Chemical Formulas Lesson

Chapter 7 Section 3.1: Formula Mass and Molar Mass

​Introduction

• ​A chemical formula tells you the elements in a compound as well as the relative number of atoms of each element

• ​This information can be used to calculate various characteristics for a compound

  • ​Formula Mass

  • ​Molar Mass

  • ​Percentage composition

​Formula Masses

• ​The formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all atoms represented in its formula

  • ​Example: the formula mass of water H₂O

    • ​2 hydrogen mass = 2

    • ​1 oxygen mass =16

    • ​Total mass = 2+16=18

​Sample Problem F on Page 238

​Find the formula mass of potassium chlorate, KClO₃

• Given Information:

  • ​Number of atoms of each element present

    • ​1 potassium, 1 chlorine, 3 Oxygen

  • Atomic Mass of each element present

    • ​K=39

    • ​Cl=35

    • ​O=16

​Sample Problem F on Page 238

​Find the formula mass of potassium chlorate, KClO₃

• Given Information:

  • ​Number of atoms of each element present

    • ​1 potassium, 1 chlorine, 3 Oxygen

  • Atomic Mass of each element present

    • ​K=39

    • ​Cl=35

    • ​O=16

• ​Doing the Math:

  • ​(1*39)+(1*35)+(3*16)=122.55

​Practice Problem 2: Find the formula mass of NaSO₃

• ​Which elements are present and how many of them

  • ​1 Sodium, 1 Sulfur, 3 Oxygen

  • ​Masses:

    • ​Na=23

    • ​S=32

    • ​O=16

  • ​Doing the math: (1*23)+(1*32)+(3*16)=126

​Molar Masses

• ​Recall: Molar Mass = mass in grams of one mole

  • ​one mole = 6.022 x 10²³ atoms

• ​One mole of water has two moles of Hydrogen atoms and one mole of oxygen atoms

  • ​Each mole of hydrogen = 1 g/mol, each mole of oxygen = 16 g/mol

    • ​(2*1)+(1*16)=18

• ​A compounds molar mass is equal to it's formula mass