Correct answer for yesterdays MCAT question is E! See explanation below. Any questions? Ask and well answer! Explanation: This question requires you to be able to differentiate the atomic number of different elements. Of the choices that are given, osmium tetroxide and uranyl acetate both contain elements with high atomic numbers, osmium and uranium, respectively. Osmium’s atomic number is 76, and uranium’s is 92. Although osmium is not as heavy as uranium, the passage suggests that heavy metals are good at providing contrast, and osmium is a heavy metal. While this question at first glance appears technical, arriving at the correct answer doesn’t actually require an intricate understanding of how TEM works. You simply need to know that TEM works best with higher atomic number elements, including the heavy metals, and then recognize which choice(s) provides such an element. Answer choice explanations: A) Osmium tetroxide contains osmium, which is a heavy metal with a high atomic number of 76. It absorbs electrons and will improve contrast, but another answer choice also includes a heavy element. B) Uranyl acetate contains uranium, which is a heavy element with atomic number of 92. It will improve contrast for TEM, but there is another answer with a heavy element too. C) Biological samples are almost entirely composed of carbon, hydrogen, nitrogen, oxygen and phosphorous. These do not provide good contrast. It is essential to treat the sample with a heavy element to improve contrast. D: While rubidium is heavier than carbon, hydrogen, nitrogen, oxygen and phosphorous (the primary elements of biology), it is not a heavy metal and still only has an atomic number of 37. It is not a good choice to improve contrast in TEM. E) Both of these treatments are routinely used in TEM preparations and greatly improve image quality. *** Review on Firecracker! Atomic weight: med.firecracker.me/topics/6924
Posted on: Tue, 03 Jun 2014 21:36:34 +0000
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