-log[2*10^-4] mental math???

[bchang57]

so a question from DAT Destroyer asks us to find pH

pH = 14 - pOH

pOH = -log[OH]

[OH] = 2*10^-4

-log[2*10^-4] = 3.7
we have to know how to do this in our head? or just memorize something like this?


the explanation in the book just goes through the steps of doing this and to me it seems like it just assumes we can do this without a calculator?

Note: i'm kinda rusty at math, haven't taken a math class since highschool...

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[The Prologue]

go to the c o u r s e savor website and view his math video on logs. He does a better job at explaining .

 

[Tunaman]

Not difficult to understand neg logs... and I'm not great at math either. Consider the following values and let me know if you see a trend:

-log 9x10^-9 is approx 8.1
-log 4x10^-6 is approx 5.6
-log 7x10^-8 is approx 7.3

The way I remembered it is by adding the number after the -log to the negative value that's raised. For the DAT you can approx these values to nearest tenth. The values will be far enough apart in your answer choices...and yes you need to know how to calculate pH and pOH for the DAT. Hope this helps

 

[sacapuntas]

so a question from DAT Destroyer asks us to find pH

pH = 14 - pOH

pOH = -log[OH]

[OH] = 2*10^-4

-log[2*10^-4] = 3.7
we have to know how to do this in our head? or just memorize something like this?


the explanation in the book just goes through the steps of doing this and to me it seems like it just assumes we can do this without a calculator?

Note: i'm kinda rusty at math, haven't taken a math class since highschool...

Seems a lot harder than it is...I was intimidated at first as well.

Do yourself a favor and calculate:

-log(1x10^-5)
-log(1x10^-4)
-log(1x10^-3)
-log(1x10^-2)


You will see the answers are 5,4,3,2 respectively.

Notice the trend, as the numbers get larger by a factor of 10, the negative log gets smaller by 1.

Now take your original question -log[2*10^-4] = 3.7

2*10^-4 is slightly larger than 1*10^-4, and remember the trend is that as the number gets larger, the -log gets smaller. So the answer must be less than 4.

Also notice that 2*10^-4 is smaller than 1*10^-3, so the answer must be greater than 3.

Lastly, notice that 2x10^-4 is closer to 1x10^-4 than 1x10^-3, so the answer should be closer to 4 than 3...probably something like 3.7 or 3.8.

Lastly, since we calculated pOH, subtract 3.7 from 14 to equal a pH of 10.3.

On the DAT, you will only be given one choice that really makes sense.

Very likely they will give you something like

a. 10.3
b. 3.3
c. 3.7
d. 4.3
e. 9.7

So you don't need to be exact, you just need to know about where that number will be.

I hope this made sense. If it didn't, get Chad's videos and watch how the gen chem videos about how to calculate pH and related videos for more assistance.

 

[ttpharm]

The way i remember is that the -log[10^-X] = X. and if the number is larger then it will decrease the answer. If my brain is not working, then I just think that if concencentration of H+ ions increases, [H+], then pH decreases.

 

[dental deezle]

Not difficult to understand neg logs... and I'm not great at math either. Consider the following values and let me know if you see a trend:

-log 9x10^-9 is approx 8.1
-log 4x10^-6 is approx 5.6
-log 7x10^-8 is approx 7.3

The way I remembered it is by adding the number after the -log to the negative value that's raised. For the DAT you can approx these values to nearest tenth. The values will be far enough apart in your answer choices...and yes you need to know how to calculate pH and pOH for the DAT. Hope this helps

That was the best strategy I have ever seen thank you so much

 

[djwon1]

You should know if it's higher or lower than X.5.

For example, -log (3.16 * 10^-5) = 4.5

if it's greater than 3.16 (4~9), it will be 4. 0~4.4
if it's less than 3.16 (1~3), it will be 4. 5~4.9

So for -log (2*10^-4) , since 2 is less than 3.16, it will be 3.5~3.9.