Topscore Chemistry titration

[americanpierg]

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What volume of HCl was added if 20 mL of 1M NaOH is titrated with 1M HCl to produce pH=2.

Getting rid of all of NaOH will take 20mL of HCl, leading to 40 mL neutral solution. pH=2 = 10^-2 H+ concentration = 10^-2 HCl concentration

(1M)(X mL)=(10^-2M)(40+X mL) = 20.4 mL

The answer came out to be 20.4mL. Answer choices were 10.2mL, 20.2mL, 30.4mL, 35.5mL, none of the above.

The correct was none of the above, NOT 20.2mL. what the heck. Is this just a stupid mistake on Topscore, or do we really have to pay attention to answers that closely. Not to mention 3/4th of the chem problems in this topscore test was difficult calculation problems and i got too dependent on rounding.

 

[alex1231]

Hi,
Can you explain how you get 20.4 mL?
(1M)(x mL)=(0.01M)(40+x)
x=0.4mL
What is x?
Thank you!

 

[UndergradGuy7]

Google gave me this http://forums.studentdoctor.net/showthread.php?t=652728
http://forums.studentdoctor.net/showthread.php?t=406065
http://forums.studentdoctor.net/showthread.php?t=646999

 

[Aschematic]

I also don't understand why the answer has to be exact.
........

If we are not allowed a calculator.. shouldn't the answer be more approximate??

 

[doc toothache]

I also don't understand why the answer has to be exact.
........
If we are not allowed a calculator.. shouldn't the answer be more approximate??

You need a calculator to solve this problem?

 

[alanan84]

I also don't understand why the answer has to be exact.
........

If we are not allowed a calculator.. shouldn't the answer be more approximate??

It should be, and topscore really needs to fix those answer choices b/c you're not going to see something like that on the real test. It's a good question with bad/confusing answer choices.

BTW, not that anyone cares, but as someone who's been on these boards for awhile, this is BY FAR the most frequently asked question. It seriously gets posted almost every other month. Kind of funny.

 

[wantVCUdental]

I came across this question on Topscore today, it has been fixed, the answer has been moved to None of the above

 

[payme2]

Hi,
Can you explain how you get 20.4 mL?
(1M)(x mL)=(0.01M)(40+x)
x=0.4mL
What is x?
Thank you!

0.4mL was the extra HCl that was added to get the concentration to 10^-2.

The first 20mL addition of HCl was to neutralize the solution and another addition of 0.4 mL brought it to pH 2.

He/she added 0.4 mL + 20 mL for the total addition.

I've never seen the algebraic equation set up this way, but I can see how useful it is when you can't use a calculator. but you'll have to remember to add the first amount.

 

[alanan84]

I came across this question on Topscore today, it has been fixed, the answer has been moved to None of the above

Then they still haven't fixed it. The confusing part is that 20.2 is so close to the answer that many gravitate towards that choice even though they have the correct answer. They need to either change the 20.2 answer choice to 20.4 and make it the correct answer or change 20.2 to something more different.

 

[Aschematic]

So do actual DAT questions allow you to be more approximate than this? This is just ridiculous.

 

[Aschematic]

You need a calculator to solve this problem?

😵O where did that come from? did you even read OP's post?

 

[dentalWorks]

I did that problem yesterday, and I think its only meant to teach us how to do the calculation by hand and not guesstimate the answer...

As soon as I read the problem, I picked 20.2 answer (B) before I wrote anything down because I knew it was going to be a number slightly higher than 20...

I HIGHLY doubt something this silly would show up IN THIS FORM on the dat, if it does, good luck lol

 

[doc toothache]

Then they still haven't fixed it. The confusing part is that 20.2 is so close to the answer that many gravitate towards that choice even though they have the correct answer. They need to either change the 20.2 answer choice to 20.4 and make it the correct answer or change 20.2 to something more different.

That's precisely why such an answer would be included in the list of choices.

 

[alanan84]

That's precisely why such an answer would be included in the list of choices.

My experience with the DAT was that the purpose was to test your ability to apply prior knowledge to the problems effectively. Not whether they can get you to pick the wrong answer because it looks close enough.

Apart from that, I don't remember there being any "none of the above" answer choices. Someone can correct me if I'm wrong on that...