Molecular Formula: Videos & Practice Problems

The molecular formula represents the actual number of each element in a compound, while the empirical formula shows the relative number of elements. To find the molecular formula, multiply the empirical formula by an n factor, which can be determined from the compound's molar mass. For example, glucose has an empirical formula of CH₂O and an n factor of 6, resulting in a molecular formula of C₆H₁₂O₆. Understanding these concepts is crucial for accurate chemical analysis and stoichiometry.

Molecular Formula

The molecular formula of a compound provides the actual number of each type of atom present, while the empirical formula indicates the relative number of those atoms. To derive the molecular formula from the empirical formula, we utilize a concept known as the n factor, which is a multiplier that adjusts the subscripts in the empirical formula to yield the molecular formula.

For instance, consider three compounds: glucose, octane, and salicylic acid. Their empirical formulas are as follows: glucose has an empirical formula of CH₂O, octane is C₄H₉, and salicylic acid is C₇H₆O₃. To find their molecular formulas, we first determine the n factor for each compound:

• For glucose, the n factor is 6. Multiplying the subscripts of the empirical formula by 6 results in C₆H₁₂O₆.
• For octane, the n factor is 2, leading to a molecular formula of C₈H₁₈.
• Salicylic acid has an n factor of 1, meaning its molecular formula is the same as its empirical formula: C₇H₆O₃.

In summary, to convert an empirical formula to a molecular formula, simply multiply the subscripts of the empirical formula by the n factor. If the n factor is not provided, the next steps involve determining it based on the compound's molar mass and the empirical formula. This process will be explored in further detail in subsequent discussions.

Molecular Formula Example 1

To determine the molecular formula of a compound, the first step is to establish its empirical formula. The empirical formula represents the simplest whole-number ratio of the elements in the compound. Once the empirical formula is known, the molecular formula can be derived if the molar mass of the compound is also available.

For example, consider lactic acid, which has a molar mass of 90.08 grams per mole. Elemental analysis reveals that it contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen. To find the molecular formula, we start by converting these percentages into grams, assuming a 100-gram sample. This results in 40 grams of carbon (C), 6.7 grams of hydrogen (H), and 53.3 grams of oxygen (O).

Next, we convert these masses into moles using the atomic masses from the periodic table: carbon (12.01 g/mol), hydrogen (1.008 g/mol), and oxygen (16.00 g/mol). The calculations yield approximately 3.3306 moles of carbon, 6.6468 moles of hydrogen, and 3.3313 moles of oxygen. To find the simplest ratio, we divide each mole value by the smallest number of moles, which is 3.3306. This results in a ratio of 1:2:1, leading to the empirical formula of CH₂O.

Following this, we calculate the empirical mass of the compound by summing the atomic masses of the empirical formula: (1 × 12.01) + (2 × 1.008) + (1 × 16.00) = 30.026 grams per mole. The next step involves determining the n factor by dividing the molar mass (90.08 g/mol) by the empirical mass (30.026 g/mol), resulting in an n factor of approximately 3.

Finally, to obtain the molecular formula, we multiply the subscripts in the empirical formula by the n factor. Thus, the molecular formula becomes C₃H₆O₃. This process highlights the importance of first determining the empirical formula, followed by calculating the n factor, which ultimately leads to the molecular formula.

What is the molecular formula for the following compound? 
Empirical Formula: NPCl₂                     Molar Mass:347.64 g/mol

• A. NPCl₂
• B. N₃P₃Cl₆
• C. N₂P₂Cl₂
• D. N₄P₄Cl₈

Answer: (B)

Cortisol (MW = 362.47 g/mol), a known steroid hormone, is found to contain 69.6% carbon, 8.34% hydrogen, and 22.1% oxygen by mass. What is its molecular formula?

• A. C₂₁H₃₀O₅
• B. C₂₁H₂₆O₅
• C. C₂₈H₄₆O
• D. C₁₈H₂₄O₂

Answer: (A)

Elemental analysis of a pure compound indicated that the compound had 72.2% C, 8.50% H and the remainder as O. If 0.250 moles of the compound weighs 41.55 g, what is the molecular formula of the compound?

• A. C₅H₇O
• B. C₁₀H₁₄O₂
• C. C₆H₈O₂
• D. C₁₂H₁₆O₄

Answer: (B)

Here’s what students ask on this topic:

What is the difference between an empirical formula and a molecular formula?

The empirical formula represents the simplest whole-number ratio of elements in a compound, while the molecular formula shows the actual number of each type of atom in a molecule. For example, the empirical formula of glucose is CH₂O, indicating a 1:2:1 ratio of carbon, hydrogen, and oxygen. However, the molecular formula of glucose is C₆H₁₂O₆, showing that each molecule contains 6 carbon, 12 hydrogen, and 6 oxygen atoms. Understanding both formulas is crucial for accurate chemical analysis and stoichiometry.

How do you determine the molecular formula from the empirical formula?

To determine the molecular formula from the empirical formula, you need the molar mass of the compound. First, calculate the molar mass of the empirical formula. Then, divide the compound's molar mass by the empirical formula's molar mass to find the n factor. Multiply each subscript in the empirical formula by this n factor to get the molecular formula. For example, if the empirical formula is CH₂O and the n factor is 6, the molecular formula is C₆H₁₂O₆.

What is an n factor in the context of molecular formulas?

An n factor is a multiplier used to convert an empirical formula to a molecular formula. It is determined by dividing the molar mass of the compound by the molar mass of the empirical formula. For example, if the empirical formula is CH₂O and the molar mass of the compound is 180 g/mol, while the molar mass of CH₂O is 30 g/mol, the n factor is 6. Thus, the molecular formula is C₆H₁₂O₆.

Why is it important to know the molecular formula of a compound?

Knowing the molecular formula of a compound is important because it provides the exact number of each type of atom in a molecule, which is essential for understanding its chemical properties and behavior. This information is crucial for tasks such as predicting reactions, calculating stoichiometry, and designing chemical syntheses. For example, the molecular formula C₆H₁₂O₆ tells us that glucose has 6 carbon, 12 hydrogen, and 6 oxygen atoms, which is vital for biochemical studies and applications.

Can the empirical formula and molecular formula be the same?

Yes, the empirical formula and molecular formula can be the same if the compound's simplest ratio of elements is also its actual number of atoms. This occurs when the n factor is 1. For example, the empirical formula of water (H₂O) is the same as its molecular formula because the ratio of hydrogen to oxygen is 2:1, and each molecule of water contains exactly 2 hydrogen atoms and 1 oxygen atom.