HELP!!!....need helps with Logs

[BushBaby]

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I can't do freaking log problems. For instance, getting the pH using the -log of a number. I have an idea of what to do but I am not sure how to really do it (the quick way).

For instance, this much I know.

-log 2.5 x 10^-6.

I know to bring the (-6) in front, making it a (+6)...then I know that 2.5 will make the pH a little less than 6 = 5.6

Why? or how do I know that the pH is a little less than 6?
I have no clue, I just do this because other students tried explaining this process to me, and this is the only part I got.


How do you guesstimate how much log of 2.5 is in the equation log 2.5 + log (-6)? Is there a scale that I need to know about?

Adding- I get the pH from simple logs of numbers like 1 x 10^8 (pH=8)...but when the number is a little complicated, this get funny.

 

[robinhudster]

yeah, i could use a quick refresher on that too. any math teachers out there willing to post a paragraph or two on the subject?
Thnaks so much.

 

[OneBadSkwerl]

-log (2.5x10^-6) can be rewritten

-log 10 ^-6 - log 2.5


now logs are to the base 10 so you're trying to think 10 to what exponent will give you 10 ^ -6? Easy, -6. So the answer is -(-6) or 6. Now for log 2.5 you try and think "10 to what exponent is 2.5"? You know it's greater than 0 since 10 ^ 0 = 1 and you know it's less than 1 since 10 ^ 1 = 10. So you know you're answer is going to be less than 6 but greater than 5.

 

[deleted9493]

Bushbaby,
Is this simply for your own satisfaction? Because what you've said in your posts is all you need for the MCAT. Just knowing how to manipulate ka,kb,[H].[OH]...to get pka,pkb,pH and pOH (just taking the -log) is pretty much the only counterintuitive thing you have to know with logs. Anything else is the simple addition/subtraction...related to multiplication and division.

 

[BushBaby]

hudsontc,

I guess knowing this can also be for my satisfaction, but it urks me to do something without knowing why I am doing it. It also appeared (in class) that the other students knew how to predict how close to (far from) 6, the pH fell into. I thought there was some system they were using...but if there isn't then I guess I shouldn't get all caught up in it.


OneBadSkwerl,

Thanks for that. You said the pH is less than 6 but greater than 5..........(I can't help but ask) how do I know if it falls closer to 5 or 6? If there is no sure method, then don't sweat it.
I know I probably won't need all the details for the MCAT as the answer choices will be clearly distinct enough for me to pick out the right answer.

 

[DrLady]

HI BB,
I don't know if you have the Ek verbal reasoning and math book, but that book shows you how to calculate the logs. The problems in there are good practice although there are very few of them. Try those, if u haven't tried them yet. I always have hard time with logs too! I always get myself confused.

 

[BushBaby]

Originally posted by DrLady
HI BB,
I don't know if you have the Ek verbal reasoning and math book, but that book shows you how to calculate the logs. The problems in there are good practice although there are very few of them. Try those, if u haven't tried them yet. I always have hard time with logs too! I always get myself confused.

Thanks DL.

For some reason I feel like I bought that book but I don't remember using it. Since I received the 101 passage book, I must have kicked the verbal/math book under my bed. I do think I remember seeing it though which means I must have ordered it somehow...uhmm...very interesting how soon the mind forgets.

I will go home and look into this.

 

[soonerpillow]

According to the Berkeley Review section on Acids and Bases, you can calculate any (most) logs with the following information:

1)When you multiply numbers, you add their logs. when you divide numbers, you subtract their logs.
2) log2 = 0.3, log3 = 0.48, log10 = 1

For example,
log4 = log(2x2) = log2 + log2 = 0.3 + 0.3 = 0.6
log5 = log(10/2) = log10 - log2 = 1 - 0.3 = 0.7
log2.5 = log(5/2) = log5 - log2 = 0.7 - 0.3 = 0.4
log2.5x10^-6 = log2.5 + log10^-6 = 0.4 + -6 = -5.6

so you only need to memorize a few values and you can calculate most logs, although it may get a little tedious if the number is too weird. The MCAT generally likes nice numbers though.

