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Mole calculation is a fundamental concept in chemistry. It helps us understand the relationships between the mass of a chemical substance and the amount of substance in terms of particles, represented in moles. To determine the number of moles in a sample, you divide the sample's mass by the molar mass of the substance.
For example, if you have a sample containing selenium and chlorine as described in the problem, you use the masses from your assumed 100 g sample.

• The mass of selenium: 52.7 g
• The molar mass of selenium: 78.96 g/mol

So, the moles of selenium are:\[\text{Moles of Se} = \frac{52.7 \text{ g}}{78.96 \text{ g/mol}} \approx 0.668 \text{ mol}\]Similarly, for chlorine with mass 47.3 g and molar mass 35.45 g/mol, the moles of chlorine are calculated as:\[\text{Moles of Cl} = \frac{47.3 \text{ g}}{35.45 \text{ g/mol}} \approx 1.334 \text{ mol}\]These calculations provide us the mole quantities needed to determine the ratio of each element in the compound.

Mass percentage is a way of expressing the concentration of an element in a compound. It tells what fraction of the compound's total mass is made up by a particular element.
In our example, selenium makes up 52.7% of the compound's mass.

• This means that in every 100 g of the compound, 52.7 g is selenium.
• The remainder, 47.3%, is chlorine.

To calculate mass percentage, you use the formula:\[\text{Mass percentage of element} = \left(\frac{\text{Mass of element}}{\text{Total mass of compound}}\right) \times 100\%\]For efficient calculations, you may assume a 100 g sample, where the percentage directly translates to mass in grams, simplifying the process of subsequent mole calculations.

Molar mass is the total mass of one mole of a substance, usually expressed in grams per mole (g/mol). It is calculated by summing up the atomic masses of all the atoms in the substance's molecular formula.
Understanding molar mass is essential for converting between the mass of a sample and the amount of substance in moles. Each element's atomic mass can be found on the periodic table.

• Selenium has a molar mass of approximately 78.96 g/mol.
• Chlorine has a molar mass of approximately 35.45 g/mol.

Using these values, you can convert between the mass of these elements in a compound and their respective mole quantities. This allows for accurate calculation of mole ratios to derive the empirical formula, as we have seen in the exercise, leading to finding the empirical formula as SeCl\( _2 \).