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Subject Test Chemistry

Question 12 of 17

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SAT Subject Test

Each of the questions or incomplete statements below is followed by five suggested answers or completions.  Select the one that is best in each case.

Analysis by mass of a certain compound shows that it contains 14 percent hydrogen and 86 percent carbon. Which of the following is the most informative statement that can properly be made about the compound on the basis of these data?

Select an Answer

It is a hydrocarbon.

Correct Answer: 
No

Its empirical formula is text CH end text subscript 2.

Correct Answer: 
Yes

Its molecular formula is text C end text subscript 2 text H end text subscript 4.

Correct Answer: 
No

Its molar mass is 28 g/mol.

Correct Answer: 
No

It contains a triple bond.

Correct Answer: 
No

This is a question on stoichiometry that tests the important skill of scientific reasoning based on experimental evidence. The question indicates that 100 percent of the composition of the compound analyzed can be accounted for with the elements hydrogen and carbon. Thus, this compound is a hydrocarbon and choice (A) is a correct statement. It is not the correct answer to the question, however, because you can deduce more specific conclusions about this compound from the information given. The relative percentage composition provides evidence that the atomic ratio of carbon to hydrogen in the compound must be 86 divided by 12 text end text colon text end text 14 divided by 1.0, or about 1 colon 2. Therefore, you can conclude that the empirical formula for the compound is text CH end text text end text subscript 2 end subscript. Thus choice (B) is a better answer than choice (A). Since you do not know the total number of moles of the compound used for analysis, you cannot calculate the molar mass or derive the molecular formula for this compound. Choices (C) and (D) cannot be determined from the information given and thus are not correct answers to the question. It is known, however, that a substance with an empirical formula of text CH end text text end text subscript 2 end subscript cannot have a triple bond. Therefore, choice (E) is incorrect.

Question Difficulty: 
medium