Molecular Formula Versus Empirical Formula – MCAT General Chemistry | MedSchoolCoach

Molecular Formula versus Empirical Formula

MCAT General Chemistry Chapter 1 - Section 1.1 - Stoichiometry

Sample MCAT Question - Molecular Formula versus Empirical Formula

A carbohydrate with the empirical formula CH2O has a molar mass of 150 g/mol. What is the molecular formula of this compound?

a) CH2O

b) C3H6O3

c) C5H10O5

d) C7H14O7

C is correct. C5H10O5.

The molecular formula is a multiple of the empirical formula, so the molar mass of CH2O times a whole number should equal 150 g/mol. The molar mass of CH2O is 12 g/mol (C) + 2 g/mol (2 H) + 16 g/mol (O) = 30 g/mol. This implies the molecular formula is 5-fold greater than the empirical formula, since 150 / 30 = 5. Therefore, we can multiply the subscripts of our empirical formula by 5 to obtain the molecular formula as such: (CH2O) x 5 = C5H10O5.

Stoichiometry

This chapter will cover the relationships between the relative amounts of substances comprising a reaction, a branch of chemistry known as stoichiometry. Stoichiometry can be used to make predictions about reactions will proceed and how much product will be created from a given amount of reactant. To begin understanding this, we must first start with the chemical formula of a molecule, which can be expressed in two separate, but related, forms: molecular formula and chemical formula.

Molecular Formula

The molecular formula represents the number of atoms in each molecule. As an example, consider glucose, which has a molecular formula of C6H12O6. This tells us that each molecule of glucose has six atoms of carbon, twelve of hydrogen, and six of oxygen. As another example, consider the molecule ribose, an important component of nucleic acids, which has a molecular formula of C5H10O5, indicating that the molecule consists of five atoms of carbon, ten atoms of hydrogen, and five atoms of oxygen.

Empirical Formula

The empirical formula is the smallest integer ratio of atoms in a molecule. That is, instead of describing the number of atoms in each molecule, it shows the ratio of the atoms to each other. Let’s return to our earlier examples of glucose and ribose. While the two sugars had different molecular formulas, we can see that the ratio between each of the atoms remains the same between the sugars, meaning that they will have the same empirical formula, CH2O. This indicates that, in both molecules, there will be one atom of carbon and one atom of oxygen for every two atoms of hydrogen. Note that while many different molecules can share the same empirical formula, each molecular formula is unique to a single molecule.
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