 

[BushBaby]

Originally posted by soonerpillow

1)When you multiply numbers, you add their logs. when you divide numbers, you subtract their logs.
2) log2 = 0.3, log3 = 0.48, log10 = 1

For example,
log4 = log(2x2) = log2 + log2 = 0.3 + 0.3 = 0.6
log5 = log(10/2) = log10 - log2 = 1 - 0.3 = 0.7
log2.5 = log(5/2) = log5 - log2 = 0.7 - 0.3 = 0.4
log2.5x10^-6 = log2.5 + log10^-6 = 0.4 + -6 = -5.6

Uhmmm....very interesting soonerpillow. Thanks..this really helps. (more things to memorize ..lol)


I just noticed how I typed the title of the thread. I said "...need helps with logs"

 

[soonerpillow]

Well, if you consider the amount of stuff you have to know for the MCAT to be equal to infinity. Then what's infinity + 2?
I found it to be really helpful, though.

 

[phatty925]

bushbaby,

i'm not sure if this is what you're asking, but are you asking how you know that the pH of 2.5x10^-6 is slight less than 6? well, the pH gives the -log of [H+ions]. so if you have slightly more than 1x10^-6 ions, this would make your pH decrease (the more H+ ions you have, the lower your pH). if you have 5x10^-6, then your pH would be 5.5, so you know that your pH level is somewhere between 5.5 and 6.0. i think that's as far as you have to know for the MCAT. hope this helps.

 

[efex101]

Yeah that is also how the TPR book explains it. If you have some number between (2-10) x 10 ^ x then the PH is x-1
So if you have 4.5 x 10^ -5 then the PH is 4

 

[heifetz]

Bushbaby,

I am not good at math either but this is how I approach your dilemma.

1st) Realize that a [H+] of 1x10^-7 = a pH of 7 (easy enough). Next any other value with 10^-7 in it such as 2x10^-7 or say 5.8x10^-7 or even 9.43x10^-7 is going to be a pH LESS than 7 but greater than 6. So think of 1x10^-7 as the max value which gives you 7 and Any other value with 10^-7 will give you a number less than 7 but greater than 6 until you get to 1x10^-6 and the whole process starts over again so that 3.54x10^-6 or 7.5x10^-6 is a pH less than our max value of 6 and greater than 5.

So basically concentrate on the number that the 10 is raised to. 10^-6 or 10^-2 or what have you. That is your base to work from and is your max value obtainable when preceded by 1. eg 1x10^-6 or 1x10^-2

I doubt the MCAT will have you differentiate the pH with regards to something like 2x10^-6 and 8x10^-6 so dont worry about that.

Hope this helps

 

[OneBadSkwerl]

Bushbaby, I'd be surprised if the answers were 5.6, 5.7, 5.8, 5.9. The quiz isn't about figuring out logs in your head, it's about concepts. What are the possible answers?

Keep in mind, a number close to 0=1 and a number closer to 1=10 if you really want to 'guesstimate' or grab a pencil and start doing full length calculations.... ie - .1 x .1 = ? .2 x .2 = ?

 

[maoeris]

Hello,
Here is a webpage that may help http://mb.sparknotes.com/mb.epl?b=75&m=167181&h=logs,math
I've been using sparksnotes to suppliment areas I am weak in or have forgotten a little like titrations and galvanic cells.

I hope this may help,
maoeris

 

[Skaterbabe74]

Ok this might be a dumb question, but do you not get to use calculators on the mcat? I'm asking because in all my math classes and science classes we can so I just know how to input the numbers to get the answer when it comes to logs and pH...

--Jessica, UCCS

 

[Tweetie_bird]

nope. no calculators. the only tools you can use for number counting are your fingers. 🙂

The MCAT is designed so you don't need to use calcs. The numbers are usually too far apart and for small things like gravity etc, you can usually use g=10 instead of 9.8. The power of rounding! That is the MCAT.