Chemistry 2

Molar Mass and % Composition

  1. Molar Mass
    1. The molar mass of a substance is defined as the mass of one mole of particles of that substance.
    2. A particle or unit of a substance is defined by its formula. When doing calculations we need to relate the mass of a substance to the mass of one mole of that substance. For monatomic elements this is easy. The mass of one mole is equivalent to the atomic mass in grams. This is commonly referred to as the molar mass. However, to calculate the molar mass of any other type of substance we use the formula to determine how many moles of atoms are contained in one mole of the substance. For example:
      a. Copper as a general substance looks like this --->
      		Its formula is Cu, so a mole of copper means a mole of these -->
      1 mol of copper atoms =
      1 x 63.55g = 63.55g
      b. Oxygen as a general substance looks like this --->
      Its formula is O2, so a mole of oxygen means a mole of these -->
      		 1 mol of oxygen molecules =
      2 x 16.00g = 32.00g
      c. Water as a general substance looks like this --->
      Its formula is H2O, so a mole of water means a mole of these -->
      1 mol of water molecules =
      2 x 1.008g = 2.016g
      1 x 16.00g = 16.00g
        18.02g
      d. Sodium Chloride as a general substance looks like this --->
      Its formula is NaCl, so a mole of sodium chloride means a mole of these -->
      1 mol of sodium chloride =
      1 x 22.99g = 22.99g
      1 x 35.54g = 35.54g
        58.44g
      (The molecular modles above were used with permission and created by C.H. Mak at Virginia Tech. University)

    3. What is the molar mass of calcium phosphate - Ca3(PO4)2?
  2. Percent Composition
    1. One way to identify compounds is to determine what percentage each element is by mass in a compound.
    2. See the example of water from the above table. The molar mass of sodium chloride is 58.44g. 22.99g of this is sodium and 35.45g is chlorine. If I want to calculate the percent composition of sodium chloride, I would do the following:
    3. What is the percent composition of calcium phosphate?