BR Physics Example 1.10

[Fing Fang Foom]

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The question asks the following: "What is the mass ratio of iron to oxygen in Fe2O3?"

For my work I multiplied 2(55.85) and 3(16). I rounded to get a ratio of 112:48. I then said that this ratio is approximately 114:50 so I could arrive at 228:100. This gets me very close to the correct answer of 2.33 g with the next closest answer choices being 1.63 g and 3.49 g.

In the explanation they kept it 111.7:48.0 which is fine but they state that this ratio is approximately equal to 116:50. The book states that both must be increased proportionally to keep the ratio the same. I understand that they are both about 2.32 but how would you know what numbers to change it to so that the ratio stays the same? I just thought a little bit on top and a little bit on bottom.

I did get the correct answer but there have been times where I have rounded and got the answer wrong. EK says that the answers will not be close enough and the mistake must be mine. I am trying to develop elimination tactics because I think it is a major factor for the MCAT score that I want.

 

[BerkReviewTeach]

The question asks the following: "What is the mass ratio of iron to oxygen in Fe2O3?"

For my work I multiplied 2(55.85) and 3(16). I rounded to get a ratio of 112:48. I then said that this ratio is approximately 114:50 so I could arrive at 228:100. This gets me very close to the correct answer of 2.33 g with the next closest answer choices being 1.63 g and 3.49 g.

In the explanation they kept it 111.7:48.0 which is fine but they state that this ratio is approximately equal to 116:50. The book states that both must be increased proportionally to keep the ratio the same. I understand that they are both about 2.32 but how would you know what numbers to change it to so that the ratio stays the same? I just thought a little bit on top and a little bit on bottom.

I did get the correct answer but there have been times where I have rounded and got the answer wrong. EK says that the answers will not be close enough and the mistake must be mine. I am trying to develop elimination tactics because I think it is a major factor for the MCAT score that I want.

Your rounding is fine in this case. It comes down to determining when to worry about proportional rounding and when it doesn't matter. When the answer choices are far apart, you don't have to be so detailed, as was the case here. In general, you should look at the answers and make the decision of how detailed you need to be in your rounding. With the choices so far apart, the difference between 114:50 and 116:50 is negligible.

I want to be politically correct and say this as politely as possible, but be careful when any book tells you something definitive about the MCAT. The test varies from exam to exam, so there is no steadfast rule as to what is true on every version of the MCAT. One test writer may make the choices far apart while another makes them so close you need to do precise math. You need to be prepared for both types of questions. A few exams ago people were complaining that they didn't do enough hard math questions in their preparation. So yeah, there are no doubt some MCATs where the answer choices are close together. The point is that you should take despotic statements in prep materials (and that's any prep materials) with a grain of salt. This is especially true for any books that haven't been updated for years. The MCAT changes, so the rules about the MCAT change too!

 

[ilovemcat]

I think they are going with the idea that:

Increasing the denominator decreases the overall answer (which they did by increasing 48 to 50). To make the answer proportional (so the answer wasn't under what it's suppose to be), they increased the numerator slightly to accomodate that change (111.7 to 116):

111.7 / 48 = (111.7 + some) / (48 + some)

Anyways, at the end of the day, what matters most is a method that gets you the correct answer. You were able to get the correct answer in a slightly different way, so do what works best for you. Also, from my experience TBR answers are always relatively close to train you be make close approximations -- however for the MCAT, answer choices won't necessarily be that close to one another (like you typically see in TBR books).

edit: TBR beat me to it 🙂

 

[Tribal]

Just curious, what edition of Gen Chem are you using?

 

[Fing Fang Foom]

I am using the 2010 edition.

I really appreciate your honesty BerkReviewTeach. I was a little hesitant about EK stressing to follow their methods with no deviation. I thought their remarks were haughty but this is my future and they are telling me that additional material could slow down my thinking and I should not pick and choose my tactics for approaching the MCAT. I was bound and determined to follow their verbal strategy to the letter. What a sigh of relief this is. I understand it is a business but that is a little underhanded in my opinion.

Do you have any advice for determining how precise I must be in my rounding? The answer choices that are very close or far apart would be obvious but I find myself hesitant in some cases. I really want to strengthen my intuition and estimation skills and the Berkeley books have certainly helped. I was dumbstruck by the tactics that are discussed to help eliminate answers.