Scientific Tutor

How is the empirical and molecular formula linked to the molar mass?

Another way that empirical and molecular formulas can be asked about is in terms of comparing their molar masses. Lets try that example we had before where the molecular formula was N₄O₁₀ and the empirical was N₂O₅. What is the molar mass of the molecular formula? It is approximately 216g/mol. What is the molar mass of the empirical formula? It is approximately 108g/mol. If we divide 216g/mol by 108g/mol what do we get? We get an answer of 2. How did we go from N₄O₁₀ to N₂O₅ before? We divided it by 2. This is not a coincidence. The same factor that is used in the subscripts of the molecular and empirical formulas is also used when it comes to their differences in molar mass. You can use this information to solve problems that involve molar masses and empirical and molecular formulas. Let us check out one style of these questions that I commonly see.

Examples: If the empirical formula is CH₂, what is the molecular formula if the molar mass is 42 g/mol?

Answer: C₃H₆

If the empirical formula is Al₂O₃, what is the molecular formula if the molar mass is 408 g/mol?

Answer: Al₈O₁₂

VIDEO Empirical and Molecular Formula Demonstrated Example 1: If the empirical formula is Ca₃P₂, what is the molecular formula if the molar mass is 546 g/mol?  Use this periodic table if needed.

Step 1:

What information do they tell us in the problem?

Answer:

empirical formula is Ca₃P₂…and its molar mass is 182 g/mol

molecular formula is ???…and its molar mass is 546 g/mol

Step 2:

What units, compounds, or information look similar or can we compare?

Answer:

182 g/mol and 546 g/mol

Step 3:

How do we compare them?

Answer:

Divide 546 g/mol by 182 g/mol = 3

Step 4:

What does that 3 represent?

Answer:

It is the number that we multiply all the subscripts of the empirical formula by.

Step 5:

What is the molecular formula?

COMPLETE ANSWER: Ca₉P₆

PRACTICE PROBLEMS:  Solve the molecular formula problems.

If the empirical formula is CH₂O, what is the molecular formula if the molar mass is 150 g/mol?

Answer: C₅H₁₀O₅

If the empirical formula is Sn₃N₄, what is the molecular formula if the molar mass is 1239 g/mol?

Answer: Sn₉N₁₂