The empirical formula, CH4, has a molar mass of 16.0 g/mol. The molar mass of its molecular formula is 64.0 g/mol. What is the molecular formula?
The correct answer and explanation is:
Correct Answer: C₄H₁₆
To determine the molecular formula of a compound, one must first understand the relationship between the empirical formula and the molecular formula. The empirical formula represents the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual number of each type of atom in a molecule.
In this case, the empirical formula is CH₄, which consists of one carbon atom and four hydrogen atoms. To calculate the molar mass of the empirical formula, we use the atomic masses:
- Carbon (C) has a molar mass of approximately 12.0 g/mol
- Hydrogen (H) has a molar mass of approximately 1.0 g/mol
So, the molar mass of CH₄ is:
(1 × 12.0) + (4 × 1.0) = 12.0 + 4.0 = 16.0 g/mol
The problem also states that the molar mass of the molecular formula is 64.0 g/mol. To find the molecular formula, we divide the molecular molar mass by the empirical molar mass:
64.0 g/mol ÷ 16.0 g/mol = 4
This tells us that the molecular formula contains four times the number of atoms in the empirical formula. We then multiply each subscript in CH₄ by 4:
- Carbon: 1 × 4 = 4
- Hydrogen: 4 × 4 = 16
Therefore, the molecular formula is C₄H₁₆.
This process is a common step in chemical analysis, especially in organic chemistry, where molecular formulas provide more precise information about the composition of a substance than empirical formulas alone. Understanding this distinction is crucial for identifying and synthesizing compounds in scientific and industrial applications.