WoC Ch 6.2 to ___

Chemical Composition

Objective: Understand atomic mass and its experimental determination

The balanced chemical equation for the reaction of solid carbon and gaseous oxygen to form gaseous carbon dioxide is as follows;

C(s) + O2(g)

CO2(g)

To determine the number of oxygen molecules required, we must know how many carbon atoms

are present in the pile of carbon.

• We must learn to count atoms by weighing samples containing large numbers of them.

• From the example earlier: 1 atom carbon reacts with 1 molecule of oxygen to yield one molecule of carbon dioxide

Atomic mass unit

The mass of a single C atom is 1.99 x 10-23g

To avoid using terms like 10-23 when describing the mass of an atom, scientists have defined a much smaller unit of mass called the atomic mass unit, which is abbreviated amu. In terms of grams,

1 amu = 1.66 x 10-24

Isotopes

What are isotopes?

What are the isotopes of carbon?

Carbon 12, carbon 13, and carbon 14All samples of carbon contain these

isotopes, always in the same proportions. We have to use an average mass for the carbon atoms.

Average atomic mass

For carbon, the average atomic mass is 12.01 amu. This means that any sample of carbon from nature can be treated as though it were composed of identical carbon atoms, each with a mass of 12.01 amu.

Mass of natural carbon

Mass of 1000 natural carbon atoms= (1000 atoms) (12.01 amu/atom) = 12,010 amu

In scientific notation?

1 carbon atom = 12.01 amu

1 carbon atom is the conversion factor

12.01 amu

Do the following problems.

1. Convert 3.00 x 1020 amu C to atoms of carbon.

2. Calculate the mass, in amu, of a sample of aluminum that contains 75 atoms.

3. Calculate the mass of a sample that contains 23 nitrogen atoms

4. Calculate the number of sodium atoms present in a sample that has a mass of 1172.49 amu.

5. Calculate the number of oxygen atoms in a sample that has a mass of 288 amu.

WoC Ch 6.3 The mole

Objectives

• understand the mole concept and Avogadro’s number

• learn to convert among, moles, mass, and number of atoms in a given sample

Recall the m&m activity

You know that a mole of anything is Avogadro’s number of anything. During the m&m activity, you determined the volume of a mole of m&m’s and compared that to a volume of something on or relative to earth.

The Pacific ocean could almost be filled with a mole of m&m’s.

2.9 moons would hold a mole of m&m’s

The Pacific Ocean and m&m’s

You could walk from California to Australia on m&m’s

The mole

• The number equal to the number of carbon atoms in 12.01 grams of carbon.

• Techniques for counting atoms very precisely have been used to determine this number to be 6.022 x 1023 (Avogadro’s number)

• A sample of an element with the mass equal to that element’s average atomic mass expressed in grams contains 1 mol of atoms

What does that mean?

• Discuss the size of the mole and the atom.

• Why do chemists use the mass unit of grams per mole rather than amu?

• How do these conversions aid chemists in a real-world setting?

Molar Mass

The molar mass of any substance is the mass (in grams) of 1 mole of the substance.

Calculate the molar mass of sulfur dioxide, carbon tetrachloride, and sulfuric acid.

Calculating mass from moles

• Calcium carbonate (also called calcite) is the principal mineral found in limestone, marble, chalk, pearls, and the shells of marine animals such as clams.

• A. Calculate the molar mass

• B. A certain sample of calcium carbonate contains 4.86 mol. What is the mass in grams of the sample?

Calculating Moles from Mass

• Juglone, a dye known for centuries, is produced from the husks of walnuts. It is also a natural herbicide (weed killer) that kills off competitive plants around black walnut tees but does not affect grass and other noncompetitive plants. The formula for juglone is C10H6O3.

• A. Calculate the molar mass of juglone• A sample of 1.56 g of pure juglone was

extracted from black walnut husks. How many moles of juglone does this sample represent?

Calculating Number of Molecules

• Isopentyl acetate, C7H14O2, the compund responsible for the scent of bananas can be produced commercially. Bees release 1 ug of this compound when they sting.

(1 x 10-6 g = 1 ug)

• How many moles and how may molecules of isopentyl acetate are released in a typical bee sting?

WoC Ch 6.6

• Objective: understand the meaning o f empirical formulas of compounds

– Empirical formula-the “simplest formula”, formula of a compound that expresses the smallest whole-number ratio of the atoms present.

WoC 6.7• Objective: Calculate empirical formulas

– Steps for determining the empirical formula of a compound1. Obtain the mass of each element present (in grams)2. Determine the number of moles of each type of atom present.3. Divide the number of moles of each element by the smallest number of moles to convert the smallest number to 1. If all of the numbers obtained are integers (whole numbers), they are the subscripts in the empirical formula. If one or more of these numbers are not integers, go on to step 4.4. Multiply the numbers you derived in step 3 by the smallest integer that will convert all of them to whole numbers. This set of whole numbers represents the subscripts in the empirical formula.

WoC 6.8

• Objective: Calculate the molecular formula of a compound, given its empirical formula and molar mass

• Molecular Formula = (empirical formula)n where n is a small whole number.

• Molar mass = n x empirical formula mass

• Solving for n gives

n = molar mass/empirical formula mass

Thus, to determine the molecular formula,

• Divide the molar mass by the empirical formula mass. This tells us how many empirical formula masses are present in one molar mass.

• This result means that n multiplied by the subscripts in the empirical formula becomes the molecular formula

• n(XaYb) = Xa(n)Yb(n)

Calculating molecular formulas

1. A white powder is analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of 283.88 g. What is the compound’s molecular formula?

2. A compound used as an additive for gasoline to help prevent engine knock has the following percent composition:

71.65% Cl, 24.27% C, and 4.07% HThe molar mass is known to be 98.96 g.

Determine the empirical formula and the molecular formula of this compound.

WoC 7.1 Evidence for a Chemical Reaction

• Objective: learn the signals that show a chemical reaction has occurred

-A chemical reaction gives a visual sign.

1. Color change

2. Solid forms

3. Bubbles form

4. Heat and/or a flame is produced or heat is absorbed.

WoC 7.2 Chemical Equations

• Objective: Learn to identify the characteristics of a chemical reaction and the information given by the chemical equation.– When methane combines with oxygen in the air

and burns, carbon dioxide and water are formed. This chemical change is a chemical reaction.

Reactants Productsis read as “yields” or

“produces”

The law of the conservation of mass

• Matter cannot be created or destroyed.

• In a chemical reaction, atoms are neither created or destroyed

• All atoms present in the reactants must be present in the products

• Making sure that the equation for a reaction obeys this rule is called balancing chemical equations.

WoC 7.3 How to Write and Balance Equations

1. Read the description of the chemical reaction. What are the reactants, products, and their states? Write the appropriate formulas.

2. Write the unbalanced equation that summarizes the information form step 1.

3. Balance the equation by inspection, starting with the most complicated molecule. Proceed element by element to determine what coefficients are necessary so that the same number of each type of atom appears on both the reactant side and the product side. Do not change the identities of any of the reactants or products.