# Questions and Answers

Is there a way to tell how many atoms are in a certain object?

The simplest way is to know two things about the object: (1) its chemical composition and (2) its mass. This is the sort of calculation you will learn to do in high school chemistry. As an example, let's say we have a very tiny drop of water, say 1 microliter, which would have a mass of 0.001 grams. The chemical formula for water is H_{2}O which means that every molecule of water has 2 atoms of hydrogen (H) and one atom of oxygen (O). Here comes the key part. From the Periodic Table of Elements, one sees that one mole of hydrogen atoms weighs 1 gram while one mole of oxygen atoms weighs 16 grams. What is this mole thing? A mole is a convenient counting unit whenever one is dealing with numbers of atoms or molecules. It is equal to Avogadro's number (NA), namely 6.022 x10^{23}. If we have one mole of water, then we know that it will have a mass of 2 grams (for 2 moles of H atoms) + 16 grams (for one mole O atom) = 18 grams. Since we only have 0.001 grams, that means that we have NA * (0.001/18) = 3.35*10^{19} molecules of water! This is a huge number, even though we only started with a tiny amount of water.

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