acid/base problem

[tank you]

in acid/base kaplan topical, this is one question:

What volume of 0.5M KOH would be necessary to neutralize 15mL of 1.0M nitrous acid?

A. 30 mL
B. 15 mL
C. 7.5 mL
D. 1.5 mL


So I used M1V1=M2V2 and solved for V1, which is 30 mL. Now I subtracted 15 from 30 because the total solution is 30mL. But the answer is A, not B.

Can someone explain please?

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[dr1day]

Yes, someone please explain. I had the same question and was going to ask one of the teachers during office hours at the center, but if I can get the answer here, that'd be great.

 

[Prophecies]

I solved the problem using stoichiometry:

N.A. = nitrous acid

(1.0 mol NA/1 L) * (0.015L) * (mol H+/mol NA) * (mol OH-/mol H+) * (mol KOH/mol OH-) * (1 L/0.5 mol KOH) = 0.03 L KOH, or 30 mL

If you write it out the correct way (stoichiometry) it will probably make more sense...I had to write it that way cuz I ran out of room

 

[liverotcod]

The question is in terms of neutralization of a strong, completely dissociated acid. You have .015 mol of H+, so you need .015 mol of OH-. That's 30mL * .0005 mol/L of the NaOH solution. The eventual volume of the solution is not important.

 

[QofQuimica]

You are confusing two types of problems: dilution problems, where you need to find the difference between the total volume and the starting volume to get the volume added to make the diluted solution; and titration problems, such as what you are doing here. Remember, for titrations the volumes and molarities are only relevant in that we are using them to get the number of MOLES of acid and base, which must be equal at the titration's end point. You can get the number of moles of each by multiplying their molarities times their volumes. First you multiply 15 mL OH- x 1 mmol/mL as you did, to get 15 mmol. So you then know that your 0.5 mmol/mL concentration of HNO3 must also have the same number of moles of acid, and you divide 15/0.5 to get 30 mL of base. That is the correct answer.

Stop a minute and rationalize this to yourself. You have a base solution with a concentration that is half of the acid's concentration. Both your acid and base are monoprotic and monobasic (important to consider, because you'd have to use twice as much base to neutralize the same amount of 1M H2SO4). So you would logically expect that you need twice as much of your base to neutralize that amount of acid. And this is indeed the case.

 

[tank you]

thanks a lot everyone, i understand it now!

 

[dr1day]

Thanks Q. That made complete sense and I'm an idiot for not thinking of it myself! Oh well, in 2 1/2 weeks it won't matter anymore.

 

[QofQuimica]

You both are welcome. One piece of advice: try to avoid the urge to plug and chug when solving PS problems. Always start by considering 1) what is the problem asking for, and 2) what kind of answer is reasonable for this problem? If you have a good grasp of where you want to go and a ballpark idea of what a reasonable answer should be, it will help you avoid making silly mistakes on the MCAT due to miscalculation. Also, if you can avoid doing calculations altogether, that will save you time as an added bonus. This problem, for example, can be solved in a few seconds once you realize that your base's volume should be twice the acid's volume, as I explained above.

 

[medicomel]

You are confusing two types of problems: dilution problems, where you need to find the difference between the total volume and the starting volume to get the volume added to make the diluted solution; and titration problems, such as what you are doing here. Remember, for titrations the volumes and molarities are only relevant in that we are using them to get the number of MOLES of acid and base, which must be equal at the titration's end point. You can get the number of moles of each by multiplying their molarities times their volumes. First you multiply 15 mL OH- x 1 mmol/mL as you did, to get 15 mmol. So you then know that your 0.5 mmol/mL concentration of HNO3 must also have the same number of moles of acid, and you divide 15/0.5 to get 30 mL of base. That is the correct answer.

Stop a minute and rationalize this to yourself. You have a base solution with a concentration that is half of the acid's concentration. Both your acid and base are monoprotic and monobasic (important to consider, because you'd have to use twice as much base to neutralize the same amount of 1M H2SO4). So you would logically expect that you need twice as much of your base to neutralize that amount of acid. And this is indeed the case.

wow! great explanation and rationalizing approach. it pays off if you just stop and think for a second

 

[Shrike]

Never mind; it was just too mean. But someone on this thread should try hard to understand that it's damaging to give answers, even correct ones, if they will lead people to the wrong conclusion about how to approach the test. Be sure you know both the right answer and the best approach before you tell the world.

 

[liverotcod]

Never mind; it was just too mean. But someone on this thread should try hard to understand that it's damaging to give answers, even correct ones, if they will lead people to the wrong conclusion about how to approach the test. Be sure you know both the right answer and the best approach before you tell the world.

Huh? You've lost me... which post are you criticizing? Was it mine?

I don't think it's mean to give "gentle correction," but you're being pretty vague.

 

[Prophecies]

Huh? You've lost me... which post are you criticizing? Was it mine?

I don't think it's mean to give "gentle correction," but you're being pretty vague.

Fear not...he was talking to me...but I'm over it. Good luck to everyone...just a few weeks =o)

 

[aye]

You both are welcome. One piece of advice: try to avoid the urge to plug and chug when solving PS problems. Always start by considering 1) what is the problem asking for, and 2) what kind of answer is reasonable for this problem? If you have a good grasp of where you want to go and a ballpark idea of what a reasonable answer should be, it will help you avoid making silly mistakes on the MCAT due to miscalculation. Also, if you can avoid doing calculations altogether, that will save you time as an added bonus. This problem, for example, can be solved in a few seconds once you realize that your base's volume should be twice the acid's volume, as I explained above.

i also had a question about this problem, but not on the volumes. i was assuming that nitrous acid was a weak acid and therefore you couldn't do the MV=MV straight up... you had to take into account the dissociation constant. y couldn't you do this?? Thanks!

 

[dr1day]

My Kaplan instructor told us to memorize the list of strong acids (there are only a few) and then you can assume that everything else is weak. They are:

H2SO4
HNO3 (I know this is nitric acid, but I would assume that nitrous acid would be as strong)
HCl
HBr
HI
HClO3
HClO4

 

[QofQuimica]

i also had a question about this problem, but not on the volumes. i was assuming that nitrous acid was a weak acid and therefore you couldn't do the MV=MV straight up... you had to take into account the dissociation constant. y couldn't you do this?? Thanks!

dr1day is correct, and you should all memorize that list of strong acids. All other acids can be considered weak. So, nitrous acid (HNO2) is a weak acid. However, you are titrating it with NaOH, a strong base, and it will be completely deprotonated. You would need the equilibrium constant if you want to figure out the molar solubility of a weak acid (or base or salt) for a given temperature. But in titration with NaOH, all acids, strong or weak, will be completely neutralized, i.e. 100% dissociated.

Shrike, you need to give Prophecies a break, at least in this case. He did do this problem correctly using dimensional analysis. In general, it's a lot better to solve it his way than to blindly use M1V1=M2V2 if the acid and base aren't monoprotic and monobasic. Guys, can we make a deal? Prophecies, will you please not post answers if you aren't sure if they are correct, and Shrike, will you please stop cracking on him? He's only trying to help. Peace, love, and happiness to all.

 

[riceman04]

is that the normality equation?
If not, what is it?