Complete the following table: (In this problem you are to determine the values for each of the empty entries. The goal of this problem is for you to become comfortable with the moles, molar mass, and Avogadro's number.)

ANSWER Table:

Formula

M, Molar
Mass (g mol-1)

m, Mass of
sample (g)

n, Moles of
sample (mol)

N, Number of atoms,
molecules or formula units

CaBr2

200.

23.8

   

Answer:

In this case the compound is calcium bromide, CaBr2, we are given the molar mass, 200 grams mol-1 and a sample with a mass of 23.8 g. Even though the molar mass was given, it could be calculated by adding the atomic mass for one calcium atom and two bromine atoms. Check it if you are not sure. As long as we know the formula of the substance the molar mass can be determined by summing the atomic masses of the elements in the formula.

The first blank cell is the moles of the sample. To determine the number of moles in the sample the mass of the sample must be converted to moles using the molar mass. This can be set-up by the following conversion,

Note: the molar mass says there are 200. g of CaBr2 in 1 mol of CaBr2. This says for CaBr2, 200. g CaBr2 = 1 mol CaBr2. We can use this equality as a conversion to change grams to moles.

Formula

M, Molar
Mass (g mol-1)

m, Mass of
sample (g)

n, Moles of
sample (mol)

N, Number of atoms,
molecules or formula units

CaBr2

200.

23.8

  0.119

 

Now we can determine the number of formula units of CaBr2 in the sample. (Note: we had to calculate the number of moles in the sample before we could calculate the number of formula units.)

To convert moles to formula units Avogadro's number is used. Recall Avogadro's number relate the number of formula units in 1 mol. 1 mol of a substance contains 6.02 x 1023 particles. In this case CaBr2 is an ionic compound so use the term formula units.

Formula

M, Molar
Mass (g mol-1)

m, Mass of
sample (g)

n, Moles of
sample (mol)

N, Number of atoms,
molecules or formula units

CaBr2

200.

23.8

  0.119

7.16 x 1022 formula units

Try another problem?