AP EquationsQuestion 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 + O2

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: (NH4)2CO3 2NH3 + H2O + CO2

• Ammonium hydroxide decomposes into ammonia and water

Ex: NH4OH NH3 + H2O

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 (NH4)2SO3.

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

116.2 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 (LiClO4)

• Li: 1(6.94) = 6.94g Li• Cl: 1(35.5) = 35.5 g Cl• O: 4(16.0) = 64.0 g O

106.4 g LiClO4

%Li =

%Cl =

%O =

Example 1How many moles of water are in 15.4 g of water?

1 mol = 6.02x1023 particles = Molar Mass (g/mol)

Molar Mass of H2O = 2(1.01) + 1(16.0) =18.0 g/mol

1 mol H2O

15.4 g H2O x ------------------ = 0.856 mol H2O

18.0 g H2O

Example 2

1 mol = 6.02x1023 particles = Molar Mass (g/mol)

How many molecules are in 5.0 moles of water?

6.02 x 1023 molecules H2O

5.0 mol H2O x ------------------------------------- = 3.0 x 1024

1 mole H2O molecules H2O

Example 3

How many moles of oxygen are in 116.4 g of carbon dioxide?

Molar Mass of CO2 = 1(12.0) + 2(16.0) = 44.0g/mol

= 5.29 mol O

Example 4

• A sample of N2O4 contains 3.49x1024 nitrogen atoms. How many moles of N2O4 are in the sample?

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

lithium perchlorate?

• Method 1 (%):

Method 2 (DA):=.00016g Li