Chemical Calculations Lesson # 1 Mass, Moles, and Molar Mass

The Mole It is important in chemistry to be able to measure the amount of a substance, whether it be in industrial applications (large scale) or in microscopic applications (tiny scale). The mole is the unit for the amount of a substance (symbol n). The term “mole” is just a name for a particular number of particles, like how the word “dozen” is equivalent to 12, and a “pair” means 2. A mole represents the number 6.02x1023 – this number is called Avogadro’s number (NA), because of the contribution of research of scientist Amedeo Avogadro.

The Mole (continued) When we say we have one mole of carbon, we in fact have 6.02x1023 atoms of carbon. The reason that we talk in “moles” in the first place, is because we cannot simply work with one atom – it is too tiny. Avogadro’s number is a more reasonable amount of atoms that can be seen, manipulated, and measured on a scale. If we want to calculate how many moles we have of a substance based on the number of atoms, or vice versa, Avogadro’s number gives us a conversion factor: 1 mole = 6.02x1023 atoms or molecules of a substance.

Moles (continued) Symbol used for the mole: n SI unit of the mole: mol

Example 1 – How many atoms are their in two moles of Helium?

Example 2 – How many moles are in 8.25x1023 molecules of H2O?

Summary Moles to Atoms: Multiply by NA Atoms to Moles: Divide by NA

Molar Mass Knowing amounts in moles can be helpful at times, but when you are working in the lab, it’s not enough to know that a compound A2B3C4 has 2 moles of A, 3 moles of B, and 4 moles of C – you need to know how much it weighs to be able to add it in to your chemical! Conveniently enough, the mass, in grams, of one mole of atoms or molecules is called the molar mass!

Molar Mass (continued) Symbol used for molar mass: M SI unit of molar mass: g/mol This means that molar mass is equal to the mass (in grams) divided by its amount (in moles), or: Molar mass = (M = m) n This value can be found on your periodic table – you called it “atomic mass” in grade 9 and 10.

Example 1 – what is the mass of water?

Example 2 – What is the mass of PCl5?

Example 3 – Calculate the molar mass of the each of the following: C6H6 A molecule of chlorine Ba(NO3)2 A calcium ion

Example 1 – calculate the mass of 0.50 mol of sulfur

Example 2 – calculate the mass of 2.50 mol of iron

Example 3 – calculate the amount of substance (number of moles) in 5 Example 3 – calculate the amount of substance (number of moles) in 5.20 g of hydrogen gas (H2)

Example 4 – calculate the amount of substance (number of moles) in 8 Example 4 – calculate the amount of substance (number of moles) in 8.75 g of iron (III) chloride (FeCl3)

Summary Mass to Moles: Divide by Molar Mass Moles to Mass: Multiply by Molar Mass

Videos Mole Explanation Molar Eclipse of the Heart