10-8, whats pH?

A = 3
b = 8

pH = B-1.10-a
pH = 8-1.10-3
pH = 7.7

 

[Queen Beach]

The DDDestroyerrrrrr taught me this and how to figure it out by just looking at it. 😀