The percent by mass formula is:

• A. %mass = (Mass of element / Molar mass) * 100
• B. %mass = (Mass of element / Mass of compound) * 100
• C. %mass = (Mass of compound / Mass of element) * 100
• D. %mass = (Total mass / Element mass) * 100

Answer: (B)

Mass percent expresses how much of the total mass of a compound comes from a specific element. It is calculated by dividing the mass of that element present in the sample by the total mass of the compound and multiplying by 100. This keeps the focus on the element’s share of the whole substance, not on molar masses or a reversed ratio. Using the element’s molar mass in the denominator would mix concepts and not reflect the actual mass fraction. Using the entire compound mass in the numerator would misrepresent what fraction belongs to the element. Flipping the ratio would invert the meaning and produce an irrelevant value.