Calculating Formula Weight

Formula Weight: sum of masses of all atoms in a given formula or compound

Formula Weight = (# atoms of A) (atomic mass of A) + (# atoms of B) (atomic mass B) …

Example: What is the formula weight of glucose: C₆H₁₂O₆?

Steps to Solve:

1. List each element and the # of atoms of that element are in the formula

C: (6 atoms of C)

H: (12 atoms of H)

O: (6 atoms of O)

2. Multiply the number of atoms of each element by that element’s respective mass (from the periodic table) to determine the TOTAL mass contributed to the formula from each element in small “chunks:”

C: (6 atoms of C) x (12 g) = 72 g

H: (12 atoms of H) x (1 g) = 12 g

O: (6 atoms of O) x (16 g) = 96 g

3. Add all accumulated masses for each element in the formula to calculate the formula weight:

72 g + 12 g + 96 g = 180 g

So formula weight of glucose = 180 g

Let’s try one more…

Example: What is the formula weight of aluminum sulfate?  Al₂(SO₄)₃    [Ans: 342 g]

Solution:

Al: 2 atoms x 27 g = 54 g
S: 3 atoms x 32 g = 96 g
O: 12 atoms x 16 g = 192 g

54 + 96 g + 192 g = 342g

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FYI: Amu and Grams

While grams are typically used to calculate formula weight because they are more practice, did you know that the first unit used to denote atomic mass–although now arbitrary—was the atomic mass unit. You may see the “amu” unit used for mass in textbooks, but not to worry, there is no need to convert from one to the other. Luckily, they mean the same thing! So 1 amu = 1 gram.

Why? Every amu is the same as a gram thanks to a clever notion: that 1 amu is equivalent to the 1/12 of the mass of a carbon atom (12 on the periodic table) in amu. So 1/12 x 12=1. That is how we attain the 1:1 ratio.