Questions 42 and 44, Morning Session, 2012 Professional Exam
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42. In a chemical reaction system, which of the following values will definitely change? Total moles in system A; total mass in system B; mass of a certain component in system C; moles of a certain component in system D. 44. Regarding the law of conservation of mass, which of the following options is not applicable? (Mass, molar mass) A. Mass of a certain component; B. Total moles in the system; C. Moles of a certain component; D. Total mass of the system. Which of these two questions should be chosen? What is the difference between them?
42.CD 44.ABC
What is the difference between the two questions? Does the total number of moles remain constant or change in the options?
This post was last edited by zshiwei2008 on 2015-7-10 at 10:06. 42 refers to something that is definitely going to change; B is the one that definitely remains unchanged. CD is something that will definitely change. A might change or might not change. For question 43, one needs to choose the option that is not necessarily unchanged, so the answer is ABC. It’s just a word game, but one needs to be careful in exams
42. The total molar amount in system A may remain unchanged; it stays the same when the sum of the stoichiometric coefficients before and after the reaction is equal. B. According to the law of conservation of mass, it must remain unchanged. 44. AC is easy to understand; for B, refer to the explanation for A in question 42
Oh, thank you, I now understand the difference
Question 42: If an inert gas is present in a chemical reaction, won’t both its mass fraction and molar fraction remain unchanged? Is it rare for me to misunderstand a question?