Inorganic chemistry Assistance Leccturer Amjad Ahmed Jumaa  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. 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: 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.

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 (CCl 4 ). Molar mass of (CCl 4 ) = (mass of C) + 4(mass of Cl). Molecular mass (CCl 4 ) = (12.01 amu) + 4(35.45 amu) = amu.

 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 (CCl 4 is g/mol), compared to its molecular mass of( amu). Example: How many moles of ethane ( C 2 H 6 ) 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 (C 2 H 6 ) = 2(12.01g) + 6( g) = g.  Hence, the conversion factor is 1 mol C 2 H g C 2 H 6

Step (3): ? Moles of C 2 H 6 = 50.3 g C 2 H 6 x 1 mol C 2 H 6 Step (2): based on the conversion factor above, you should come up with the following strategy. Grams C 2 H 6 → moles of C 2 H g C 2 H 6 = 1.67 moles