Which quantity must be known to compute a component's percent by mass in a compound?

Which quantity must be known to compute a component's percent by mass in a compound?

• A. The empirical formula only
• B. The molecular mass and the element's molar mass
• C. The number of moles of each element
• D. The density of the compound

Answer: (B)

Percent by mass tells you what fraction of the compound’s mass comes from a specific element, so you need to know how much mass is in one mole of the compound and how much mass of that element is present in that mole. The mass from the element is (number of atoms of that element in the formula) × (element’s molar mass), and the total mass is the compound’s molecular (molar) mass. Therefore, percent by mass = [(n × M_element) / M_total] × 100%. Knowing the molecular mass of the compound and the element’s molar mass gives you the needed numbers. The density is not involved, and the empirical formula alone doesn’t determine the exact percentages without the full molecular mass.

Percent by mass tells you what fraction of the compound’s mass comes from a specific element, so you need to know how much mass is in one mole of the compound and how much mass of that element is present in that mole. The mass from the element is (number of atoms of that element in the formula) × (element’s molar mass), and the total mass is the compound’s molecular (molar) mass. Therefore, percent by mass = [(n × M_element) / M_total] × 100%. Knowing the molecular mass of the compound and the element’s molar mass gives you the needed numbers. The density is not involved, and the empirical formula alone doesn’t determine the exact percentages without the full molecular mass.