To determine the empirical formula of a compound, you need to find the mass of each element present in the compound and convert it to moles. Then, calculate the individual mole ratios and finally write out the empirical formula.
There are several ways to represent the molecular structure and composition of a chemical compound. This includes the commonly used molecular formula, which shows the number of atoms of each element in the compound, as well as the structural formula, which illustrates the arrangement and bonds between different atoms in the compound.
Another important way to convey information about a compound is through its empirical formula. The empirical formula of a compound shows the elements that make up the compound in their simplest possible integer ratio.
How To Determine The Empirical Formula?
The empirical formula of any compound can be determined using a few simple steps. The procedure involves finding the amount of each element in the compound and converting that amount to moles. Then, individual mole ratio calculations are performed. Once the individual mole ratios are calculated, they can be converted to whole numbers (if necessary) and the empirical formula of the compound can be written. However, let’s take a closer look at the steps involved in calculating the empirical formula of a compound.
Explanation of empirical formula calculation using a flowchart
Step 1: Find The Mass (Amount) Of Each Element In The Compound
We begin by determining the exact amount (in grams) of each element present in the compound under study. Let’s assume that the compound we are analyzing is XaYbZc. Chemical analysis of the compound XaYbZc provides information about the percentage of each element (X, Y, and Z) in the compound. We convert these percentage values into simple numbers by assuming that we have 100g of the sample compound.
For example, let’s say the chemical analysis reveals that the compound XaYbZc contains 40.0% of X, 6.7% of Y, and 53.3% of Z. If we have 100 grams of the sample compound, this implies that X is 40 g, Y is 6.7 g, and Z is 53.3 g.
This step is only necessary when the composition of the compound is provided in percentage form. If the amount of each element is already given in grams, this step can be skipped.
Step 2: Convert The Amount Of Each Element To Moles
In Step 2, we convert the mass values obtained in the previous step to moles. In case you’re unaware, moles are a unit of measurement used to quantify the quantities of small particles, such as atoms, molecules, electrons, protons, neutrons, etc. One mole of an element is equivalent to approximately 6.022×1023 atoms of that element. The formula to find the number of moles of an element from its mass is:
Number of moles = Mass of the element (in grams) / Molar mass of the element
Returning to our sample compound…the molar mass of X is 12.0107 g/mol, Y is 1.00784 g/mol, and Z is 15.999 g/mol. (Note: The molar mass of any element can be found through a simple Google search.)
The quantity of moles is as follows: the moles of X are 3.33 mol, the moles of Y are 6.6 mol, and the moles of Z are 3.33 mol.
Step 3: Calculate Mole Ratios Of Each Element
Now, find the element with the smallest number of moles in the compound. From the previous step, we see that both element X and Z have 3.33 moles in the compound, while Y has 6.66 moles.
We can determine the mole ratios of each element by dividing the individual mole values by the smallest mole value found. Here, the smallest value is 3.33 moles. Thus, we find:
Number of moles of X/3.33 = 1
Number of moles of Y/3.33 = 2
Number of moles of Z/3.33 = 1
Step 4: Convert Mole Ratios Into Whole Numbers
The second-to-last step in the procedure is converting the mole ratios to whole numbers, if they are not already. The mole ratios we found in the previous step are 1, 2, and 1, all of which are whole numbers. Therefore, we can skip this step and move on to the final step.
However, if the mole ratios were not whole numbers, multiplying them by an integer would give us the desired values. Let’s say the mole ratios for another compound are 1.5 and 2. 1.5 is not a whole number and needs to be converted. Simply multiplying it by 2 would give us a whole number.
Note: Make sure to multiply all the mole ratios by the integer, not just the non-whole number mole ratio.
1.5 x 2 = 3 and 2 x 2 = 4
Step 5: Write The Empirical Formula
Now that we have all the necessary data, it is time to write the empirical formula for the compound XaYbZc. Write down the symbol of each element in the compound, followed by its mole ratio as a subscript.
For our sample compound, the empirical formula is X1Y2Z1 or simply XY2Z.
Final Words
The sample compound XaYbZc that we have been using as an example is actually glucose. The molecular formula for glucose is C6H12O6, but as we concluded in the final step of the procedure, its empirical formula is CH2O.
Therefore, empirical formulas only provide information about the ratios in which the elements are present, not the actual number of atoms in the compound. Information about the actual number of atoms of an element present in a compound is provided by its molecular formula. To determine the molecular formula of a compound, multiply the subscripts of each element in the empirical formula by the ratio of molecular weight to empirical formula mass.