# TBR Chem

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#### Dochopeful13

##### Full Member
7+ Year Member

Can someone please explain the solution for 8.21? I have no idea where the math is being pulled from.

Thank you.

##### Full Member
5+ Year Member

Ugh ... ummm

pH = (1/2)pKa - (1/2)log[HA] Right?

So, what they're asking you to do is guestimate a bit.

1/2 pKa = 1.82

1/2 log [HA] is kind of like 1/2 (-1)

so, 1.82 -(-.5) = 1.82 + .5 ish... or 2.32ish which leaves you B and C

BUT because we rounded .07 to .1 the pH should be slightly higher not to the power of 10 higher... so B is slightly higher...

It's really a bad question that you'll never see on the MCAT but if you do, just rationalize it out, pick and move on... easy for me to say, I know

Last edited:
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#### Dochopeful13

##### Full Member
7+ Year Member

Ugh ... ummm

pH = (1/2)pKa - (1/2)log[HA] Right?

So, what they're asking you to do is guestimate a bit.

1/2 pKa = 1.82

1/2 log [HA] is kind of like 1/2 (-1)

so, 1.82 -(-.5) = 1.82 + .5 ish... or 2.32ish which leaves you B and C

BUT because we rounded .07 to .1 the pH should be slightly higher not to the power of 10 higher... so B is slightly higher...

It's really a bad question that you'll never see on the MCAT but if you do, just rationalize it out, pick and move on... easy for me to say, I know
Your amazing thank you very much. Where did they pull .010 M from though??

##### Full Member
5+ Year Member
Your amazing thank you very much. Where did they pull .010 M from though??
It's one of those questions where you know you don't have a calculator so you sort of have to improvise and make the question easier.

I think the 0.010 M is a typo, it should have been 0.1 M and not with the extra sig fig on there

#### PlsLetMeIn21

##### Full Member
2+ Year Member
It's one of those questions where you know you don't have a calculator so you sort of have to improvise and make the question easier.

I think the 0.010 M is a typo, it should have been 0.1 M and not with the extra sig fig on there

It's not a typo. That's their method for solving math questions. Use the easiest number you can find that is greater than the weird number in the question and the easiest number you can find that is smaller than the weird number in the question. That gives you the range in which the correct answer fits. Then pray the test writers weren't mean.

That equation is super helpful. They'll explain how to use it in their passage explanations. You'll master that by the time you finish the chapter.

Their trick is this: for a weak acid, cut the pKa in half and then add 0.5 for every decimal place in the molarity. For this example, half of the pKa is 1.821. So 1.0M would have a pH = 1.821, 0.1M would have a pH = 2.321, 0.01M would have a pH = 2.821, and so on. That's why they say the pH falls between 2.321 and 2.821, but is closer to 2.321 because 0.07 is closer to 0.1 than 0.01.

It may not seem clear the first time you do it, but stick with their tricks and they get super helpful. Have you gotten to their isoelectric point trick yet? I totally didn't get it at first and then the light went on and it was bam, this is super easy.

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#### Dochopeful13

##### Full Member
7+ Year Member
It's not a typo. That's their method for solving math questions. Use the easiest number you can find that is greater than the weird number in the question and the easiest number you can find that is smaller than the weird number in the question. That gives you the range in which the correct answer fits. Then pray the test writers weren't mean.

That equation is super helpful. They'll explain how to use it in their passage explanations. You'll master that by the time you finish the chapter.

Their trick is this: for a weak acid, cut the pKa in half and then add 0.5 for every decimal place in the molarity. For this example, half of the pKa is 1.821. So 1.0M would have a pH = 1.821, 0.1M would have a pH = 2.321, 0.01M would have a pH = 2.821, and so on. That's why they say the pH falls between 2.321 and 2.821, but is closer to 2.321 because 0.07 is closer to 0.1 than 0.01.

It may not seem clear the first time you do it, but stick with their tricks and they get super helpful. Have you gotten to their isoelectric point trick yet? I totally didn't get it at first and then the light went on and it was bam, this is super easy.
You are awesome. Taking a break from chem right now. I’m diving back in next week and will let you know. I appreciate the help!

#### BerkReviewTeach

##### Company Rep & Bad Singer
Vendor
10+ Year Member
PleaseLetMeIn, that is a wonderful explanation.