What is the standard condition measurement for mass when calculating percent composition?

When calculating percent composition, the standard measurement for mass is in grams. This is because percent composition is calculated as the mass of a specific element in a compound divided by the total mass of the compound, multiplied by 100 to express it as a percentage. Using grams as the unit allows for consistency and ensures that the calculations reflect the actual proportions of elements in the compound accurately.

The other choices, while they are units of mass, do not align with the conventional approach to percent composition calculations in chemistry. For example, kilograms would lead to a larger number that could complicate the percentage calculations, and milligrams would provide a less practical or harder-to-work-with number for most substances. Moles, on the other hand, are a measure of quantity rather than mass and do not fit the definition needed for calculating percent composition, which requires absolute mass values.