What is Empirical Formula Mass Class 11?

The empirical formula mass is the sum of the atomic masses of all the atoms present in the empirical formula of a compound. It represents the simplest whole-number ratio of elements in the compound.

Understanding Empirical Formula and Empirical Formula Mass

Here's a breakdown:

• Empirical Formula: This is the simplest whole-number ratio of atoms in a compound. For example, the molecular formula of glucose is C₆H₁₂O₆, but its empirical formula is CH₂O.

• Empirical Formula Mass: Once you have the empirical formula, you calculate its mass by adding up the atomic masses of each element present, multiplied by its subscript in the empirical formula.

Calculating Empirical Formula Mass: An Example

Let's use the example of glucose (CH₂O) to calculate the empirical formula mass.

1. Identify the Empirical Formula: CH₂O

2. Find the Atomic Masses:

   • Carbon (C): Approximately 12.01 amu
   • Hydrogen (H): Approximately 1.01 amu
   • Oxygen (O): Approximately 16.00 amu

3. Calculate the Empirical Formula Mass:
   (1 x Carbon's atomic mass) + (2 x Hydrogen's atomic mass) + (1 x Oxygen's atomic mass)
   (1 x 12.01 amu) + (2 x 1.01 amu) + (1 x 16.00 amu) = 12.01 + 2.02 + 16.00 = 30.03 amu

Therefore, the empirical formula mass of glucose (CH₂O) is approximately 30.03 amu (atomic mass units).

Relationship to Molecular Formula Mass

The molecular formula mass is a multiple of the empirical formula mass. The molecular formula represents the actual number of atoms of each element in a molecule of the compound. The ratio between the molecular formula mass and the empirical formula mass gives you the multiplying factor to convert the empirical formula to the molecular formula.

For glucose:

• Molecular Formula: C₆H₁₂O₆
• Molecular Formula Mass: (6 x 12.01) + (12 x 1.01) + (6 x 16.00) ≈ 180.18 amu
• Empirical Formula: CH₂O
• Empirical Formula Mass: ≈ 30.03 amu

The ratio is 180.18 / 30.03 ≈ 6. This confirms that you multiply the subscripts in the empirical formula (CH₂O) by 6 to get the molecular formula (C₆H₁₂O₆).

In summary, the empirical formula mass is a fundamental concept in chemistry used to represent the mass of the simplest ratio of elements within a compound.