How To Tackle MCAT style Logarithm Questions

When it comes to mathematical calculations on the MCAT shortcuts are the way to go. But with most shortcut you have the option of falling back on the long yet proven pen-and-paper calculations to get your results.

But this is not the case when it comes to logarithms. Logs or negative logs, unlike multiplication, division and similar cannot be solved in a simple written out manner.

Therefore, learning a non-calculator trick for solving log questions is a must for every MCAT student. And that’s why my newest video shows you how to quickly tackle simple negative log questions, and a trick for solving the more difficult questions, all without a calculator.

MCAT Style Question Mentioned In This Video:

Chemistry Question: Find the pH of a 2.3 x 10¯⁴ molar solution of NaOH

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This is Video #8 in my MCAT Math Without A Calculator tutorial series. Click HERE for the entire series

1. Vikas says:

I like your tutorial. Can you please help me in simplifying 10 to the power -4.2.

• Leah4sci says:

Estimate and round down to 10^4

2. Juliane says:

Hi Leah

How would you find the pH give the [H+] 9 x 10^-6 M.
I am trying to apply your concept and my answer would be ~ 6 since it’s just 9 by itself.

Can you explain how you would approach something like this?

Thanks!

• Leah4sci says:

Round 9 up to 10. Did you ask the same question on YouTube?

3. Very helpful, thank you. What’s the trick if I want to find the [H+] concentration? What is 10^ -10.36?

• Leah4sci says:

Estimate between 10^-10 and 10^-11

4. MJ says:

Hi Leah, thanks for sharing your knowledge. But I’m having trouble with one question.. how come this rule doesn’t apply to -log(1.5×10^-4)? From what I’ve understood, it should be between 3 and 4 and since 5 is #.3, shouldn’t it be 3.3?? But the answer is 3.8. am I missing something here?:S
Thank you in advance!

• Leah4sci says:

MJ: 1.5 is between 1 and 2, not 5

5. faiza says:

how would you ( log 4.5 X 10^5) and (log 4.5 X 10^-5)

6. Thanks for sharing your knowledge!

• Leah4sci says:

You’re welcome. Are you pre-med?

7. Imani Agard says:

Hey Leah, I love your videos so far, but I’m having problems understanding how you make the estimations. Take for example -log (2.3* 10^-4).

What is your reasoning behind deducing that the pH will lie between 3 and 4? Why are you rounding down to one and rounding up to ten? I think you rushed on that concept and it missed me.

Thanks for your help.

• Leah4sci says:

The log trick only works on numbers like 1 and 10. So if you estimate that 2 lies betwen 1 and 10 both times 10^-4 you get your range

8. jagan says:

how will you find -log(1.8 X 10^ -5 ) ?
what will you do for .8 after finding the range which is between 5 and 4..?

• Leah4sci says:

Don’t overthink it Jagan. On the MCAT an answer that is close enough is good enough. Estimate your equation as -log(2×10^-5) or even better, 1 x 10 ^ -5

9. cameus says:

Hi Leah,
AS a sophomore university don’t you think it’s soon for me, for the MCAT Exams.?
I don’t want to go for it twice.

• Leah4sci says:

Cameus it depends when you want to test. I think you’re a bit early

10. Chris says: