Final answer:
The mass percent composition of an element in a molecule is the fraction of the molecule's mass that comes from that element's mass, calculated using the element's atomic mass and the compound's molar mass. Option A is correct.
Step-by-step explanation:
The mass percent composition of an element in a molecule is A. This can be calculated by taking the atomic mass of the element and dividing it by the molar mass of the compound, then multiplying the result by 100% to find the percentage. This calculation can be done for any quantity of molecules, such as a single molecule, a dozen, or a mole (which is the most common practice).
To determine the percent composition of a compound from its chemical formula, one uses the subscripts in the formula to calculate the mass of each element in one mole of the compound. This number is then divided by the molar mass of the compound and multiplied by 100%. This method is frequently used to determine the empirical formula and the molecular formula of a compound, thereby revealing the chemical identity of the substance.