5.2 – The Avogadro constant and Mole Ms. Munir

Mole The mole is defined as the amount of substance that contains as many elementary entities (atoms, molecules, or formula units) as exactly 12g of Carbon-12. One mole of a substance contains 6.022 x 1023 particles of the substance. This value is called the Avogadro constant(NA).

Example 1 The distance from St. Johns in NF to Vancouver in BC is 5046 km. Suppose that you had 1 mol of peas, each of diameter 1 cm. How many round trips could be made between these cities, laying the peas from end to end?

Solution A round trip is 2(5046 km) = 10,092 km convert to cm 10092 x 105 = 1.01 x 109 cm. In 1 mol of peas, you have 6.022 x 1023 peas. Because each pea is 1 cm long, 6.022 x 1023 peas would be 6.022 x 1023 cm long in a line. Round trip | Distance(cm) ---------------|-------------------- 1 | 1.01 * 109 x | 6.022 * 1023 1/x = (1.01 * 109) / (6.022 * 1023) x = 5.96 * 1014 round trips

Converting moles to number of particles # of particles (atoms, molecules, formula units) Number of moles Multiply by 6.02 x 1023

Formula N = number of particles n = number of moles NA = Avogadro constant N = n x NA

Example A sample contains 1.25 mol of nitrogen dioxide, NO2. Solution: How many molecules are in the sample? How many atoms are in the sample? Solution: 1 mole has 6.02 x 1023 molecules Therefore, 1.25 mole of NO2 will have 1.25 x 6.02 x 1023 molecules = 7.52 x 1023 molecules. In a molecule of NO2, there are 3 total number of atoms. As such there are 3 x 7.52 x 1023 = 2.26 x 1024 atoms in 1.25 moles of NO2.

Converting number of particles to moles Formula: n = N/NA Example: How many moles are present in a sample of carbon dioxide made up of 5.83 x 1024 molecules? Solution: n = 5.83 x 1024 / 6.02 x 1023 = 9.68 mol of CO2 Therefore, there are 9.68 moles of carbon dioxide in the sample.

Homework P 179 #1-10 McGrawHill P 270 #1 – 8 Nelson