Calculation questions

[Avinash]

Here's one question I got on a Kaplan sample test. I know how to solve it, and I got the correct answer, but I had to use my calculator Are we expected to calculate something like this in our head? For the first step I calculated how many moles of H2 there were in the 13.4L, and I had to do:

 

PV/RT=n

(1)(13.4)/(0.082)(273) = 0.6 . Are we expected to do this all with old fashion multiplication/division?

 

51. B2H6(g) + H2O(l) → B(OH)3 (aq) + H2 (g)

Consider the above unbalanced equation. For this

reaction, how many grams of diborane are required to

produce 13.4 L of H2 at STP?

A. 27.6 g

B. 13.8 g

C. 4.6 g

D. 2.76 g

[THX]

No, you don't have to. You should realize that at STP, 1 mole of any gas = 22.4L. So, since the MCAT is all about approximations, 13.4 is roughly 0.5 moles. Then you simply do moles * molar mass and locate the closest answer that is higher than whatever the calculation gives you.

[Avinash]

No, you don't have to. You should realize that at STP, 1 mole of any gas = 22.4L. So, since the MCAT is all about approximations, 13.4 is roughly 0.5 moles. Then you simply do moles * molar mass and locate the closest answer that is higher than whatever the calculation gives you.

 

Thanks, I figured it had to do with approximations

[Superking25]

Yeah, that question isn't too bad, all those answers have a difference of around a factor of 2...

 

I think I remember much more similar answers on the MCAT... but then again, I might be wrong...

[PreciousBodilyFluids]

PV/RT=n

(1)(13.4)/(0.082)(273) = 0.6 . Are we expected to do this all with old fashion multiplication/division?

 

The other posters are right about the 22.4 liters. But even without that, you should be able to do the above in your head. Just fudge the numbers so that they're easier to work with.

 

In this case, approximate 0.082 to 0.1, making the denominator something near 27. Then, 13.4/27 is roughly 0.5. That's close enough for the MCAT.

 

Don't be afraid to change the numbers you're working with by a little bit in order to make the calculations easier. The people who write these questions expect you to do this, and they're not going to try to trick you and rob you of points just because you aren't a mental whiz. At least, the writers for the AAMC do - I sometimes think the Kaplan writers have their heads up their asses.

 

If you still need reassuring, pick a calculation question on one of the AAMC exams and examine it in detail. Try to figure out what mistake you'd need to make in order to get each of the three incorrect answers, and you'll find that they're trying to trap people who forget to include the (1/2) in the equation for kinetic energy, or who forget that you're supposed to square the distance in the gravitational force equation - things like that. They don't really care how good you are at doing accurate, fast math out to whatever decimal place.

 

The exception to this would be powers of ten, though. You should definitely be comfortable with scientific notation. I actually prefer working in it, wherever possible.

 

Anyway. Ramblings of a Physics major who has taken the MCAT way too many times.