Once we know the mass of a sample we can use the mass of the atoms or molecules in the sample to determine how many atoms or molecules are in the sample.

**How many ^{12}C atoms are in a 1.00 kg block of pure ^{12}C isotope?**

Obviously atoms and molecules are not a convenient unit of measure when we're working with macroscopic (*i.e.*, human size) objects. For this reason chemist define a new unit of measure called **the mole**.

1 mole is defined as the number of carbon atoms in exactly 12.000000 grams of pure ^{12}C.

From our definition of the atomic mass unit we get

1 mole = 6.022140857 x 10^{23} chemical units

The number 6.022140857 x 10^{23} is called **Avogadro's number**.

**How many moles are in a 1.00 kg block of pure ^{12}C isotope?**

Of course we know a 1.00 kg block of naturally occuring carbon will contain a mixture of ^{12}C, ^{13}C, and even some ^{14}C isotopes. So the number of moles of carbon atoms in a 1.00 kg block of naturally occuring carbon is

We can also calculate the mass of a mole of molecules.

**What is the mass of a mole of methane (CH _{4}) molecules?**

mass of 1 mole of C is | 12.011 g |

mass of 4 moles of H is | 4 X 1.00g |

mass of 1 mole of CH _{4} is | 16.043 g |

In other words the **molecular weight** of CH_{4} is 16.043g.

**Molecular weight** = the mass in grams of 1 mole of a molecule.

For ionic compounds, which do not exist as individual molecules we use the term Formula weight

**Formula weight** = The mass in grams of 1 mole of the chemical formula.

**What is the formula weight of CaCO _{3}?**

mass of 1 mole of Ca is | 40.08 g |

mass of 1 mole of C is | 12.011 g |

mass of 3 moles of O is | 3 X 15.999 g |

Formula weight of CaCO _{3} is | 100.09 g |

Let's look at another example.

**How many hydrogen atoms are there in 2.50 g of NH _{3}?**

Now, let's try some sample quiz questions on:

Chemisty, The Central Science, 10th Ed.

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