# How to calculate formula mass

Calculating the formula mass of a compound is a crucial skill for every chemistry student. Formula mass, also known as molecular weight or molar mass, is the sum of the atomic weights of all the atoms that make up a molecule or compound. By learning how to calculate formula mass, you can solve various chemistry problems and gain a fundamental understanding of chemical reactions.

In this article, we will explain step-by-step the process of calculating formula mass for various compounds and provide examples to help you master this essential concept.

**Step 1: Identify the Elements and Their Atomic Weights**

The first step in calculating the formula mass of a compound is to identify its constituent elements and their respective atomic weights. The periodic table is your best friend here. Locate each element involved in your compound on the chart and note their individual atomic weights.

For example, let’s calculate the formula mass of water (H2O). We have two hydrogen atoms (H) with an atomic weight of 1 unit each and one oxygen atom (O) with an atomic weight of 16 units.

**Step 2: Determine the Number of Each Atom in the Compound**

Now that you have identified each element’s atomic weight, it’s time to determine how many atoms of each element are present in the compound. This information can typically be found within the chemical formula itself.

For our water example, we have two hydrogen atoms (indicated by the subscript ‘2’ after ‘H’) and one oxygen atom (since there is no subscript after ‘O’, it is assumed to be ‘1’).

**Step 3: Multiply Each Atom’s Atomic Weight by Its Quantity in the Compound**

Next, multiply each element’s atomic weight by its corresponding quantity within the compound. This process will give you a weight total for each element present.

For water, multiply two hydrogen atoms’ atomic weights (2 x 1 = 2 units) and one oxygen atom’s atomic weight (1 x 16 = 16 units).

**Step 4: Add the Weight Totals for Each Element**

To find the compound’s formula mass, add the weight totals of each element calculated in Step 3.

For water, add the hydrogen weight total to the oxygen weight total (2 + 16 = 18 units). Hence, the formula mass of water is 18 units.

**Examples**

1. Carbon Dioxide (CO2)

– Carbon: Atomic weight = 12 units; Quantity = 1

– Oxygen: Atomic weight = 16 units; Quantity = 2

Formula mass = (1 x 12) + (2 x 16) = 12 + 32 = 44 units

2. Sodium Chloride (NaCl)

– Sodium: Atomic weight = 23 units; Quantity = 1

– Chlorine: Atomic weight = 35.5 units; Quantity = 1

Formula mass = (1 x 23) + (1 x 35.5) = 23 + 35.5 = 58.5 units

**Conclusion**

Calculating formula mass is a fundamental skill in chemistry and essential for understanding chemical reactions and solving various chemistry problems. By following these four simple steps – identifying the elements and their atomic weights, determining the number of each atom in the compound, multiplying each atom’s atomic weight by its quantity, and adding these values – you can easily calculate formula mass for any molecule or compound.