Praxis II: Chemistry Exam Prep

Answer: 12.6 percent

To determine percentage composition by mass, divide the molar mass of the given element by the molar mass of the compound. In the context of the question, the molar mass of hydrogen is 1.0079 g/mol and the molar mass of the compound is 32.0416 g/mol. However, 1.0079 g/mol must be multiplied by four to account for all of the hydrogens. This new value gives a percentage composition by mass of hydrogen of 12.6 percent.