I'd really appreciate help with this . . .

[sonybaby]

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I am having major problems remember how to solve logarithms in my head! For instance, how would I solve that -log9 x 10^-4 is approximately equal to 3? OR that -log (2 x 10^-4) equals 3.7? Maybe I'm completely missing something . . . but I'm doing practice DAT questions and stressing out because I can't remember how to do these problems! Please help me if you can . . . thank you!

 

[Ex_EE]

For -log 9x10^-4 remember that -log 1X10^-4 = 4 and that -log 1x10^-3 = 3
Since 9 X 10 ^-4 is much closer to 1 x 10-3 (.1) than 1X10^-4 (.8) then the answer is close to 3.

Same goes for the other question. Hope this helps.

 

[Streetwolf]

I am having major problems remember how to solve logarithms in my head! For instance, how would I solve that -log9 x 10^-4 is approximately equal to 3? OR that -log (2 x 10^-4) equals 3.7? Maybe I'm completely missing something . . . but I'm doing practice DAT questions and stressing out because I can't remember how to do these problems! Please help me if you can . . . thank you!

-log(2x10^-4) = -log(2) - log(10^-4)
= -log(2) + 4
= 4 - log(2)

It will be close to 4 since 2 is closer to 1 than it is to 10 (so log 2 is closer to 0 than it is to 1).

For the other:

-log (9x10^-4) = -log(9) - log(10^-4)
= -log(9) + 4
= 4 - log(9)

It will be close to 3 since 9 is closer to 10 than it is to 1 (so log 9 is closer to 1 than it is to 0).

 

[askadds]

just do yourself a favor and just memorize log 1 through log 10. its pretty simple and its one less thing to worry about. thats what i did and it helped me out tremendously. almost every gchem practice exam and the real DAT had like 1 of those problems

-log(2x10^-4) = -log(2) - log(10^-4)
= -log(2) + 4
= 4 - log(2)

It will be close to 4 since 2 is closer to 1 than it is to 10 (so log 2 is closer to 0 than it is to 1).

For the other:

-log (9x10^-4) = -log(9) - log(10^-4)
= -log(9) + 4
= 4 - log(9)

It will be close to 3 since 9 is closer to 10 than it is to 1 (so log 9 is closer to 1 than it is to 0).