Video Transcript
Which of the following best describes an empirical formula? (A) The total mass of one mole of a compound. (B) The simplest positive integer ratio of different types of atoms in a compound. (C) The percentage by mass of each element present in a compound. (D) The total number of atoms in a compound. Or (E) the connectivity between each atom present in a compound.
In chemistry, it’s often useful to be able to represent the same compound in different ways. One way to represent a compound is with its molecular formula, which is a chemical formula that expresses the exact number of atoms of each element in the molecule. The molecular formula of water is H2O, which shows that in each water molecule there’s two hydrogen atoms and an oxygen atom. As another example, the molecular formula of glucose is C6H12O6.
Another kind of formula, the empirical formula, gives the simplest positive integer ratio of different types of atoms in a compound. For example, in glucose, carbon, hydrogen, and oxygen atoms appear in a one-to-two-to-one ratio. So the empirical formula of glucose is CH2O. Instead of giving the exact number of atoms, it gives the ratio of those atoms. As it turns out, the empirical formula for water is H2O, the same as its molecular formula. Since the ratio of atoms in the molecular formula, two to one, happens to be as simplified as possible, the molecular formula and the empirical formula are the same.
Looking at the answer choices, one of them matches our definition of empirical formula, choice (B) the simplest positive integer ratio of different types of atoms in a compound. Choice (B) is the correct answer. The other answers are incorrect. The empirical formula does not tell us anything about the molar mass of a compound, the percentage by mass composition of a compound, the number of atoms in a compound, or the connectivity within the compound. So which of the following best describes an empirical formula? That’s choice (B) the simplest positive integer ratio of different types of atoms in a compound.
