Now let's look at a couple of example problems.

Calculate the number of moles in 31.4 g of Fe.

Procedure to Follow:

Remember our timeline for solving these problems:
Timeline

Also, remember our 3-step approach to problem solving:

  1. Determine what information we have or can use for this calculation

  2. Determine what it is we need to calculate

  3. Set up equation to solve using appropriate conversion factors

The nice thing about the timeline is that we can see where we are starting (step #1), see where we are going (step #2), and find the appropriate conversion factors to get from step #1 to step #2 (step #3).

  1. We are given the following information

      We have 31.4 g of Fe

  2. We want to calculate the number of moles of Fe

  3. So, if we look at our timeline, we are starting on the left-hand side (mass) and we want to get to the middle (moles). The conversion factor that allows us to do this is molar mass. We know the molar mass of Fe is 55.85 g/mol.

    Now, we can setup the equation to solve:

    Calculation

Notice that the conversion factor will always be a fraction! It is merely a relationship between two quantities of information. For example, molar mass is the relationship between mass of a substance and 1 mole of that substance.

Let's try another example.

Calculate the number of moles in 5.0 x 108 atoms of 14C.

  1. We are given the following information

      We have 5.0 x 108 atoms of 14C

  2. We want to calculate the number of moles of 14C

  3. So, if we look at our timeline, we are starting on the right-hand side (atoms) and we want to get to the middle (moles). The conversion factor that allows us to do this is Avogadro's Number.

    Now, we can setup the equation to solve:

    Calculation

Let's do one more that involves multiple conversion factors.

Calculate the mass (in grams) of 1.0 x 1032 molecules of water, H2O.

  1. We are given the following information

      We have 1.0 x 1032 molecules of H2O

  2. We want to calculate the number of grams of this many molecules of water

  3. So, if we look at our timeline, we are starting on the right-hand side (molecules) and we want to get to the left-hand side (mass). Now, we will need two conversion factors. The first conversion factor, Avogadro's Number, will convert molecules to moles. Then we need the conversion factor between moles and mass which is molar mass.

    Now, we can setup the equation to solve:

    Calculation

Now it's your turn to work a few examples. You will need some scratch paper, a calculator, and a periodic table handy to do these examples.

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