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TBR 5 ...2 quick ps questions

Discussion in 'MCAT Study Question Q&A' started by Joker88, Jul 29, 2011.

  1. Joker88

    5+ Year Member

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    3. How many grams of glucose must be added to .5kg of water to raise the boiling point by .1 degrees C, given that the value of i is 1 and the value of Y for glucose is roughly equal to that of formic acid?

    Answer B. 15 Grams

    Explanation

    B is the best answer. Glucose is most like formic acid of all the compounds listed in the Table 1. Neither compound dissociates in water to a significant extent (although formic acid undergoes about 1% dissociation, which we can regard as negligible), resulting in an i-value of 1 for each. A 1.0 molal solution of formic acid is shown in Table 1 to have a boiling point of 100.6˚C, so a 0.16 molal solution (one-sixth molal) would have a boiling point elevated by 0.1˚C. To form a 0.16 molal solution in water with 0.50 kg of water, one-twelfth of a mole of glucose must be added. We ultimately need to determine the mass of glucose necessary to make one-twelfth of a mole. The molecular mass of glucose is 180 grams per mole, meaning that one-twelfth of a mole of glucose has a mass of 15 grams, choice B. You could also get the right answer by working backwards from the answer choices. For instance, consider choice D: 180 g of glucose is equal to one mole of glucose, which when added to 0.50 kg of water results in a 2.0 m solution. By comparison to 1.0 m formic acid in Table 1, we can estimate that the boiling point of a 2.0 molal glucose solution would be around 101.2 ˚C. The same reasoning eliminates choices A and C. The best answer is B.

    18.Which of the following drawings accurately depicts the direction of flow in the magnetic field generated by a current (i) flowing clockwise through a wire loop?


    A. [​IMG]

    A is the best answer. The problem on this question is that the pictures can play tricks on your mind, much like a drawing by the late M. C. Escher, a famous Dutch graphic artist. We can start by noting that the arrows must all curl in the same direction, either pointing into the page within the loop or pointing out of the page within the loop. Choices C and D can be eliminated, because they are a mixture of arrows rather than all the same type. We now must decide between choice A and choice B. This is a case for the right-hand rule: Placing your right thumb in the direction of the current results in your fingers curling in the direction of the magnetic field. Choice A represents the magnetic field that is generated by a clockwise current while choice B represents the magnetic field that is generated by a counterclockwise current. Choice A is the best answer, if you follow your fingers from knuckles to fingertips at each of the points along the loop. The best answer is A.

    B. [​IMG]

    C. [​IMG]

    D. [​IMG]

    Usually the right hand rule problems are pretty easy but this one i cant really see how they got A. If someone who has TBR 5 physical can explain, I'd really appreciate it!
     
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  3. wish2bMD

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    the mag field one is really simple. point your thumb in the direction of the current. then curl your fingers. your fingers represent the mag field.
     
  4. mitchlucker

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    The equation you're using for 1 is deltaT=iKbm everything should just be plug and chug.

    For the 2nd one, it can't be B, C, or D. The field is changing as the current is going around the loop. The only possible answer is A. Also, point your thumb in the direction that the current is moving. Then curl your fingers in like you're hitch-hiking. The field will go in the direction your fingers are curled in.
     
  5. iliveinmyguitar

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    The magnetic field and current one is really a question of perception. Every "curled arrow" around the loop can be either seen as pointing inward or outward. Im a very visual person and this question was impossible for me to figure out. Even though the explanation does address this issue in the beginning and then continues to explain the answer I still can't see how one can definitively arrive at an "objectively" correct answer.
     
  6. NextStepTutor_3

    NextStepTutor_3 MCAT Tutor
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    @iliveinmyguitar , that's a really good point. I think A is the objectively correct answer, but only because of tiny, barely-visible differences in the way they've drawn the diagrams. If I understand you correctly, you're saying that it all depends on how you look at it (for example, in diagram A, we can either see the arrows being overlapped by the loop and going out of the page or see the arrows overlapping the loop and going into the page).

    sdn magnetism.png
    I tried to draw this out to clarify (bear with me, my drawing is terrible). Basically, if the diagram resembled the one on the left, it would show the arrow coming out of the page. If it resembled the one on the right, the arrow would be depicted as going into the page. If we look really closely at the diagrams given, they're slightly more similar to the picture on the right. For A specifically, this must mean that the arrows are going in the same direction as our curled fingers and A is the correct answer.

    But it's definitely super ambiguous, almost like an optical illusion. I'd be curious to hear if anyone else had this same problem!
     
  7. Padfoot

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    Bump? I don't see how my fingers are curling in the right direction, especially for the left vertical part of the loop. My fingers are curling in the opposite direction..
     
  8. Cawolf

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    If you wrap your right hand around the left-vertical portion your thumb should be pointing in the direction of the current - which is upwards.

    Your fingers are then "wrapping" around the wire as shown in the solution.
     

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