HTML University

Science Dept.

Chemistry CHE 100 Notes
J. Chemical Calculations

1. Definition of Atomic Mass Unit
   1. 1amu = 1/12 ¹²C atom

2. Calculation of Formula or Molecular Masses (or Weights)
   1. H₂O

   2. NaCl

   3. H₂CO₃

   4. CH₃OH

3. Calculation of Moles
   1. Give the Atomic Weight of K in amu

   2. Give the Atomic Weight of Na in amu

   3. Notice the Weight Ratio between K and Na
      1. Would there be the same number of K atoms in 39 amu of K
         as there would be number of Na atoms in 22.99 amu of Na?

      2. Would there be the same number of K atoms in 39 grams of K
         as there would be number of Na atoms in 22.99 grams of Na?

   4. Avogadro's Number = 6.022 x 10²³

   5. How many moles of O in 20.0 grams of O?

   6. How many moles of O₂ in 20.0 grams of O₂?

4. Molar Volume Of A Gas
   1. One Mole of a Gas occupies 22.4 liters at STP

   2. Definition of STP or Standard Temperature and Pressure
      1. Standard Temperature is 0^oC

      2. Standard Pressure is One Standard Atmosphere (14.7 psi or 760 mm Hg)

5. Calculation of Per Cent Composition of Compounds
   1. Na in NaCl
      1. Na = 22.99 amu; Cl = 35.45 amu

      2. [(Na)/(NaCl)] x 100 = Na%

      3. [(22.99 amu)/(22.99 + 35.45 amu)] x 100 = Na%

      4. [(22.99 amu)/(58.44 amu)] x 100 = 39.34% = Na% in NaCl

   2. H in H₂O
      1. H = 1.0 amu; O = 16 amu

      2. [(2H)/(2H + O)] x 100 = H%

      3. [(2 amu)/(2 + 16 amu)] x 100 = H%

      4. [(2 amu)/(18 amu)] x 100 = 11.1% = H% in H₂O

6. Calculation of Simplest Formula
   1. Information: Na = 32.4%; S = 22.6%; O = 45.1%

   2. Change per cent to grams: Na = 32.4g; S = 22.6g; O = 45.1g

   3. How many moles?
      1. of Na = 32.4g/22.99 = 1.41 moles Na

      2. of S = 22.6g/32.1 = 0.704 moles of S

      3. of O = 45.1g/16.0 = 2.82 moles of O

   4. Divide all three values of moles by the smallest value
      1. for Na: 1.41/0.704 = 2

      2. for S: 0.704/0.704 = 1

      3. for O: 2.82/0.0704 = 4

   5. The Formula is: Na₂SO₄

7. How About a Quick Quiz?