Ch 3 Chemistry The Central Science

Front 1

What is stoichiometry?

Back 1

The quantitative nature of chemical formulas and reactions is called stoichiometry.

Front 2

What is law of conservation of mass?

Back 2

Lavoisier observed that mass is conserved in a chemical reaction.
This observation is known as the law of conservation of mass

Front 3

What are Chemical equations?

Back 3

Chemical equations give a description of a chemical reaction.

Front 4

What are the two parts of chemical equations?

Back 4

There are two parts to any equation:
• reactants (written to the left of the arrow) and
• products (written to the right of the arrow):
2H2 + O2 ---- 2H2O

Front 5

What are stoichiometric coefficients?

Back 5

numbers in front of the chemical formulas

Front 6

What do the Stoichiometric coefficients show?

Back 6

Stoichiometric coefficients give the ratio in which the reactants and products exist.

• 2H2O means that there are two water molecules present.

Front 7

What do the subscripts in an equation show?

Back 7

The subscripts give the ratio in which the atoms are found in the molecule.
• H2O means there are two H atoms for each one molecule of water.

Front 8

What happens when balancing a chemical equation?

Back 8

When balancing a chemical equation we adjust the stoichiometric coefficients in front of
chemical formulas.
• Subscripts in a formula are never changed when balancing an equation.
• Example: the reaction of methane with oxygen:
CH4 + O2 ---- CO2 + H2O
• Counting atoms in the reactants yields:
• 1 C;
• 4 H; and
• 2 O.
• In the products we see:
• 1 C;
• 2 H; and
• 3 O.
• It appears as though an H has been lost and an O has been created.
• To balance the equation, we adjust the stoichiometric coefficients:
CH4 + 2O2 --- CO2 + 2H2O

Front 9

What are combination reactions?

Back 9

In combination reactions two or more substances react to form one product.
Combination reactions have more reactants than products.

Front 10

What are decomposition reactions?

Back 10

In decomposition reactions one substance undergoes a reaction to produce two or more other
substances.
Decomposition reactions have more products than reactants.

Front 11

What are combustion reactions?

Back 11

Combustion reactions are rapid reactions that produce a flame.
Most combustion reactions involve the reaction of O2(g) from air.

Front 12

What is the Formula weight?

Back 12

(FW) is the sum of atomic weights for the atoms shown in the chemical formula.

Front 13

What is Molecular weight?

Back 13

Molecular weight (MW) is the sum of the atomic weights of the atoms in a molecule as shown in the
molecular formula.

Front 14

How is Percentage composition obtained?

Back 14

Percentage composition is obtained by dividing the mass contributed by each element (number of atoms times AW) by the formula weight of the compound and multiplying by 100.

%element = (number of atoms of that element)(atomic weight of element)(100)/ (formula weight of compound)

Front 15

What is the mole?

Back 15

The mole (abbreviated "mol") is a convenient measure of chemical quantities. 1 mole of something = 6.0221421 x 1023 of that thing.

Front 16

What is molar mass?

Back 16

The mass in grams of 1 mole of substance is said to be the molar mass of that substance. Molar mass
has units of g/mol
• Molar masses for elements are found on the periodic table.
• The formula weight (in amu) is numerically equal to the molar mass (in g/mol).

Front 17

What are the methods of Interconverting Masses and Moles?

Back 17

To convert between grams and moles, we use the molar
• Mass: g
• Moles: mol
• Molar mass: g/mol

Front 18

Method of Interconverting Masses and Number of Particles?

Back 18

To convert between moles and molecules we use Avogadro’s number.
• Number of particles: 6.022 x 1023 mol–1 (Avogadro’s number).
• Note: g/mol x mol = g (i.e. molar mass x moles = mass), and
• mol x mol–1 = a number (i.e. moles x Avogadro’s number = molecules).

Front 19

What is an empirical formula?

Back 19

Recall that the empirical formula gives the relative number of atoms of each element in the molecule.

