Figuring out pH from log

[Ramil]

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ok here is my confusion i understant how to take a log of the number and get the pH. What i'm confused about when to round up and when to round down

Ex. -log 5 X 10^-3 = 3
Also
-log 1 X 10^-3 = 3

in which case the pH will be over 3 and in which case it would be under 3? How do the 5 & 1 affect the (3)?

 

[Kneecoal]

anytime you see a "log 1 x 10^---" whatever the 'blank' is will be your pH.

anytime the number directly after "log" is not 1, your pH will be lower than the number in 'blank,' but to varying degrees based on what your "not 1" number is. "after log" numbers closer to 1 will give you a pH closer to what your 'blank' number is, "after log" numbers closer to 10 (though it doesn't go higher than 9) will give you a pH closer to whatever your 'blank' number is minus 1.

for example:
-log 2 x 10^-3 could give you a pH around maybe 2.8. after checking my calculator it says 2.7

-log 8 x 10^-7 could give you a pH around 6.2. calculator says 6.1

if this sounds confusing, just read it over one or 2 more times, but hopefully it helped. the test won't give you two answers to choose from that are within a couple tenths of each other, so it's a good, quick method to use.

 

[Ramil]

God bless you.

 

[Pat Kelly]

if you really want to speed up through this section, just memorize 10 numbers from the log table, it's so useful and you can usually get pretty accurate up to 2 decimal places

for example -log .2 = .7, -log .3 = .53... memorize up to .9

so then -log 3 *10^-5 = 4.53 -> you take the power -1 and then add -log .3 to it

 

[EagleDream]

I think someone shows me a method to find Log.
So let says log 4.0x10-3
first log 10-3=3
second u take 4/2=2 (always divided by 2)then take 3.0+2.0=5.0 now move the decimal to the left one place .5----->>>.5 and take 3-.5 and the answer for log 4.0x10-3 = 3-.5 = approximate 2.5

 

[Ramil]

where do i get that table

 

[Ramil]

is it always gona be the number to power of 10 minus the adjustment you just found?

 

[ktpinnock]

ok here is my confusion i understant how to take a log of the number and get the pH. What i'm confused about when to round up and when to round down

Ex. -log 5 X 10^-3 = 3
Also
-log 1 X 10^-3 = 3

in which case the pH will be over 3 and in which case it would be under 3? How do the 5 & 1 affect the (3)?

So I just checked the -log(5E-3) and the answer is 2.3, even if you round it, it will be 2..not 3
and what others have said, the -log of 1x10^-_ will yield the answer of the exponent.