Any way to do DECMIAL LOGARITHMS in your head?

[DrTacoElf]

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If i get a log from 1-10 i'm fine however on some acid/base problems i keep running into hydrogen concentrations of like .015, .025, is there ANY WAY to get the pH in your head?

Essentially to solve -log(.0225)

 

[drat]

When given a 0.0225 M H+ and you need to calculate the pH, I find it easiest to rewrite the concentration as 2.25 x 10^-2 and then rewrite the equation like this:

-log(0.0225) = -log(2.25 x 10^-2) = -(-2 + log 2.25)

log 2.25 is roughly 0, because you know the log 10 = 1, log 100 = 2, log 1000 = 3, etc.

So you can guesstimate the pH to be about 2 (The exact answer is 1.65). This is sufficient for the DAT. In no way are you expected to know what the log (0.0225) is.

Another example: [H+] = 0.0011 M

-log (0.0011) = -log (1.1 x 10^-3) = -(-3 + log 1.1)

pH ~ 3.

exact = 2.95

Hope that helps you out!

drat

 

[DrTacoElf]

Great Reply this is going to be invaluable to me 🙂

Thanks! 😀

 

[HBomb]

drat,
That was excellent and very informative!!! Now I know how to do it in my head too.

DrTacoElf,
FYI, this type of calculation isn't need for the DAT. It might be coming up in some of your problem sets, like in the Kaplan material, but from my experience (and from what I've heard), I have never seen a problem that required you calculate out logarthims (either base 10 or natural).

 

[drat]

HBomb, I'm going to have to disagree! *wink, wink* I encountered pH problems on the DAT.

drat

Glad this post helped you guys out.

 

[edkNARF]

all my logarithm questions very very simple (ie-log1000=3). if i were you, i would focus on learning all the trig stuff, that is what killed me!

 

[DrTacoElf]

what kinda trigonometry stuff?
how in depth?

 

[edkNARF]

sin of x degrees equals this, - sin x = sin -x, etc. if you look at the questions in the released dat, it shoud give you a good indication

 

[DrTacoElf]

Link to released DAT please?

 

[edkNARF]

I don't know if it is available online. I got mine from my school's pre-professional health committe office.

 

[sxr71]

what kinda trigonometry stuff?
how in depth?

It is essential to know the values of Cos, Sin, Tan, Cot, Cosec, sec for 0,30,60, and 90 degees. This is very easily remembered in chart form.

 

[edkNARF]

If you know that, you will be fine.

 

[HBomb]

HBomb, I'm going to have to disagree! *wink, wink* I encountered pH problems on the DAT.

drat

Glad this post helped you guys out.

Just to clarify (sorry for being vague before)...I didn't mean to say that pH problems or log or ln problems won't be on the DAT, I meant to say that I didn't think you would have to convert a log or ln to decimal.

 

[drat]

Just to clarify (sorry for being vague before)...I didn't mean to say that pH problems or log or ln problems won't be on the DAT, I meant to say that I didn't think you would have to convert a log or ln to decimal.

But you may have to convert a pH to a decimal concentration.

 

[edkNARF]

those calculations very simple though

 

[HBomb]

Maybe others can help out here and clarify...but when I took my actual DAT, I did not have to convert any type of log to decimal...the answers were all left in log format. I do remember having to do some of that on the Kaplan material but not on the actual DAT. Not that it's not a good thing to know because it definitely is...and your approach seemed very easy to apply!!!

 

[edkNARF]

to the original OP: if there are logs, they will be either VERY simple or left in the log format. don't go crazy worrying about this. spend your time practicing the PAT!