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 percent hydrogen and 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.
Its empirical formula is .
Its molecular formula is .
Its molar mass is g/mol.
It contains a triple bond.
This is a question on stoichiometry that tests the important skill of scientific reasoning based on experimental evidence. The question indicates that 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 , or about . Therefore, you can conclude that the empirical formula for the compound is . 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 cannot have a triple bond. Therefore, choice (E) is incorrect.