Sarah C. answered • 10/11/20
Chemistry Major (and TA) with 1.5 years of Research Experience
To find the formula weight of C8H10N4O2 (caffeine) or NH4Br, first you need to find the empirical formula, which means to write the molecular formula using the simplest ratios. For example, fructose (C6H12O6), a sugar found in fruits and high-fructose corn syrup, has the empirical formula C1H2O1 (the "1"s aren't necessary). I derived the empirical formula by dividing the numbers (6 and 12) in the original fructose molecular formula by 6--their highest common denominator. If the numbers are already in their simplest ratio, like NH4Br, then skip this step.
To use the empirical formula to find the formula weight, refer to your periodic table and locate the atom's average atomic mass (typically at the bottom of that element's square). Once you've located the average atomic mass, multiply that number by the number of that atom in the empirical formula. Referring back the the fructose example, I'd multiply Carbon's average atomic mass (12.01) by 1. Repeat for each atom and sum up the results. For fructose, the formula weight would be (12.01 x 1) + (1.01 x 2) + (16.00 x 1) = 30.03 amu.
Calculating the molar mass of a compound is easy, because we just did it while we found the formula weight! To find the molar mass, count the number of atoms of one element at a time in the compound. For Ca(OH)2, remember the 2 applies across the parentheses, so there's 1 Ca, 2 Os, and 2 Hs. From there, find an elements average atomic mass on the periodic table (40.08 for Calcium) and multiply that number by that element's atoms in the compound (one for Ca in this case). Repeat for the remaining atoms and sum up at the end. For Ca(OH)2 it would look like: (40.08 g/mol x 1 Ca) + (16.00 g/mol x 2 O) + (1.01g/mol x 2 H)
