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

unknown

  

56.8

0.476

  

Answer:

For this problem the formula of the compound is not given!

Does this make calculating the molar mass of the substance impossible? NO!

Notice the units on molar mass are grams per mol. The grams of the sample and moles of the sample are given. All we need do to calculate the molar mass is to divide the mass of the sample by the moles of 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

unknown

119

56.8

0.476

  

To calculate the number of formula units (we do not know whether the substance is an atom, molecule, or ionic) the number of moles of the substance are know and Avogadro's number is known. Recall Avogadro's number is the number of particles in per mol,

6.02 x 1023 formula units = 1 mol

Convert moles of the substance to formula units using the above equality,

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

unknown

119

56.8

0.476

2.87 x 1023