Inorganic chemistry
 Converting mass in grams to number
of atoms.
 Calculating involving molecular mass,
Calculating molecular mass.
 Calculating the number of moles in a
given amount of a compound.
Assistance Leccturer Amjad Ahmed Jumaa
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1
Converting mass in grams to number of atoms:
To converting the following conversion, you need to use both molar mass and
Avogadro's number as conversion factors.
Example:
Zinc is the main component of pennies minted after 1981, how many zinc atoms are
present in (20.0 g) of (Zn)?
Solution:
Step(1): the two conversion factors needed are:
 You should come up with following strategy:
Grams of Zn → moles of Zn → atoms of Zn.
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Step(2):
Calculating involving molecular mass:
Calculating molecular mass:
The molecular mass is simply the sum of the atomic masses ( in amu) of all the
atoms in the molecule.
Example:
Calculate the molecular mass of carbon tetrachloride (CCl4).
Molar mass of (CCl4) = (mass of C) + 4(mass of Cl).
Molecular mass (CCl4) = (12.01 amu) + 4(35.45 amu) = 153.8 amu.
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Calculating the number of moles in a given amount of a compound:
To complete this conversion, the only conversion factor needed is the molar
mass in units of ( g/mol). Remember, the molar mass of a compound ( in grams) is
numerically equal to its molecular mass ( in atomic mass units). For example, the
molar mass of (CCl4 is 153.8 g/mol), compared to its molecular mass of( 153.8
amu).
Example:
How many moles of ethane ( C2H6) are present in (50.3 g) of ethane.
Solution:
Step(1): first, the molar mass of ethane must be calculated in order to have the
appropriate conversion factor for the problem.
Molar mass of (C2H6) = 2(12.01g) + 6( 1.008 g) = 30.07 g.
Hence , the conversion factor is
1 mol C2H6
30.07 g C2H6
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Step (2): based on the conversion factor above,
you should come up with the following strategy.
Grams C2H6 → moles of C2H6
Step (3):
= 1.67 moles
? Moles of C2H6 = 50.3 g C2H6 x 1 mol C2H6
30.07 g C2H6
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