GS-2 Physical Sciences Question 23

[fergus_to5501]

Should the answer be A since answer B would tell us how many grams of Fe are rusted per 80 minutes rather than per hour?

[admin]

B is the correct answer.

Another student asked a similar question, here is the question that the student posed but was placed in the wrong thread:



li5504552

Joined: 20 Jun 2007
Posts: 80

Posted: Thu Jun 28, 2007 10:53 pm Post subject: GS 2 physical sciences #23

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-I just wanted to clarify why the grams calculated is already for the hour. Is this because iron rusts in one hour all the iron turns into Fe 2+ in one hour. I am still kinda confused then what the 80 min signifies as we use this in our calculation. Thanks.


admin
Site Admin
Posts: 679

Posted: Fri Jun 29, 2007 1:50 pm Post subject:

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[quote]There are 2 times described in the question:

(1) the time that the iron was in the water = 1 hour. This is the time that rusting occurred and that Fe++ was left in solution. This is the per hour done at the end of the problem. After the one hour, the iron was removed from solution so clearly no further rusting could occur.

(2) 80 minutes is the time for the electrolysis which was being used to determine how much Fe++ was in solution. [/quote]

"I just wanted to clarify why the grams calculated is already for the hour. Is this because iron rusts in one hour all the iron turns into Fe 2+ in one hour."

Almost true, the only difference is that in the question it says that the remaining iron was removed. In other words not "all" the iron rusted but some did and the iron that did not was removed before the experiment continued.

" I am still kinda confused then what the 80 min signifies as we use this in our calculation."

Now the second part of the experiment begins by running current through the sample of Fe ions over a period of 80 minutes. This is why the equation Q = I t can be used where I is the current running through over a period of time t equivalent to the 80 minutes but in units of seconds. Thus the total charge Q is calculated.

[dwumi1604]

This is still not clear to me. I thought the "t" in eqn: Q=It means how long in secs the current (I) passes through the beaker.

Shouldnt the Q = 0.2 x 80mins, since the current, I, was passed thru AFTER the first one hour of the beaker's exposure to the atmosphere?

Secondly, how can dimensional analysis be used to answer this question.


Thanks.

[mcat_premed3832]

Your first question:

"This is still not clear to me. I thought the "t" in eqn: Q=It means how long in secs the current (I) passes through the beaker.

Shouldnt the Q = 0.2 x 80mins, since the current, I, was passed thru AFTER the first one hour of the beaker's exposure to the atmosphere? "

You are right! BUT the 80 minutes needs to be converted to seconds using dimensional analysis (your 2nd question):

(80 minutes) x (60 seconds/minutes) = 4800 seconds

Your understanding and diligence with units (= dimensional analysis) is VERY important for the MCAT. In fact, sometimes it will help you find a solution even if you forgot the equation. Besides the fact that DA was used for the above, you can follow the rest of the Explanation by maintaining careful attention to the units.

[bronsonyal2863]

Is the formal It = nF used?

[dcherian8173]

This was a tricky question. It is easy to select choice A (and do more work than necessary) if you don't read the question carefully.

The question asks for the rusting per hour and not the electrolysis per hour. The rusting was done over 60min, while the electrolysis was done over 80min. Dividing 0.28g by 4/3 will give you the rate of electrolysis, which is not what you want.