How to Find Empirical Formula Mass?

The empirical formula mass is found by determining the simplest whole-number ratio of elements in a compound and then calculating the mass of that formula. This process involves several steps.

Steps to Determine Empirical Formula Mass

Here's a step-by-step guide to finding the empirical formula mass using the information provided:

1. Convert Mass to Moles:

   • First, you need to know the mass of each element in the compound.
   • Then, using the molar mass from the periodic table, change the mass of every element to moles. This is done by dividing the mass of each element by its respective molar mass.
   • Example: If you have 24g of Carbon and Carbon's molar mass is 12g/mol, then you would have 24g / 12g/mol = 2 moles of Carbon.

2. Determine the Mole Ratio:

   • Next, divide every mole value by the lowest number of moles computed from the previous step. This will give you the mole ratio of each element relative to the element with the smallest number of moles.
   • Example: If you have 2 moles of Carbon and 4 moles of Hydrogen, divide both by 2, resulting in a 1:2 ratio.

3. Round to Whole Numbers:

   • Now, round the resulting mole ratios to the nearest whole number. These whole numbers become the subscripts in your empirical formula, representing the mole ratio of the elements.
   • Example: A mole ratio of 1:2.03 might be rounded to 1:2.

4. Write the Empirical Formula:

   • Using the whole number ratios, you can now write the empirical formula, indicating the ratio of each element.
   • Example: From the previous step's ratio of 1:2 of Carbon and Hydrogen, the empirical formula is CH₂

5. Calculate the Empirical Formula Mass:

   • Once you have the empirical formula, you can calculate its mass.
   • Multiply the atomic mass of each element by its subscript in the empirical formula and add the results together.
   • Example: For CH₂, it is (1 12.01 g/mol) + (2 1.01 g/mol) = 14.03 g/mol

Practical Insights

• Dealing with Non-Whole Numbers: If the mole ratios are not close to whole numbers after the division, you may need to multiply all of the ratios by a common factor to obtain whole number subscripts.
• Context is Key: The empirical formula provides the simplest ratio of elements, not necessarily the actual molecular formula. The molecular formula could be a multiple of the empirical formula.

In summary, finding the empirical formula mass involves converting mass data to moles, determining mole ratios, and then writing the empirical formula before calculating the empirical formula mass.