AP Equations Question 4 Molar Mass and % Composition Chapter 3, Part 1

Directions: For each of the following three reactions, write a balanced equation for the reaction in part (i) and answer the question about the reaction in part (ii). In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You may use the empty space at the bottom of the next page for scratch work, but only equations that are written in the answer boxes provide will be scored.

Good News! All the reactions on the test occur!!!

The example on the test I don’t expect you to know the answer to this one….yet! Just note the format…

An Example you know! (a)Solid mercury(II) oxide decomposes as it is heated in an open test tube in a fume hood. (ii) After the reaction is complete, is the mass of the material in the test tube greater than, less than, or equal to the mass of the original sample? Explain. _________________________________ (i)Balanced equation less; gaseous oxygen escapes 2 HgO  2 Hg + O 2 from the test tube

Add to Equation Sheet Composition Reaction A & B If there is a metal that can have multiple charges (like Fe +2 and Fe +3 ): If there is limited amount of the metal (or an excess amount of the other element) the ion will have the higher charge.

Add to equation sheet! Ammonium carbonate decomposes into ammonia, water and carbon dioxide Ex: (NH 4 ) 2 CO 3  2NH 3 + H 2 O + CO 2 Ammonium hydroxide decomposes into ammonia and water Ex: NH 4 OH  NH 3 + H 2 O

Note: We are going to skip the acid reactions for now! We will come back to them in Chapter 4 don’t worry

Molar Mass The mass of 1 mole of a substance (i.e., g/mol) – The molar mass of an element is the number we find on the periodic table

Molar Mass Numerically equal to the: - Formula weight (for ionic compounds) - Molecular weight (for molecular compounds) The formula weight or molecular weight (in amu’s) will be the same number as the molar mass (in g/mol)

Calculate the molar mass of ammonium sulfite. The formula is (NH 4 ) 2 SO 3. In 1 mole of the compound there are: 2 moles of N X 14.0 g/mole = 28.0 g N 8 moles of H X 1.01 g/mole = 8.08 g H 1 mole of S X 32.1 g/mole = 32.1 g S 3 moles of O X 16.0 g/mole = 48.0 g O g/mole (SD done by place value) (units are always g/mole)

Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation: molar mass of element in the compound % of element = x 100% molar mass of compound

Find the percent composition of lithium perchlorate (LiClO 4 )

Example 1 How many moles of water are in 15.4 g of water? 1 mol = 6.02x10 23 particles = Molar Mass (g/mol) Molar Mass of H 2 O = 2(1.01) + 1(16.0) =18.0 g/mol 1 mol H 2 O 15.4 g H 2 O x = mol H 2 O 18.0 g H 2 O

Example 2 1 mol = 6.02x10 23 particles = Molar Mass (g/mol) How many molecules are in 5.0 moles of water? 6.02 x molecules H 2 O 5.0 mol H 2 O x = 3.0 x mole H 2 O molecules H 2 O

Example 3

Example 4

Example 5 How many grams of lithium can be obtained from.0025 g of lithium perchlorate?