Atomic Mass The Mole Molecular Weight Unit 8 – Chemical Quantities 1

Upon completion of this unit, you should be able to do the following: – Calculate the total mass of each element in a compound. – Calculate the molecular weight of compounds 2 Unit 8 – Chemical Quantities

Atomic Mass Most elements occur in nature as a mixture of isotopes. ElementNumber of stable isotopes C 2 Cl 2 O 3 Fe 4 This is one reason why atomic masses are not whole numbers. They are based on averages. 3

Atomic Mass As a reference, we use the atomic mass unit (amu), which is equal to 1/12 th of the mass of a 12 C atom. One atomic mass unit = x gram Using this relative system, the mass of all other atoms can be assigned. Example 35 Cl = amu 4

Atomic Mass The average atomic weight of an element can be calculated if the abundance of each isotope for that element is known. Example - Chlorine is a mixture of two isotopes. Isotope Mass (amu) Abundance 35 Cl % 37 Cl % 5

Atomic Mass x ( amu) = amu x ( amu) = 8.95 amu 35 Cl 37 Cl Average atomic mass for chlorine = amu 6

The Mole The number of atoms in grams of 12 C can be calculated. One atom 12 C = amu = amu x (1.661 x g/amu) 1 atom = x g Number of atoms = g (1 atom / x g) = 6.02 x atoms The number of atoms of any element needed to equal its atomic mass in grams will always be 6.02 x atoms, a quantity known as the mole. (6.02 x is also known as Avogadro’s number) 7

The Mole One mole equals the atomic mass in grams of an element mass of 1 mole of 12 C = grams mass of 1 mole of C = grams mass of 1 mole of Na = grams mass of 1 mole of H = grams mass of 1 mole of O = grams 8

Mass and Moles Atomic mass or molecular weight (MW) The average mass of an atom in an element, expressed in atomic mass units (amu) or grams / mole. Formula mass (or formula weight FW) The total mass for all atoms in a compound. 9

Mass and Moles Example - Formula weight of NaOH MW of Na = g/mol MW of O = g/mol MW of H = g/mol FW = ( ) g/mol = g/mol 10

Pg Suppose that after you graduate from college, you go to work for a chemical company. You are working with a chemist to improve the process that produces methanol from the reaction of hydrogen and carbon monoxide. Your first assignment is to order enough hydrogen and carbon monoxide to produce 6 kg of methanol (CH 3 OH). How would you determine how much of each material to order? 11 Chemical Equations

Write the unbalanced equation for the reaction. CO (g) + H 2 (g) → CH 3 OH (l) Next, balance the equation. CO (g) + 2 H 2 (g) → CH 3 OH (l) Understand that you can multiply this equation by any number and it is still balanced. For example, multiply by 12 to get 12 CO (g) + 24 H 2 (g) → 12 CH 3 OH (l) This is still balanced. Since 12 is a dozen we could also write this as 1 dozen CO (g) + 2 dozen H 2 (g) → 1 dozen CH 3 OH (l) 12 Chemical Equations

We could also multiply the equation by a very large number, say x x [CO (g) + 2 H 2 (g) → CH 3 OH (l)] or x CO (g) + 2 (6.022 x ) H 2 (g) → x CH 3 OH (l) Just as 12 is called a dozen, x is called a mole so we could rewrite the equation as 1 mol CO (g) + 2 mol H 2 (g) → 1 mol CH 3 OH (l) 13 Chemical Equations

Example 9.1, page 248 Propane (C 3 H 8 ) is a fuel commonly used for cooking on gas grills. Propane reacts with oxygen to produce heat and the products of carbon dioxide and water vapor. This combustion reaction is represented by the unbalanced equation C 3 H 8 (g) + O 2 (g) → CO 2 (g) + H 2 O(g) Give the balanced equation and state the meaning of the equation in terms of moles. 1C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O (g) 1 mol of C 3 H 8 reacts with 5 mol of O 2 to form 3 mol of CO 2 and 4 mol of H 2 O 14 Chemical Equations

Now use the balanced equation to predict the moles of products that a given number of reactants will yield. Consider the decomposition of water into hydrogen and oxygen. 2 H 2 O (g) → 2 H 2 (g) + O 2 (g) The equation tells you that 2 mol of H 2 O will decompose to yield 2 mol of H 2 and 1 mol of O Chemical Equations

Molecular Weight Practice Problems Page ; 28, 30, 32, 36 16