Front 20

How do you Find empirical formula from mass percent data?

Back 20

• We start with the mass percent of elements (i.e. empirical data) and calculate a formula.
• Assume we start with 100 g of sample.
• The mass percent then translates as the number of grams of each element in 100 g of sample.
• From these masses, the number of moles can be calculated (using the atomic weights from the
periodic table).
• The lowest whole -number ratio of moles is the empirical formula.

Front 21

How do you Find the empirical mass percent of elements from the empirical formula?

Back 21

• If we have the empirical formula, we know how many moles of each element is present in one
mole of the sample.
• Then we use molar masses (or atomic weights) to convert to grams of each element.
• We divide the number of grams of each element by the number of grams of 1 mole of sample to
get the fraction of each element in 1 mole of sample.
• Multiply each fraction by 100 to convert to a percent.

Front 22

What is law of conservation of mass?

Back 22

Law of conservation of mass: matter cannot be lost in any chemical
reactions.

Front 23

What is Stoichiometry?

Back 23

Stoichiometry: quantitative nature of chemical formulas and chemical
reactions.

Front 24

What is Lavoisier famous claim?

Back 24

Lavoisier: mass is conserved in a chemical reaction.

Front 25

What are Chemical equations?

Back 25

Chemical equations: descriptions of chemical reactions.

Front 26

What are the Two parts to an equation?

Back 26

Two parts to an equation: reactants and products:

Front 27

What are Stoichiometric coefficients?

Back 27

Stoichiometric coefficients: numbers in front of the chemical formulas;
give ratio of reactants and products.

Front 28

What is combustion and its products?

Back 28

Combustion is the burning of a substance in oxygen from air

CO2 and H20.

Front 29

How much is 1 amu?

Back 29

1 amu = 1.66054 x 10^-24 g

Front 30

How many amu is in 1g?

Back 30

1 g = 6.02214 x 10^23 amu (mol)

Front 31

What is the mass of 1 12C atom in amu?

Back 31

12 amu

Front 32

Mass of 1 mol of 12C atom in g?

Back 32

12 g

Front 33

What is the Relative atomic mass?

Back 33

Relative atomic mass: average masses of isotopes:
Naturally occurring C: 98.892 % 12C + 1.108 % 13C.
Average mass of C:
(0.98892)(12 amu) + (0.0108)(13.00335) = 12.011 amu.
Atomic weight (AW) is also known as average atomic mass (atomic weight).
Atomic weights are listed on the periodic table.

Front 34

What is Percent composition?

Back 34

Percent composition is the atomic weight for each element divided by the formula
weight of the compound multiplied by 100:

% Element = [(Atoms of Element)(AW)]/(FW of Compund) times 100

Front 35

What is a mol and how much is it?

Back 35

Mole: convenient measure chemical quantities.
1 mole of something = 6.0221367 x 1023 of that thing.

Front 36

What is the molar mass?

Back 36

Molar mass: mass in grams of 1 mole of substance (units g/mol, g.mol-1).

Molar mass: sum of the molar masses of the atoms:
molar mass of N2 = 2 x (molar mass of N).

Front 37

How do you find FW?

Back 37

Formula weights are numerically equal to the molar mass.

Front 38

How to find molecular formula from empirical formula?

Back 38

Once we know the empirical formula, we need the MW to find the molecular
formula.
Subscripts in the molecular formula are always whole-number multiples of subscripts in
the empirical formula

Front 39

What is Molecular weight?

Back 39

Molecular weight (MW) is the sum of the atomic weights of the atoms in a molecule as shown in the
molecular formula.
• Example: MW (C6H12O6)
• = 6(12.0 amu) + 12 (1.0 amu) + 6 (16.0 amu)
• = 180.0 amu.

Front 40

How do you find % Yield?

Back 40

%Yeild= (Actual Yield/ Theoretical Yield) x 100