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. If you would like some help use the Help table below. To check your answers use the Answer table.)

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

Cr2O3

  

  

 

9.63 x 1023

Answer:

In this case the compound is chromium (III) oxide, Cr2O3 and only the number of formula units are known. The molar mass can be determined by adding the atomic masses of two chromium atoms (52.0 u) and three oxygen atoms (16.0 u).

2 x 52.0 u + 3 x 16.0 u = 152 u

Changing the unit 'u' to 'grams' gives the molar mass of Cr2O3.

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

Cr2O3

152 

  

 

9.63 x 1023

Next calculate the number of moles of the sample. Moles must be calculated before the mass of the sample. To calculate the moles of the sample we'll use Avogadro's number, 6.02 x 1023 = 1 mol. We are given 9.63 x 1023 formula units of Cr2O3 in the sample.

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

Cr2O3

152 

  

  1.60

9.63 x 1023

Knowing the number of moles of the sample use the molar mass (the number of grams in a mole of Cr2O3) to convert from moles of sample to grams,

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

Cr2O3

152

243

1.60

9.63 x 1023