# AAMC 8 - PS, #10, 20, 55, 57 expand on explanation?

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#### ssa915

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Can any1 give an expanded explanation to the following problems? (I have bought these exams, and have the solutions with me, but I needed clarification on the answers...can any1 help?

10 - ??

20 - why does the explanation say that the path (distance) travelled by the sphere is greater on the inclined plane...the question states that they are both 10-m in length? The only way I got the correct answer to this was by using a value of 10m for both spheres.

55 - I can see how 'C' makes sense, but why is the answer not 'D'?

57 - How do we know that HCl reacts with CO3 and not SO4?

#### chaeymaey

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10. C. The formula for &#969;d is given in the fourth paragraph of the passage: &#969;d = 4&#960;&#956;B1/h. If B1 is nonzero, then the nuclear magnetic dipole moment, &#956;, must be nonzero in order for &#969;d to be nonzero. So, the question becomes, Which one of the four nuclei given as choices here has a nonzero &#956;? The magnetic moments of paired nucleons cancel each other out, so any nucleus with an even number of protons (even Z) and an even number of neutrons (even N) will have &#956; = 0. But even if you didn&#700;t know this fact from nuclear physics, you can still choose the correct answer. All of the choices have an even Z (and an even A, which implies an even N), except choice C, which has an odd Z. We are told in the passage that each H atom, which we know has an odd number of protons (namely, 1) has a nonzero &#956;. We conclude, then, that nucleus C, the only choice with an odd number of protons has, like an H atom, a nonzero &#956;.

20. C. First, we eliminate choices B and D because they&#700;re not true. The experiments of Galileo that are described in the passage were used to measure d (distance traveled) and t (the time of travel) and then determine the ratio d/t2. It seems easy enough to measure a distance of 10 m with a reasonable degree of accuracy. However, it would have been far more difficult for Galileo to measure very short time intervals (less than, say, a couple of seconds) with a comparable degree of accuracy. (A sphere dropped from a height of 10 m would reach the ground in less than a second and a half.) Furthermore, any error incurred in measuring t would be amplified because t is squared in the ratio Galileo was calculating. Therefore, the main advantage in using an inclined plane (whose angle could be made as small as desired, thereby making the sphere&#700;s travel time as long as desired) is that the spheres would take longer to reach the bottom. Note that while choice A is generally true, Galileo was not measuring the final velocity of the spheres, only the distance traveled and their travel time, so choice C is a better response than A.

55. C. Sodium ion, Na+, is neutral in solution, and carbonate ion, CO3 2, is a polyprotic base. Therefore, a solution of Na2CO3 is basic and thus turns red litmus to blue. Since the reaction between carbonate and water is CO3 2 + H2O &#8594; HCO3
 + OH, choice C is best.

57. C. The carbonate ions in solution react with the strong acid to produce carbon dioxide: CO3 2 + 2 H+ &#61510; CO2(g) + H2O.

OP
S

#### ssa915

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chaeymaey said:
10. C. The formula for &#969;d is given in the fourth paragraph of the passage: &#969;d = 4&#960;&#956;B1/h. If B1 is nonzero, then the nuclear magnetic dipole moment, &#956;, must be nonzero in order for &#969;d to be nonzero. So, the question becomes, Which one of the four nuclei given as choices here has a nonzero &#956;? The magnetic moments of paired nucleons cancel each other out, so any nucleus with an even number of protons (even Z) and an even number of neutrons (even N) will have &#956; = 0. But even if you didn&#700;t know this fact from nuclear physics, you can still choose the correct answer. All of the choices have an even Z (and an even A, which implies an even N), except choice C, which has an odd Z. We are told in the passage that each H atom, which we know has an odd number of protons (namely, 1) has a nonzero &#956;. We conclude, then, that nucleus C, the only choice with an odd number of protons has, like an H atom, a nonzero &#956;.

20. C. First, we eliminate choices B and D because they&#700;re not true. The experiments of Galileo that are described in the passage were used to measure d (distance traveled) and t (the time of travel) and then determine the ratio d/t2. It seems easy enough to measure a distance of 10 m with a reasonable degree of accuracy. However, it would have been far more difficult for Galileo to measure very short time intervals (less than, say, a couple of seconds) with a comparable degree of accuracy. (A sphere dropped from a height of 10 m would reach the ground in less than a second and a half.) Furthermore, any error incurred in measuring t would be amplified because t is squared in the ratio Galileo was calculating. Therefore, the main advantage in using an inclined plane (whose angle could be made as small as desired, thereby making the sphere&#700;s travel time as long as desired) is that the spheres would take longer to reach the bottom. Note that while choice A is generally true, Galileo was not measuring the final velocity of the spheres, only the distance traveled and their travel time, so choice C is a better response than A.

55. C. Sodium ion, Na+, is neutral in solution, and carbonate ion, CO3 2, is a polyprotic base. Therefore, a solution of Na2CO3 is basic and thus turns red litmus to blue. Since the reaction between carbonate and water is CO3 2 + H2O &#8594; HCO3
 + OH, choice C is best.

57. C. The carbonate ions in solution react with the strong acid to produce carbon dioxide: CO3 2 + 2 H+ &#61510; CO2(g) + H2O.