MOLE - AMOUNT OF SUBSTANCE BASIC CHEMISTRY INFORMATION AND TUTORIALS

What is inside 1 mole?

Sometimes we need to keep track of the number atoms or molecules in a sample. We can’t go strictly by mass; different atoms and molecules have different masses.

One gram of water has over 2½ times as many molecules as one gram of ethanol because ethanol molecules are much more massive than those of water. It is not possible to count atoms and molecules because they are invisibly small and uncountably numerous.

To understand how chemists keep track of particles, it is helpful first to consider how builders keep track of nails.

A building project uses many more nails than anyone wants to count, so instead, nails are weighed. To plan a project, the builder consults a table that gives the weight of 1000 nails. For example, 1000 1½ inch tinned nails weighs 4 pounds. Based on this information, the builder knows how to count nails by weighing. It should be clear that this method will only work if all the nails are about the same mass.

To put this into a chemical perspective, a single one of these nails contains about 19,600,000,000,000,000,000,000 iron atoms. Clearly we can’t deal with atoms in thousands; the weight of a thousand atoms would not register on the most sensitive scale. The standard number of units—that is, atoms, molecules, or ions—used by chemists is 6.022 × 10^23, which is 6.022 multiplied by 1 followed by 23 zeros.

The reason for this clumsy-looking number is that it is about the number of atoms in one gram of hydrogen, the lightest of the atoms. An amount of stuff with this many particles is called a mole of the stuff. In chemical jargon, we say that the molar mass (also called atomic weight) of hydrogen atoms is one gram per mole.

The mass of a mole of something is called its molar mass or molecular weight (in the special case of the atoms of an element, it is sometimes called the atomic weight). The molar masses of atoms of the elements are determined by the numbers of protons and neutrons in the atoms.

Different elements have different molar masses, because their atoms weigh different amounts. The molar masses of the elements are listed on the periodic table.


Rather than think of H2O as a formula representing a single water molecule weighing 3.0 × 10-23 gram, we can think of it as representing a mole of water molecules weighing 18 grams. The formula tells us that each mole of water has two moles of hydrogen atoms (1 gram each) and one mole of oxygen atoms (16 grams each). We can think of chemical equations in terms of moles. The equation H2 + ½O2 → H2O makes sense if we read it as one mole of H2 reacts with half a mole of O2 and yields one mole of H2O.