Chapter 3: Mass, formula, and reactions

Counting term:

states specific number of items grouped together

Counting terms are used

when items are too small or there are too many to easily count one-by-one

24 cans =

1 case

144 pencils =

1 gross

500 sheets =

1 ream

12 eggs =

1 dozen

Moles are used to

count atoms, ions, and molecules (too small to count one-by-one)

1 mole =

6.02 x 10^23 atoms, molecules, or ions

Avogadro’s number =

6.02 x 10^23

Molar Mass (aka…

Molecular Weight, MW)

Molar Mass is

the mass in grams of 1 mole of an element or compound

1 mole C =

12.01 g

1 mole Li =

6.941 g

To find the molar mass of a compound:

Multiply the molar mass of each element by the subscript in the chemical formula then

add the masses.

Conversion factor to get from particles to mole:

1 mole/6.02 × 10^23 particles

Conversion factor to get from mole to particles:

6.02 × 10^23 particles/1 mole

Molar mass can be written as an equality and therefore as

conversion factors!

It converts grams to moles and vice versa.

Conversion factor to get from grams to mole:

ex. 16.05g CH4/1 mole of CH4

Conversion factor to get from mole to grams:

ex. 1 mole CH4/16.05g CH4

If your Given is atoms or molecules and your Need is moles use:

Avogadro’s #: 1 mole = 6.02 × 10^23 atoms or molecules

If your Given is moles and your Need is atoms or molecules use:

Avogadro’s #: 1 mole = 6.02 × 10^23 atoms or molecules

If your Given is grams and your Need is moles use:

Molar Mass: 1 mole = ____ grams

If your Given is moles and your Need is grams use:

Molar Mass: 1 mole = ____ grams

If your Given is atoms or molecules and your Need is grams use:

Avogadro’s # and Molar Mass

If your Given is grams and your Need is atoms or molecules use:

Avogadro’s # and Molar Mass

Chemical reaction:

transformation of one or more substances into different substances

Chemical equations:

use chemical formulas to express the identities of substances involved in a reaction and use coefficients to indicate their proportions

Reactants -

the substances that are consumed

Products -

the substances that are formed

Physical state of products/reactants:

(s) solid, (ℓ) liquid, (g) gas, (aq) aqueous solution

Coefficients:

indicate proportions of reactants and/or products

Stoichiometry:

- Quantitative relation between reactants and products in a chemical

equation

- Indicated in chemical equation by coefficients

Law of Conservation of Mass:

- The sum of the masses of the reactants in a chemical equation is equal to the sum of the masses of the products

Chemical equations must be balanced.

The number of atoms on the reactant side is equal to the number of atoms on the product side for each element.

To balance a chemical equation: Step 1

Write a chemical equation.

To balance a chemical equation: Step 2

Count the atoms of each element on the reactant side and product side

To balance a chemical equation: Step 3

Use ONLY coefficients to “balance.” You cannot change subscripts or chemical formulas. Balance elements that appear in only one reactant and product first

To balance a chemical equation: Step 4

Double check

Combustion Reactions:

heat-producing reactions between oxygen (O2) and another element or compound.

Combustion of hydrocarbons

(molecules composed of carbon and hydrogen) produces CO2 and H2O.

Rules for Balancing Combustion Reactions of Hydrocarbons: Step 1

Balance C first by adjusting coefficient in CO2

Rules for Balancing Combustion Reactions of Hydrocarbons: Step 2

Balance H next by adjusting coefficient for H2O

Rules for Balancing Combustion Reactions of Hydrocarbons: Step 3

Balance O last by adjusting coefficient for O2

Stoichiometric Calculations

To calculate the mass of a product from the mass of a reactant:

Step 1

Convert mass of reactant into moles using its molecular weight

Stoichiometric Calculations

To calculate the mass of a product from the mass of a reactant:

Step 2

Convert moles of reactant to moles of product using the mole ratio from the balanced chemical equation

Stoichiometric Calculations

To calculate the mass of a product from the mass of a reactant:

Step 3

Convert moles of product in mass of product using its molecular weight

Limiting reagent (aka limiting reactant limiting reactant):

reactant that is completely consumed in the chemical reaction and determine the amount of product that can be formed.

Theoretical Yield:

the maximum amount of product possible in a chemical reaction for given quantities of reactants.

_{Determined by the limiting reagent}

Actual Yield:

the measured amount of product formed

Percent Yield: *hint* is a formula

= actual yield/theoretical yield x 100%

Percent Composition

Mass percent (%) = mass of element in compound/mass of compound x 100

We can calculate the empirical formula of a compound if we know the percent composition of each element.

1. Assume 100 g of substance.

2. Convert mass of each element to moles.

3. Compute mole ratios.

4. If necessary, convert to smallest whole-number ratios.

Empirical formula:

simplest whole-number molar ratio of elements in a compound

Molecular formula:

actual molar ratio of elements in a compound

_{• Equal to some multiple of empirical formula}

_{• Determined from empirical formula and molecular mass}