#20 - Oxygen-18 is a heavier-than-normal isotope of oxygen.

[Administrator]

Complete Question Explanation

Must Be True. The correct answer choice is (A)

Answer choice (A): This is the correct answer choice. The passage states that in a rain cloud, oxygen-18 are rarer than normal oxygen. Thus we know for certain that rain clouds contains more ordinary oxygen than oxygen-18.

Answer choice (B): The stimulus states that the oxygen-18 content of rain clouds remains fairly constant along their entire routes. If that is the case, then there is no reason why once the rain clouds pass over the Amazon, they would contain a greater-than-normal percentage of oxygen-18. They should contain a relatively normal percentage of oxygen-18.

Answer choice (C): The stimulus states that in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. That does not mean, however, that the rainfall contains more oxygen-18 than ordinary oxygen. Suppose, for example, that 80% of all water molecules containing oxygen-18 and 60% of all water molecules containing ordinary oxygen descend to earth in rainfall. If there is a lot more ordinary oxygen than oxygen-18, however, then the rainfall could contain more ordinary oxygen than oxygen-18. For example, the rain cloud could be composed of the following:

• Ordinary Oxygen Oxygen-18
  
  1000 units 10 units

If during rainfall, 80% of all water molecules containing oxygen-18 and 60% of all water molecules containing ordinary oxygen descend to earth, then the rainfall will contain 600 units of ordinary oxygen and 8 units of oxygen-18. Thus we can see that the cloud’s rainfall does not have to contain more oxygen-18 than ordinary oxygen.

Answer choice (D): The stimulus does not state that the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18. In fact, it states that a higher proportion of all molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth during rainfall.

Answer choice (E): The stimulus does not state that rain clouds must surrender more oxygen-18 than it retains during rainfall. It merely states that rainfall contains a higher proportion of its total oxygen-18 than its total ordinary oxygen.

[Sweetgpeach]

LR Question Training (Prep Test 1-20)"-p.50 Q 94

...this is paradoxical for me. What is your recommended strategy to correctly deconstruct
this passage?


Cheers,
Georgia

[Nikki Siclunov]

Hi Georgia,

Indeed, it is paradoxical: you'd expect the oxygen-18 content of the clouds to decrease over time. And indeed, the first question following the stimulus asks us to explain the paradox. Answer choice (B) is correct for that question: if tropical rain forest can replenish the rain clouds, this would explain why their oxygen-18 content remains constant.

I believe your question concerns the second question following the stimulus, asking us to identify what must be true. A prephrase would be difficult to formulate, so I'd just proceed by the process of elimination: if you can't prove an answer choice by the information contained in the stimulus, it is incorrect. It is imperative to fully understand the facts before attacking the answer choices.

Answer choice (A) is correct, as it states a provable claim: once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18. This can be proven by direct reference to the second sentence in the stimulus.

Answer choice (B) is incorrect, because the levels of oxygen-18 remained constant as the clouds passed over the Amazon.

Answer choice (C) is also incorrect. The stimulus does not say that rainfall contains more oxygen-18 than ordinary oxygen: all we know is that it contains a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen. In other words, the cloud surrenders a higher percentage of its oxygen-18 content than of its regular oxygen content. Let' say, for instance, that there are 50M ordinary oxygen molecules in a rain cloud, but only 10M oxygen-18 molecules. In rainfall, we could have 5M molecules of ordinary oxygen (10% of all such molecules) and 2M oxygen-18 molecules (20% of all such molecules). We have satisfied the parameters in the stimulus, but disproven answer choice (C).

Answer choice (D) is false. See explanation above.

Answer choice (E) makes an improper comparison. We are given no information as to how much oxygen-18 is retained by the cloud after a rainfall, so we cannot compare the amount of oxygen surrendered vs. retained.

This is a tough question! Let me know if my explanation helps.

[ckrachmer]

Answer choice (A) is correct, as it states a provable claim: once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18. This can be proven by direct reference to the second sentence in the stimulus.

actually, that's not true; the passage reads, "Oxygen 18 is a heaver-than-normal isotope of oxygen. In a rain cloud, WATER MOLECULES containing oxygen-18 are rarer than WATER MOLECULES containing normal oxygen"

Answer choice A states that once a rain cloud is formed over the atlantic, it contains more ORDINARY OXYGEN than OXYGEN-18; not more molecules, just "More."

