Video Transcript
Which of the following molecules does not have the same empirical formula as the rest? (A) CH2O, (B) C2H4O2, (C) C4H8O3, (D) C5H10O5, or (E) C6H12O6.
Each of the answer choices given represents a different molecular formula. A molecular formula is a chemical formula that expresses the exact number of atoms of each element in a molecule. This question is asking us about empirical formulas. An empirical formula is a chemical formula that expresses the simplest whole-number ratio of atoms of each element in a molecule or ionic compound.
To better understand the difference between a molecular formula and an empirical formula, let’s use the molecular formulas of ethene and butene to determine their empirical formulas. As an empirical formula expresses the simplest whole-number ratio of atoms of each element, we’ll need to simplify the subscripts in the molecular formula by dividing by their greatest common factor. In the case of ethene, the greatest common factor of two and four is two and the greatest common factor of four and eight is four.
To simplify the molecular formula to the empirical formula, we simply divide each subscript by the greatest common factor. So the empirical formula of ethene is C1H2. But as we don’t write subscript values of one, we can represent the empirical formula as CH2. The empirical formula of butene is also CH2. So, although the molecules are different and have different molecular formulas, their empirical formulas are the same.
To answer the question, we’ll need to determine the empirical formula for each of the molecular formulas given. Then, we can identify which of the molecules does not have the same empirical formula as the rest. In answer choice (A), the molecular formula contains one atom of carbon, two atoms of hydrogen, and one atom of oxygen. The greatest common factor of these values is one. Dividing each subscript by the greatest common factor gives us the empirical formula CH2O, the same as the molecular formula.
The greatest common factor of the subscripts shown in the molecular formula in answer choice (B) is two. To determine the empirical formula, we divide each subscript by the greatest common factor, which gives us the empirical formula CH2O. The greatest common factor of four, eight, and three is one. So, as was the case with answer choice (A), the empirical formula of answer choice (C) is the same as the molecular formula.
The greatest common factor of five, 10, and five is five. Dividing each subscript by the greatest common factor gives us the empirical formula CH2O. And finally, the greatest common factor of six, 12, and six is six. Dividing each subscript by the greatest common factor gives us the empirical formula CH2O. Looking at the empirical formulas, we can see that the molecule that does not have the same empirical formula as the rest is the molecule shown in answer choice (C) C4H8O3.
