1.7.3. Molecular Mass

Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

Molecular mass refers to the total mass of a molecule calculated by adding the atomic masses of each element in that molecule, weighted by how many times each element appears in the molecule. For example, in a water molecule (H₂O), there are two hydrogen atoms and one oxygen atom. Each oxygen atom has an atomic mass of 16.00 atomic mass units (amu), while each hydrogen atom has an atomic mass of approximately 1.008 amu. Therefore, the molecular mass of water can be calculated as follows:

• From two hydrogen atoms: 2 × 1.008 amu = 2.016 amu
• From one oxygen atom: 1 × 16.00 amu = 16.00 amu
• Total molecular mass = 2.016 amu + 16.00 amu = 18.016 amu.

Imagine you are baking cookies. You have a recipe that calls for 2 cups of flour, 1 cup of sugar, and 1 cup of butter. To know the total amount of ingredients you have, you would add them together—just like adding the atomic masses of each element to find the molecular mass of a compound.

For example, molecular mass of methane, which contains one carbon atom and four hydrogen atoms, can be obtained as follows: Molecular mass of methane, (CH₄) = (12.011 u) + 4 (1.008 u) = 16.043 u.

To calculate the molecular mass of methane (CH₄), you consider that it consists of one carbon atom and four hydrogen atoms. The atomic mass of carbon (C) is approximately 12.011 amu, and the atomic mass of hydrogen (H) is approximately 1.008 amu. The calculation works as follows:

• For carbon: 1 × 12.011 amu = 12.011 amu
• For four hydrogen atoms: 4 × 1.008 amu = 4.032 amu
• Total molecular mass of methane = 12.011 amu + 4.032 amu = 16.043 amu.

Consider a shopping cart where you are buying a single avocado (C) and four tomatoes (H). If the avocado costs $12.011 and each tomato costs$1.008, to find out how much you spend in total for the ingredients, you would add together the costs: $12.011 (avocado) + 4 ×$1.008 (tomatoes) = total costs, similar to calculating the molecular mass.

Similarly, molecular mass of water (H₂O) = 2 × atomic mass of hydrogen + 1 × atomic mass of oxygen = 2 (1.008 u) + 16.00 u = 18.02 u.

The molecular mass of water is calculated by looking at its molecular formula, which consists of two hydrogen atoms and one oxygen atom. The calculation is done as follows:

• For hydrogen: 2 × 1.008 amu = 2.016 amu
• For oxygen: 1 × 16.00 amu = 16.00 amu
• Therefore, the total molecular mass of water (H₂O) is 2.016 amu + 16.00 amu = 18.016 amu, which is often rounded to 18.02 amu.

Think of making a fruit salad with 2 cups of strawberries and 1 cup of blueberries. If strawberries weigh 1.008 ounces each and blueberries weigh 16.00 ounces per cup, you would calculate the total weight by multiplying and adding their respective weights. This is similar to how we calculate the molecular mass of H₂O.

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