Since the answer choice states "more," but does not specify exactly what "more" means in this case, we really can't conclude that "more" means "more water molecules," and we really can't rule out a scenario in which a rain cloud is 51% normal oxygen molecules, and 49% oxygen-18 molecules, HOWEVER, due to oxygen-18 having a higher atomic weight, there is "more" oxygen-18 in the cloud, in terms of atomic weight.

In truth, this is really a sloppy question; what makes it worse is that question 20 states, "which one of the folloiwing INFERENCES about an individual rain cloud..."

Since the passage outright states, "in a rain cloud,oxygen-18 molecules are rarer than normal oxygen, molecules" we really can't "INFER" that there are more normal oxygen molecules than oxygen-18; this information was already explicitly provided as a fact, and no inferences are neccesary to derive it.

Just a terrible, terrible question.

[Nikki Siclunov]

Hi ckrachmer,

Thanks for your comment regarding this question. The way in which the term "more" is used in this question is contextual: a reasonable person would justly interpret the statement "a rain cloud contains more ordinary oxygen than oxygen-18" to be referring to the water molecules in the cloud, not the atomic weight of each type of oxygen. If the author wanted to compare the proportional weight of each type of oxygen, the sentence would have been constructed differently.

And indeed, the passage outright states that in a rain cloud, "oxygen-18 molecules are rarer than normal oxygen," i.e. there are fewer oxygen-18 molecules than normal oxygen molecules. Consequently, answer choice (A) states a provable claim. On the LSAT, restatements qualify as inferences. I realize that in the real world, the term "inference" has a slightly different connotation; however, on the LSAT an inference refers to any "provable claim," including claims that are a direct restatement or a paraphrase of an original claim.

One last piece of advice: don't argue with the LSAT. Yes, not all questions are perfect, and rarely - very rarely - questions get thrown out after release, usually due to an ambiguity in language that lends itself to multiple reasonable interpretations. That said, each question is repeatedly tested for logical cohesion, feasibility, bias, etc. I can't tell you how many students try to dispute question validity after the release of each LSAT, but rest assured that LSAC has an excellent, bullet-proof defense for every single question that they publish. If that weren't the case, they wouldn't be in business for this long, and the test wouldn't be as well-respected as it is today.

Just my 2c

[ckrachmer]

Hey Nikki, thanks for your response. I especially appreciated the info on how the LSAT defines "infer/inference." I have taken a lot of practice tests, and had not yet put that together, so thanks!

as for the question and how a reasonable person surely must interpret it, I can't say I really agree with you. I mean...part of the context provided in this question comprises a statement discussing relative density/weights of oxygen and O-18. Furthermore, the question prompt explicitly uses the phrase "water molecules containing..." whenever it is water molecules that are being discussed.

Answer choice A not only conspicuously omits this phrase, it refers to content of "oxygen" and "oxygen-18", NOT "oxygen ATOMS" and "O-18 ATOMS," creating the impression that choice A is referring to aggregate atomic content. Indeed, I could employ your reasoning tactic and conclude that, "given the context of the question, any reasonable person would interpret answer choice A as referring to atomic weight, not the number of molecules." To wit, take the statement "a molecule of water contains more hydrogen than oxygen," which is identical in form/syntax to the statement in answer choice A. By your logic, I am supposed to consider that a true statement, since a water molecule is made up of two hydrogen atoms and only one oxygen atom. But in reality, oxygen comprises almost 90% of a water molecule, and if you made that statement to a chemist, they would laugh in your face.

Sorry, I'm not trying to be antagonistic, but it just seems unfair and unwarranted to conclude that any reasonable person would simply disregard significant distinctions/inconsistencies between the prompt and answer choices, as well as the provided information regarding density/weights, in their interpretation of the question/answer choices. I guess if what you say is true, we can infer that I am an unreasonable person, since I had some uncertainty over how parts of this question should be interpreted.

Also, I know it doesn't seem like it, but I am typically not one to argue with the test; I've taken about 40-50 practice tests, and while there are several questions that I thought were a little ambiguous yet ultimately fair, this is the only question I have come across that I feel should have been removed from the test and wasn't. Not only is it overly ambiguous, but it seems to penalize people for thinking about things carefully and identifying distinctions that may not be readily apparent; qualities which strike me as being fundamental to success in law school/as an attorney. That, I think, is why the question is so disconcerting to me: answering this question correctly would require me to think in a way that would adversely impact my ability to answer all other questions correctly. In light of this, I think arguing with the LSAT in this instance -- at least for me -- is productive, because it hones rather than dulls the faculties that TYPICALLY contribute to correctly answering questions/ scoring well.

