Final answer:
The number of moles in a given sample can be calculated by dividing the mass of the sample by the substance's molar mass. For example, 21.6 g of boron equals approximately 2.00 moles, and 54.9 g of manganese equals about 1.00 mole. This method applies to other elements by using their specific molar masses.
Step-by-step explanation:
Calculating Moles from Masses of Elements
To calculate the number of moles of a substance, you need to divide the mass of the sample by the molar mass of the substance. The molar mass is the mass of one mole of a substance and is usually given in grams per mole (g/mol). Each element has a unique molar mass that can often be found on the periodic table or in a chemistry reference.
For example, to find the number of moles in a 21.6 g sample of boron (B), which has a molar mass of approximately 10.81 g/mol, you would divide the mass of boron by its molar mass:
Number of moles of B = 21.6 g / 10.81 g/mol ≈ 2.00 moles
Applying this method to each sample given, you use their respective molar masses to determine the number of moles. Here is how you would calculate it for manganese (Mn), which has a molar mass of about 54.94 g/mol:
Number of moles of Mn = 54.9 g / 54.94 g/mol ≈ 1.00 moles
This same calculation method can be used for all other samples given by replacing the sample mass and the molar mass for each specific element.