# Quick refresh: molecular weight

A quick refresher on molecular weight: if you look at a periodic table, every element has a number and a molecular weight. The molecular number tells you how many proton an atom has, and the molecular weight gives you the average weight of an atom (including protons and a range of possible numbers of neutrons). So, no matter how your periodic table is organized, the molecular wieght is the larger of the numbers listed in the element's box:

blurry table of elements some excerpts

By definition, one mole of an element weighs the same (in grams) as the molecular weight.

For those of you who are thinking, hold on, what's a mole ... a mole is a more convenient way to count up molecule. As everyone knows, your average fist-sized object contains a billion-gazillion molecules, which makes them rather inconvenient to count. So instead, we talk about a "mole" of molecules, which means 6.022 * 1023, or about 60 trillion trillion. Kind of like a "dozen" eggs, but a much bigger number.

So anyway, one mole of carbon is the same as 60 trillion trillion carbon atoms and weighs all of 12 grams. Likewise, a mole of hydrogen weighs a paltry 1 gram, and a mole of oxygen tops out at 16 grams. (We're rounding a little here, which is no big deal).

To get a single glucose molecule (C6H12O6), we need 6 carbon atoms, 12 hydrogen atoms, and finally 6 oxygen atoms. Likewise, to get a mole of glucose, we need 6 moles of carbon, 12 moles of hydrogen, and 6 moles of oxygen. All of this weighs

6*12 + 12*1 + 1*16 = 180 grams.

So, the molecular weight (or weight of a mole) of sugar is 180g.

### The chemical formula of caffeine is C8H10N4O2. Using the periodic table excerpts from below, what is its molecular weight?

(To make this problem interactive, turn on javascript!)

• I need a hint ... : A caffeince molecule has 8 carbon atoms and each weights about 12 g/mol.

• ...another hint ... : Add up the weight of each atom type separately.