MCAT Math Vid 9 - Antilogs in pH and pKa Without A Calculator

[@bbazarov176]

(1) 4.19+ 0.15=4.33 (Always add constant of 0.15 to your pKa value) (2) 5.00-4.33=0.67 (increase the given pKa to the whole next number and subtract from that whole number answer you get from step 1) (3) 0.67 x10 = 6.7 (always multiply whatever number you get by 10) (4) 10^-5 ( one thing to note your 10^- number will be alway one digit higher than given pKa value) (5) 6.7x10^-5 Hope it helps everybody. Calc: 6.46x10^-5 .

[@ghostmedic86]

This works great unless the number is like 3.82.

[@MsPslater]

@@ghostmedic86 I found this method only works on negative numbers such as 10^-4.22 but like 10^3.86 it doesn't work, not sure why.

[@Leah4sciMCAT]

I came up with a much simpler method recently, see my newer Logs video for the simpler shortcut

[@alexanderpineschi3222]

​@Leah4sciMCAT hi I'm looking for this new simpler way but I'm not seeing any recent log videos can u maybe attach a link???

[@alexanderpineschi3222]

​@@Leah4sciMCATkzbin.info/www/bejne/hp-XlKNjls6SeZosi=HDYUb15mc6j-_IHp Is this it?

[@toluwalaseilesanmi9635]

This video series has been so helpful to me and given me more confidence to tackle the MCAT. Thank you so much for making this video series

[@Leah4sciMCAT]

You are so welcome!

[@KarlKarsnark]

At 4:15 mark I believe you misspoke when you said "point 1 gives me an 8 and point 5 gives me a 5". I think you meant to say "point 1 gives me an 8 and point 3 gives me a 5". Due to the fact that you rounded 4.19 to 4.2 and (.2) lies half way between (.1) and (.3), which would then give the (8 x 10^-x) and (5 x 10^-x) values you use later in the problem.

[@thenightdances21]

yes, @Leah4sci can you confirm this because I'm struggling at this point to understand if you misspoke and how you got the final answer?

[@Leah4sciMCAT]

Yes, thanks for catching that. I misspoke, it should be between 0.1 gives me 8 and 0.3 gives m 5

[@rickykay2024]

Omg i got so confused , your previous video was clearer...

[@Leah4sciMCAT]

Ricky Kalendarov what did you find confusing?

[@bownic12]

I have the same question!

[@cmhardin37]

I thought it was Crystal clear...

[@Mahdi25009]

same

[@lirimsopaj9071]

You are the best online teacher I ever known.Thank you.You really helped me

[@Leah4sciMCAT]

lirim sopaj thank you

[@meowismeify]

Leah4sciMCAT Hi Leah! I love your videos! I was a bit confused in this video around 4:04. You didn't mention how you determined that the exponent is going to be -5 and not -4 or some other number. Thanks!

[@Leah4sciMCAT]

I show you to round it

[@hp8309]

I just figured out why. We have to refer to the video before this one to see why. In the previous video, she says for -log ( 1 E -4), the pH has to be between 3 &4. That is, for any pH= - log (1 x 10 ^ -#) , the pH must be between (# - 1) & the #. So if we have pH = - log ( 1 x 10 ^ -7), the pH would be between 6 and 7. If we have pH = - log (1x 10^-5), the pH would be between 4 & 5 etc etc. Now, back to your question: 1. In the current video we know the pKa (or pH) is between 4 & 5 because 4.19 is between 4 & 5. 2. We pick the # (the exponent on Ka= y x 10^-#) to have a value of -5 because if we picked -4, the pH would have to be between 3 & 4, not 4&5 like we need. Remember earlier how I said for any pH= - log (1 x 10 ^ -#) , the pH must be between (# - 1) & the #. 3. At this point, we know our Ka has to end with 10^-5. Next, we can figure out what the coefficient in front of the x10^-5 is. 4. Next, we use the fact that 4.19 --> 4.2 and 4.1

[@biancarosas409]

@heli god bless you this helped me so much

[@briana6547]

oh thank you i just saw this after i commented lol

[@therealreels1121]

@@hp8309 you are a goddess!!! thank you!

[@bonenettle]

Basically what people are asking (I think), and which I'm also a little confused by, is at 4:27 where the answer is somewhere between 5x10^5 - 8x10^-5. I would have reasoned the range to instead be 5x10^4 - 8x10^4. I know this isn't right, but I don't see a logic behind why you chose the former (and correct) answer.

