Final answer:
To calculate the mass percentage of an element in an impure sample, the total mass of the sample must be measured in addition to knowing the number of moles of the element.
Step-by-step explanation:
In order to calculate the mass percentage of an element in an impure sample, besides knowing the number of moles of the element, it is essential to measure the total mass of the impure sample. The mass percentage is determined by dividing the mass of the element by the total mass of the sample and then multiplying by 100%. This principle is based on the concept that the mass of the element in grams is numerically equivalent to its mass percentage when considering a sample weighing 100 g, due to the scale for percentages being 100. Therefore, the additional quantity needed to calculate the mass percentage of the element is the total mass of the impure sample of the substance.