Para-dichlorobenzene

Question

Para-dichlorobenzene is an ingredient in mothballs that has a molar mass of 147.00 g/mol and an empirical formula of C3CLH2. What is its molecular formula

Answer 1

Hello Emalia. To determine the molecular formula in this problem, we can do the following:

Take the given molar mass and divide by the mass of the empirical formula. Use the periodic table to determine the mass of the empirical formula, C₃ClH₂. The mass of the empirical formula is 73.499 g/mol.

molar mass / empirical formula mass

(147.00 g/mol) / (73.499 g/mol) = 2

This tells us that the molecular formula must be twice as large as the empirical formula. So we double all the subscript number in the empirical formula.

molecular formula: C₆Cl₂H₄

Answer 2

Hi Emalia!

For this question, its important to remember that the empirical formula is the most simplified form of the molecular formula in the sense that we reduce the ratios of the numerical values associated with the atoms to their smallest amount. For example, a C₂H₄ molecular formula would have an empirical formula of CH₂ because we can divide both atoms by 2.

With that in mind, the factor for determining our molecular formula is (molar mass of molecular formula / molar mass of empirical formula). This should get us a whole number. Since we are looking for the molecular formula, we will multiply this number to the entire empirical formula almost like we are distributing it to each atom. If we were given the molecular formula and were looking for the empirical formula, we would then divide by that number.

Since we have the molar mass of the molecular formula, we just need to get the molar mass of the empirical formula. C₃ gets us 12 g/mol x 3 = 36 g/mol, Cl is 35.45 g/mol, and H₂ gets us 1 g/mol x 2 = 2 g/mol for a total of 73.45 g/mol. We then would plug our values into the equation. (147 g/mol / 73.45 g/mol) = 2. Since we got 2, our molecular formula is the empirical formula multiplied by 2 to make C₆Cl₂H₄