[@Leah4sciMCAT]

+Alexandra Anne Did you see my math cheat sheet on the values to memorize?

[@Chebe2]

This wasn't as clear, I got lost once we got to the 10^-4.19=Ka. I wasn't sure what was happening. I have rewatched taken notes over and over. :(

[@israelsanchez1225]

Watch the previous video multiple times and very carefully to understand the trick. Its basically the trick from the first video in reverse.

[@Leah4sciMCAT]

I’m sorry there has been some confusion with this concept. The idea is that a pKa of 4.19 lies between the whole numbers 4 and 5. Since Ka = 10^-pKa, the Ka for this acid must fall between 10^-5 and 10^-4. If only one of your MCAT answer choices falls within that range, that’s great! You’ve got your answer. If not, it can be helpful to memorize a set of decimal values that give slight variations in your answers to anti-log calculations. Thanks for commenting!

[@thebigcat2390]

I analyzed this video for well over an hour and I'm still lost in the sauce

[@Leah4sciMCAT]

Say what?

[@stephaniecantu6973]

I think what people are confused is to why the -4 wasnt selected and why -5 was. Yes its because the pH would of have to been 3.19 but how can we calculate to know that? And eliminate -4.

[@Leah4sciMCAT]

At which point of the video please?

[@jburag2]

im confused on why it is to the -5th power not -4th

[@Leah4sciMCAT]

Jennifer Buraga which part?

[@Sunshine-vt7kh]

@@Leah4sciMCAT I would also like to know this! It confused me too. Around your 4 minute time stamp, how did you know your answer was going to be to the -5th power instead of the -4th power?

[@pkibera]

I realize this question has been asked before, but I don't think you gave a satisfactory answer. Since our desired number has the exponent lying between 4 and 5, why did you pick 5 instead of 4 as the exponent? In other words, why didn't you pick our value ranges to be between: 5 x 10^-4 and 8 x 10^-4? How did you know to go with -5 instead of -4 as the exponent?

[@Leah4sciMCAT]

pkibera which part of the video are you asking about? I don't really understand your question

[@Leah4sciMCAT]

Leah4sciMCAT I just rewatched the video and found your question. 8x10^-4 is between 1x10^-3 and 1x10^-4. But since we have 4.19 and not 3.19 we can't use the range of 3-4

[@JoeBased08]

Leah 3:58 please explain why you chose 8 x 10^-5 instead of 8x 10^-4. I watched all the other videos in this series and took notes based on your videos, but I don't understand why you chose -5 instead of -4.

[@Leah4sciMCAT]

Since 4.19 is between 4 and 5 we have to look at both.

[@samirmuhammad6145]

this is amazing for gamsat prep!!

[@Leah4sciMCAT]

thank you! glad to help Good luck on the GAMSAT

[@gabbyorozco5966]

at 4:45, you said the answer was between 5 x 10^-5 and 8 x 10^-5; why is it raised to negative 5 and not negative four? other people asked this question below but you do not provide an explanation as to why?? In the video before this one, i thought i had to stay in the range you give us, therefore between negative 4 and negative 5 exponents.

[@Leah4sciMCAT]

Sorry for the late reply. You must remember that a number raised to the power of -5 is smaller than a number raised to the power of -4. Therefore, the range we must stay in for this problem is between 1 x 10^-5 (smaller) and 1 x 10^-4 (larger). If I were to say my answer is between 5 x 10^-4 and 8 x 10^-4, I would be exceeding my expected range. Both of those values are greater than 1 x 10^-4.

[@andrewdunn777]

Hello, love your videos, but I am very confused. I get up to the point where the Ka is in the range between 1x10^-4 and 1x10^-5 and am confused with the final answer you get. Isn't 6.46x10^-5 larger than 1x10^-5 therefore being out of the range??

[@Leah4sciMCAT]

+Andrew Dunn I challenge you to turn each of these numbers into a decimal and compare

[@andrewdunn777]

6.46x10^-5 = 0.0000646. 1x10^-5 = 0.00001. To me it still looks like the 6 is larger and therfore not in the range. I know I must be wrong, but cannot see how.

[@nafisomohamud8600]

Your videos are amazing, what a huge help. Thank you Leah :)

[@Leah4sciMCAT]

Glad you like them! You're welcome!

[@jyothi26rab]

this video is very confusing last one is very clear kindly explain once again please

[@Leah4sciMCAT]

I'm sorry that you're finding this video confusing. I'm Thinking of redoing this topic but not anytime soon. Please see if other youtubers have posted similar explanations so that you can find another explanation quickly

[@scottbrydle8758]

Goes a little off the rails in the second half.

