1. Lecture 52 – Lecture 53 Mass and the Mole Ozgur Unal 1

2.  What is amu (atomic mass unit)? 2  amu is a unit we use to measure the mass of individual atoms.  The mass of a Carbon-12 atom is 12 amu.  The mass of a Helium-4 atom is approximately 4 amu.  The masses of elements listed in the periodic table are not integers. Do you remember why?  One mole of C contains 6.02 x 10 23 C atoms.  One mole of Cu contains 6.02 x 10 23 Cu atoms.  Are the masses of one mole of C and one mole of Cu the same?

3. 3  Remember the definition of a mole: the number of Carbon- 12 atoms in 12 g of Carbon-12.  Therefore, one mole of Carbon-12 has a mass of 12 g.  The mass in grams of one mole of any pure substance is called its molar mass.  Molar mass of Carbon-12 is 12 g/mol.  How about the other elements?  The molar mass of any element is numerically equal to its atomic mass and has the units g/mol.  Example:  Molar mass of iron is 55.847 g/mol  1 mole of iron has a mass of 55.847 grams.  What is the molar mass of potassium?

4.  Example: Chromium is a transition element anda component of chrome plating. Chrome plating is used on metals and in steel alloys to control corrosion. Calculate the mass in grams of 0.045 mol Cr. 4  Example: Determine the mass in grams of each of the following. a- 3.57 mol Al b- 42.6 mol Si

5.  Example: Calcium is the fifth most abundant element on Earth and always found combined with other elements because of its high reactivity. How many moles of calcium are in 525 g Ca? 5  Example: Determine the number of moles in each of the following. a- 25.5 g Ag b- 300.0 g S

6.  Example: Gold is one of a group of metals called the coinage metals (copper, silver and gold). How many atoms of gold are in a US Eagle, a gold bullion coin with a mass of 31.1 g gold? 6  Example: Helium is an unreactive noble gas often found in underground deposits mixed with methane. The mixture is separated by cooling the gaseous mixture until all but the helium has liquified. A party balloon contains 5.50 x 10 22 atoms of helium gas. What is the mass, in grams, of the helium?