Understanding Molecular Formulas from Empirical Formulas and Molecular Mass

In chemistry, understanding how to derive a molecular formula from an empirical formula and a given molecular mass is a fundamental skill. This guide will walk you through the process, using examples to illustrate the steps involved.

What is an Empirical Formula?

The empirical formula represents the simplest whole-number ratio of the elements in a compound. For instance, if a compound contains 17.2% carbon, 32.1% hydrogen, and 50.7% oxygen by mass, it can be calculated that the simplest whole-number ratio of these elements is 1:2:1, which would yield the empirical formula CH2O.

Deriving the Molecular Formula from Empirical and Molecular Mass

Once we have the empirical formula, we can use the molecular mass to find the molecular formula. The molecular formula is a whole-number multiple of the empirical formula. Here's the step-by-step process:

Calculate the empirical formula mass: C1H2O1 has an empirical formula mass of 12 2 16 30 g/mol. Divide the given molecular mass by the empirical formula mass:

n  Molecular mass / Empirical formula mass
   150 g/mol / 30 g/mol
   5

Multiply the entire empirical formula by this ratio to get the molecular formula: - The molecular formula of a compound with the empirical formula CH2O and a molecular mass of 150 is C5H10O5.

Example Calculations

Example 1

Empirical formula mass: 12 (C) 2 (H) 16 (O) 30 g/mol

Molecular mass: 150 g/mol

Multiplying factor:

n  150 / 30
   5

Molecular formula: C5H10O5

Example 2

Empirical formula mass: 12 2 16 30 g/mol

Molecular mass: 180 g/mol

Multiplying factor:

n  180 / 30
   6

Molecular formula: C6H12O6

Example 3

Another compound with a molecular mass of 180 and an empirical formula of CH2O can be calculated similarly:

Empirical formula mass: 30 g/mol

Multiplying factor:

n  180 / 30
       6

Molecular formula: C6H12O6

This is the formula for glucose.

Practice Calculations

Try calculating the molecular formula for a compound with an empirical formula of CH2O and a given molecular mass of 180:

Empirical formula mass: 30 g/mol Molecular mass: 180 g/mol Multiplying factor:

n  180 / 30
   6

Molecular formula: C6H12O6

Conclusion

Understanding how to derive the molecular formula from an empirical formula and molecular mass is crucial for those studying organic chemistry. The process involves calculating the empirical formula mass, dividing the given molecular mass by the empirical formula mass to find the multiplying factor, and then multiplying the empirical formula by that factor to obtain the molecular formula.

Recommended Reading:

How to Calculate Molar Mass Empirical vs. Molecular Formulas Understanding Stoichiometry

For further practice, consider working through molecular formula and empirical formula problems from your textbook or online resources. This will help reinforce your understanding and improve your problem-solving skills in chemistry.