[@Leah4sciMCAT]

Thanks for the feedback

[@bgeer8452]

Great video. May I recommend, for continuity with the previous video (MCAT Math Vid 8,) that at 3:40 you flip the table? i.e. in the previous video, the "#.1" was on the bottom, though in this video, it is on the top. That made this video a bit more challenging to follow (since I just watched the previous before this.) Thank you again for all of your help.

[@Leah4sciMCAT]

Alas KZbin does not allow video editing

[@bgeer8452]

I understand. All the same, very helpful information. I find the videos very helpful for visual learning, and the commentary helps reinforce the concepts. Great stuff!

[@briana6547]

Hi Leah, why did you end up going for 10^-5 for the pH= 4.19 problem? We had two options of 10^-4 and 10^-5 when you explained that we need to round down and up to 4 and 5, respectively. but it wasn't explained why -5 was a better answer choice. Why did we go for 5?

[@FreakishWolf]

-4 would give the value of 4.0 while -5 would give any of the values between 4 and 5. 10^-4 is 10 x 10^-5 and results in the 4 value, 8 x 10^-5 gives 4.1, 5 x 10^-5 gives 4.3, 1 x 10^-5 gives 5

[@stephaniecantu6973]

How would you find this out by hand? @@FreakishWolf

[@TheSab003]

@@FreakishWolf Thank you so much! Got it.

[@Leah4sciMCAT]

10^-4 is 10x bigger than 10^-5, so if you're choosing a number between 1 and 10, you have to keep this in mind. That being said, I realize that this method is quite confusing and have since come up with a MUCH easier shortcut that I teach in my newer logs video. You can find it here: leah4sci.com/mcatmath

[@1993LAlice]

Hi Leah, I really like your video very much and they help a lot! But I just have one question toward the end of the video: isn't 6.46 x 10^-5 actually closer to 5 x 10^-5 instead of closer to 8 x 10^-5 (I think you said it's closer to 8 x 10^-5)? In this case if there are 2 answer choices between 5 x 10^-5 and 8 x 10^-5 how do I know which one it is?

[@Leah4sciMCAT]

Alice Liou if you take the midpoint between 5 and 8 you get 6.5 which means that yes, you are correct. Can't believe I missed this ;) That being said, the MCAT is not out to trick you. The topics/passages are difficult but the answers are straight forward if/when you have the right background and setup. They won't trick you like this

[@saranieto4963]

Thank you seriously so much. This was unbelievably helpful

[@Leah4sciMCAT]

You're very welcome!

[@zubiakhan7417]

is nice this tips improve my math

[@Leah4sciMCAT]

Glad it was helpful to you!

[@jared394]

Will you be rerecording this soon? The video is mostly helpful but there are some areas where I have trouble following due to speed. Thanks!

[@Leah4sciMCAT]

I don't have plans to re-record. Have you gone to settings to change it to a slower speed when I talk too fast for you?

[@msfreeable]

Thank you, this was so helpful!

[@Leah4sciMCAT]

I'm so glad!

[@lukemiller5719]

You have a soothing voice, you should do narrating

[@Leah4sciMCAT]

Thank you, I appreciate your kind words. Hope you found this video helpful

[@kandyappleseed]

Hi Leah. I understood up until around the 3:40 minute mark. I think I’m confused on the exponent x part & how the answer came out to be around 5 & 8. Can you please clarify & explain?

[@Leah4sciMCAT]

The pKa of 4.19 is somewhere between 4.1 and 4.3. Based on the numbers in green that I ask my students to memorize, that puts our Ka value somewhere between 5 x 10^-5 and 8 x 10^-5. To understand more about how I got these values to memorize or why they are important for logarithmic calculations, make sure to watch my full MCAT math tutorial at Leah4sci.com/MCATmath or download the accompanying cheat sheet at leah4sci.com/mcat-math-study-guide-cheat-sheet/

[@danakarko3346]

Hello, how can I approximate 1x10^-5.4 or any 10 to a negative decimal? the answer is 3.9 x10^-6 but how?

[@Leah4sciMCAT]

+Dana Kar You have to approximate using the 2 closest whole number values. 1x10^-5 and 1x10^-6

[@kumaradityamamc6262]

Nice vedio it really help me thanks a lot

[@Leah4sciMCAT]

You're welcome!

[@koolbunnykinz]

why did you go with the ^-5 and not ^-4?

[@Leah4sciMCAT]

At which specific point in the video?

