gc question

[mg777]

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i was doing a gen chem problem in barron;s here it is

How many atoms of carbon are in 4e-8 grams of C3H8?




this problem seems really easy, but i keep getting a different asnwer from barron's. i think they have a misprint. anyways, someone want to crunch this number, and tell me what they get? thx

 

[ems5184]

Okay, so to solve this problem, I first would find the moles of propane (C3H8) that are going to be used. In this case, we taje 4e-8 g * (mole/44g) which gives us 9.091e-10 moles of propane.

In each mole of propane there are 6.022*10e23 molecules of propane according to Avogadro. So, we do a stoichiometric calculation as follows:
9.091e-10 moles * 6.022e23 molecules/mole = 547454545454545.45
molecules of propane.

Next, we know that there are 3 atoms of carbon per molecule of propane (from the molecular formula), so multiply that last answer by 3 and the solution for number of carbon atoms present should be about 1.642*10e15.

Let me know what you got, and what you think of my calculations. Also, if you dont mind, could you put up the answer choices given and say which is correct according to the book and what answer you were getting.

 

[mg777]

yea, i got the same thing u did. heres what i did

4e-8mol C3H8 (1 mol C3H8/ 44g) ( 3 mol C/1mol C3H8)(6.022e23/1mol C)

=1.64e15

now, the choices in barrons for this problem. its problem 60 on p.313

A. 1.64e15
B. 2.34E11
C.1.89E14
D. 4.66E2
E.4.00E4

the answer in the book is E. now im just about sure its wrong, since u got the same thing. there's no other way to do it.

 

[Utes]

Excuse my ignorance, I am just starting with my DAT preparations, but how would you do this problem on the DAT without a calculator?

Utes

 

[mg777]

on the dat, gc section and qr section we have to estimate. however some of the calculation problems are in a form that doesnt represent the final answer. i have gathered this from practice tests and DAT sample test 2004 provided by the ADA as I have not taken the real dat yet. for example say the question is

what is 4 from 10.

A. 10-4
b.4-10
c. 10x4

 

[ems5184]

Yeah I would be almost 100% certain that Barron's is wrong on this one, it happens sometimes. Looking back on it, this problem isnt bad to do mentally, just rounding like 9e-10*6.02e23 can be rounded to 9e-10 and 6e23, so multiply the numbers and add the exponents which gives you 54e13. Then, multiply by 3 and this gives you 162e13 which is equivalent to 1.62e15, so not too terrible to work out.

 

[dane4695]

I am also sure they are wrong. I posted another thread with further questions... but i'm not sure... check me.