Scientific Understanding

**How do you calculate the molar mass?**

After we have learned how to form these ionic compounds, we can move on to calculating the mass of a compound. This is what teachers will call a molar mass, or atomic mass, or molar weight, or an atomic weight. Most chemistry teachers will use all of those phrases in the previous sentence interchangeably. As far as you need to know, the molar mass, atomic mass, molar weight, or atomic weight are all talking about the same thing and you use the same resource to answer these problems. That resource is the periodic table. At the bottom of each individual element box on the periodic table will be the molar mass of each element. The way to solve the problems is to total up all the masses of the individual atoms. In order to keep the numbers simple and easy to read, I will usually round to the nearest whole number. If you are confused as to what the parenthesis mean then look back to a previous section representing compounds and molecules with subscripts.

**Examples**: Give the molar mass of the compounds or molecules.

H_{2}O |
18 g/mol |

CO_{2} |
44 g/mol |

N_{4}S_{3} |
152 g/mol |

(H_{3}O)_{3}As |
130 g/mol |

Ra(BrO_{3})_{2} |
482 g/mol |

**VIDEO Calculating Molar Mass Demonstrated Example 1****: **Give the molar mass of the compound or molecule: Al_{2}S_{3}

**Step 1:**

What is the mass of a single Aluminum?

Answer: 27 g/mol

**Step ****2****:**

How much Aluminum do we have?

Answer: 2

**Step ****3****:**

What is the total mass of the Aluminum?

Answer: 2 * 27 g/mol = 54 g/mol

**Step ****4****:**

What is the mass of a single Sulfur?

Answer: 32 g/mol

**Step ****5****:**

How much Sulfur do you have?

Answer: 3

**Step ****6****:**

What is the total mass of the Sulfur?

Answer: 3 * 32 g/mol = 96 g/mol

**Step ****7****:**

What is the total mass of the compound?

96 g/mol + 54 g/mol = 150 g/mol

COMPLETE ANSWER: 150 g/mol

**VIDEO Calculating Molar Mass Demonstrated Example 2****: **Give the molar mass of the compound or molecule: Be(NO_{3})_{2}

**Step 1:**

What is the mass of a single Beryllium?

Answer: 9 g/mol

**Step ****2****:**

How much Beryllium do we have?

Answer: 1

**Step ****3****:**

What is the total mass of the Beryllium?

Answer: 1 * 9 g/mol = 9 g/mol

**Step ****4****:**

What is the mass of a single Nitrogen?

Answer: 14 g/mol

**Step ****5****:**

How much Nitrogen do you have?

Answer: 2

**Step ****6****:**

What is the total mass of the Nitrogen?

Answer: 2 * 14 g/mol = 28 g/mol

**Step ****7****:**

What is the mass of a single Oxygen?

Answer: 16 g/mol

**Step ****8****:**

How much Oxygen do we have?

Answer: 6

**Step ****9****:**

What is the total mass of the Oxygen?

Answer: 6 * 16 g/mol = 96 g/mol

**Step ****10****:**

What is the total mass of the compound?

96 g/mol + 28 g/mol + 9 g/mol = 133 g/mol

COMPLETE ANSWER: 133 g/mol

**PRACTICE PROBLEMS**: Give the molar mass of the compounds or molecules. Use the periodic table.

BaSe | 216 g/mol |

Si_{5}As_{8} |
740 g/mol |

K_{3}P |
131 g/mol |

CH_{4} |
16 g/mol |

C_{6}H_{12}O_{6} |
180 g/mol |

BClF_{2} |
84 g/mol |

(NH_{4})_{3}P |
85 g/mol |

Pb(Cr_{2}O_{7})_{2} |
639 g/mol |

Ba(C_{2}H_{3}O_{2})_{2} |
255 g/mol |

We will be reviewing the molar mass continuously throughout the first half of chemistry, so make sure you know how to solve the above problems and keep the techniques in mind.