[@sibinmathews4244]

how did u round to X10^-5 .... ????? 4.19 is closer to X10^-4

[@Leah4sciMCAT]

Sibin Mathews you have to round up AND down to find the in between

[@bbazarov176]

nice. Its really helpful for my future MCAT exam.

[@Leah4sciMCAT]

awesome, when are you testing?

[@bbazarov176]

Leah4sciMCAT I wish I could take it now, but I have not finished my undergraduate yet. Probably I am going to take it in 2-3 years, and definitely gonna use your tutorials. Thank you so much. Glad that I found these videos. I will use your videos to survive my CHEM classes.

[@Leah4sciMCAT]

Wow 2-3 years is quite a ways away. Are you currently a Freshman or Sophomore?

[@bbazarov176]

Yeah I know. I have quite a lot of time, and I am freshman. Just finished my 2nd semester.

[@Leah4sciMCAT]

Haha no worries then. Right now focus on getting good grades, keeping your GPA high, and building a strong foundation. There will be a new MCAT format by the time you test.

[@nikkinyarko]

I am confused how we are sure that the exponent will be 10^-5 and not 10^-4 (3:53)

[@Leah4sciMCAT]

Since the starting value is 4.19, it is already larger than 4. So we must upgrade our exponent to the more negative number of -5. Just to check, you could do the -log of 8x10^-4 in your calculator and find that it would give us around 3.1 (smaller than our original value) while doing the -log of 8x10^-4 gives us 4.1.

[@nikkinyarko]

@@Leah4sciMCAT Makes sense, thank you!

[@ns8253]

Your videos have been very helpful, thank you! Can you help me understand how you decided that the x exponent would equal 5 at 3:53. Please advise, thanks!!

[@Leah4sciMCAT]

Nathalie S 4.19 is between 4 and 5

[@lavarwilliams3284]

Leah4sciMCAT I get that but 4.19 is closer to 4 than it is to 5 so it seems like the exponent would be 4

[@eazya1523]

NICCCEEEE !!! Thank you.

[@Leah4sciMCAT]

You're welcome!

[@jyothi26rab]

i understood the first part now am confused how to calculate negative log of 5,7 or any decimal am not able to fig out please explain

[@Leah4sciMCAT]

At which point in the video?

[@GCC507]

where is you video on buffers?

[@Leah4sciMCAT]

non-existent at this time, however I will be happy to teach you this information via private tutoring session. Details on my website leah4sci.com/mcat-tutoring

[@pkibera]

Same question Jennifer, Natalie, and meowismeify asked below. You do not explain how you arrived at the exponent 5 instead of 4. I'm not sure how else to ask the question. What part of my questions don't you understand? The desired value lies between two exponents (4 and 5), and you picked 5. However, you do not provide a rationale for why you picked 5 instead of 4 or why you did not pick 4. As mentioned. the same question has been asked by 3 other people, but you haven't yet answered the question.

[@Leah4sciMCAT]

pkibera As explained in the video, if the value is between 4 and 5 you have a number between 1x10^-4 and 1x10^-5. the first is the same as saying 10x10^-5 and so we choose a number between 1 and 10, and put that to the exponent of -5

[@Vunami]

Hey Leah, I think you should delete this video and remake an updated version to cover some important points that you might have left out

[@Leah4sciMCAT]

I appreciate your feedback, but what do you think I left out?

[@DrTanika]

Get the gist thanks, just got confusing at the end

[@Leah4sciMCAT]

You're welcome! What was confusing about it?

[@terrissas2240]

Log has never been this easy for me

[@Leah4sciMCAT]

Glad I could help!

[@bradyrule5696]

GOAT

[@Leah4sciMCAT]

Thanks!

[@kat2600]

why 10^-5???

[@Leah4sciMCAT]

I'm sorry, but I don't understand the context of your question. If you have a question about the example in this video, please contact me through my website at Leah4sci.com/contact and provide a time stamp along with any additional comments and/or questions.

[@ewwxaca]

Confusing.

[@Leah4sciMCAT]

What specifically about the video was confusing? I'm always looking for feedback to help me improve future videos.

[@ginamarcus5095]

This video got confusing after 5:02

[@Leah4sciMCAT]

Thank you for the feedback, but I'm confused. That's just where I tell you where you can find additional resources.

[@hp8309]

she picks the -5 because if she picked -4, the pH would be between 3 & 4, not 4 & 5 like we discovered earlier on in this video.

[@stephaniecantu6973]

How do you figure that out @@hp8309