Reference: Mole Calculations & Percent Compositions printable pdf download

Pre-AP Chemistry

Reference: Mole Calculations & Percent Compositions

Mass (g) to Moles Calculations

Total Number of Molecules to Moles Calculations

1 mole of any atom or molecule

[a] One mole of sodium has a mass of 23.0 grams (This

6.02x10 23 particles

value is directly from the periodic table). How much would

2.50 moles weigh?

23.0 grams

à

= 57.5 grams

× 2.5moles

1 mole

[e] How many atoms of Pb are present in 1.3 moles?

à 6.02x10

atoms

×1.3 moles =7.8x10

atoms

1mole

[b] Calculate the how many moles of gold are present in

12.5 grams.

Notice that the type of particle (Pb) did NOT matter. A

à 1 mole Au

×12.5 g =0.0635 mole

mole of anything is just a really big number. When you are

197.0 g

just counting particles, it does not matter what you are

counting. Only mass (grams) depends on the type of

Notice that in these examples, each value was given to three

material. A dozen eggs is the same number as a dozen

bowling balls. A dozen eggs weighs less than a dozen

significant figures. The answer is also given to three

significant figures. Don’t round your work in the middle of

bowling balls, however.

the calculations. Round your final answer to have the same

[f] If you have 2.50x10

molecules, how many moles

number of significant figures as the original problem.

are present?

1 mole

[c] Calculate the mass of one mole of carbon tetrachloride.

à

× 2.50x10

molecules

à CCl

6.02x10

molecules

à Look up each element on the periodic table: C=12.0, Cl =

à = 0.0415 moles or 4.15x10

–2

moles

35.5 grams per mole

à Add them up based on how often they repeat in the

molecule:

Chart/Summary

12 + 4(35.5) = 154.0 g/mole CCl

à

1 mole CCl

154.0 g

[d] How much would 0.750 moles of Na

weigh?

àNa = 23.0, S=32.1, O=16.0

à2(23)+32.1+4(16) = 142.1 gram/mole of Na

142.1 g

à

× 0.75 mole =106.575 = 107 grams

1 mole Na

When a compound is ionic, such as NaCl, it does not occur

as separate molecules. It is a big, repeating crystal of Na’s

and Cl’s. The basic ratio of atoms or “building blocks” is

called a formula unit instead of a molecule. The

calculations are the same.

A formula unit of NaCl is Na=23.0, and Cl=35.5

NaCl = 58.5 gram/mole

Handout: Mole Calculations

11/20/14

Reference: Mole Calculations & Percent Compositions

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