Also, not that it matters, and I don't really have anything against the LSAT at all, I actually love this test and think it's very fair 99.9% of the time, but, given that LSAC performs all this testing and has an excellent, bullet-proof defense for every single question they publish, why are questions thrown out, albeit rarely? Regardless of how long LSAC have been in business, the only pertinent inferences we can draw from the available evidence are these: 1) LSAC is capable of putting questions on tests that are ultimately deemed unsatisfactory/invalid; 2) there are instances in which rigorous testing of logical cohesion etc. do not adequately determine a question's ultimate validity; which leads to 3) that a given question has passed rigorous testing of logical cohesion etc., and a bullet-proof defense of said question's validity has been formulated, does not in fact establish the question's ultimate validity.

anyway, thanks again!

[andriana.caban]

Answer choice (C): The stimulus states that in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. That does not mean, however, that the rainfall contains more oxygen-18 than ordinary oxygen. Suppose, for example, that 80% of all water molecules containing oxygen-18 and 60% of all water molecules containing ordinary oxygen descend to earth in rainfall. If there is a lot more ordinary oxygen than oxygen-18, however, then the rainfall could contain more ordinary oxygen than oxygen-18. For example, the rain cloud could be composed of the following:

• Ordinary Oxygen Oxygen-18
  
  1000 units 10 units

If during rainfall, 80% of all water molecules containing oxygen-18 and 60% of all water molecules containing ordinary oxygen descend to earth, then the rainfall will contain 600 units of ordinary oxygen and 8 units of oxygen-18. Thus we can see that the cloud’s rainfall does not have to contain more oxygen-18 than ordinary oxygen.

Hi!

While I understand why (A) is correct after examining the other answer choices, I'm confused as to the relationship between proportions and percentages you give in your example.

If a proportion is the number or amount of a group when compared to a whole, how can rainfall not contain more oxygen 18 than regular normal oxygen water molecules? Is there anyway you can provide an example explaining how a higher proposition does not equal more of something (but not using molecules, perhaps something simpler?)

[James Finch]

Hi Andriana,

The issue here is that we don't know the exact ratio in the clouds between water molecules with oxygen-18 and regular oxygen, only that there are more with regular oxygen. Let's say this ratio is 10:1. The stimulus also tells us that the ratio goes down when it rains, as the actual rain contains more oxygen-18 compared to regular oxygen than this 10:1 ratio. So maybe the rain ratio is 5:1, where oxygen outnumbers oxygen-18 by only 5-to-1, whereas up in the clouds it's 10-to-1; either way, there's still more oxygen than oxygen-18 in both the clouds and the rain, although this doesn't necessarily have to be true of the rain, since we don't know the ratio. It could become 1:1 or even 1:2.

Hope this clears things up!

[ericsilvagomez]

Hi,

From my understanding, A is correct because the second sentence in the stimulus mentions the word "rarer" when comparing oxygen-18 and normal oxygen, correct? I think this would mean there would be more ordinary oxygen. What confuses me is the part about it forming over the Atlantic because my thinking goes to the end of the stimulus. Even though I chose B, I did not fully understand why answer choice C was wrong. How could there be more ordinary oxygen when the stimulus says a higher proportion of oxygen-18 falls to Earth? And even if there could be more water molecules for normal oxygen, is there a different way you can describe it? I did not understand the math.

[srusty]

Hi,

From my understanding, A is correct because the second sentence in the stimulus mentions the word "rarer" when comparing oxygen-18 and normal oxygen, correct? I think this would mean there would be more ordinary oxygen. What confuses me is the part about it forming over the Atlantic because my thinking goes to the end of the stimulus. Even though I chose B, I did not fully understand why answer choice C was wrong. How could there be more ordinary oxygen when the stimulus says a higher proportion of oxygen-18 falls to Earth? And even if there could be more water molecules for normal oxygen, is there a different way you can describe it? I did not understand the math.

Hi Eric,

You're right that answer choice (A) is correct because of what we see in the second sentence: oxygen-18 molecules are rarer than normal molecules, so answer choice (A) makes sense that the rain cloud has more ordinary oxygen than oxygen-18.

Answer choice (C) says "more" which is about amount, meanwhile the information given to us about the rainfall in the stimulus is about proportion. For example, if we use real numbers, there could be 99 ordinary oxygen molecules and 1 oxygen-18 molecules in the cloud, making oxygen-18 about 1% of the molecules. However, when rainfalls, oxygen-18 could be 20% of the molecules falling to Earth. Answer choice (C) confuses percent (or proportion) with amount, and makes that incorrect.

Hope this helps!