Lecture 52 – Lecture 53 Mass and the Mole Ozgur Unal 1
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 C atoms. One mole of Cu contains 6.02 x Cu atoms. Are the masses of one mole of C and one mole of Cu the same?
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 g/mol 1 mole of iron has a mass of grams. What is the molar mass of potassium?
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 mol Cr. 4 Example: Determine the mass in grams of each of the following. a mol Al b mol Si
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 g Ag b g S
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 atoms of helium gas. What is the mass, in grams, of the helium?