Revision

Topic 5: Formulae, Equations and Amounts of Substance

How we know how much stuff we have in Chemistry?: The mole

Revision: The Avogadro constant (NA or L), sometimes known as Avogadro’s number is:

602,000,000,000,000,000,000,000 mol-1OR 6x1023 mol-1and 1 mole of any substance (atoms, ions or molecules) contains this many particles.

Why is it useful?

It is the bridge between the ratios expressed in a chemical equation and the amounts that we need to use in the lab.

Empirical and Molecular Formulae

Formulae and equations constitute the internationally recognised shorthand of chemistry, and tell us what and how much stuff is involved in a chemical reaction.

Any new compound is analysed to determine how much of each element is present (% mass). The simplest ratio of the constituent elements is called the…EMPIRICAL FORMULA (GCSE)

To determine the MOLECULAR FORMULA we need more information (usually the Mr of the compound).

Remember:

- Ionic substances are always represented using their empirical formulae.

- The formula of a substance is sacred. If we change the atoms present in any way, we have a different substance.

What is the molecular formula?

Combustion analysis

A 1.50 g sample of hydrocarbon undergoes complete combustion to produce 4.40 g of CO2 and 2.70 g of H2O. What is the empirical formula of this compound?

We burn the compound in XS oxygen and we find the mass of the compounds produced. We use this information to determine how much of each element there is in the original compound

CH3

A 0.2500 g sample of a compound known to contain carbon, hydrogen and oxygen undergoes complete combustion to produce 0.3664 g of CO2 and 0.1500 g of H2O. What is the empirical formula of this compound?

CH2O

PV = nRT“The ideal gas equation”

• P is the pressure of the gas IN PASCALS, Pa

• V is the volume of the gas in cubic meters, m3

• n is the amount of substance of gas (in moles)

• R is the ideal, or universal, gas constant, 8.31 Jmol-1K-1

• T is the absolute temperature of the gas in KELVIN, K

Make sure that you are happy with all of these units and their conversion!!

1. Calculate the volume occupied by one mole of a gas at 25 oC and 100 kPa.

2. Calculate the pressure of a gas given that 0.2 moles of the gas occupy 10 dm3 at 20 oC.

3. Calculate the temperature of a gas if 0.5 moles occupy 1.2 dm3 at a pressure of 200 kPa.

4. Calculate the mass of a sample of carbon dioxide which occupies 20 dm3 at 27 oC and 100 kPa.

5. Calculate the relative molecular mass of a gas if a 500 cm3 sample at 20 oC and 1 atm has a mass of 0.66 g.

Questions p121/124

Equations and Ionic Equations

When balancing equations we must keep in mind 2 criteria:

1. They must balance for mass, i.e. the number of each of the atoms on the left hand side must equal that of the right hand side. Eg…

2. The total charge on each side of the equation must be the same. Eg…

Rules for writing ionic equations

1. Separate the ions of soluble ionic compounds (but which are soluble?). Eg….

2. Covalent or insoluble ionic compounds are written as the complete formula, eg…

3. Cancel spectator ions.

Try:1. Nitric acid and potassium hydroxide

2. Silver nitrate and sodium chloride

3. Lithium bromide and chlorine gas.

Name the types of reaction involved in these examples.

Eg. Fe(s) + S(s) FeS(s) Ar 56 32 88 Formula mass

Molar masses 56g 32g 88g

Note: molar mass has units, relative masses don’t.

Reacting masses

We work out the molar mass of a substance by adding up the relative atomic masses of the constituent elements and representing this number in grams.

Eg. 1 mole of carbon atoms (ignoring isotopes) has a mass of 12g1 mole of methane has a molar mass of 16g.

Number of moles of a substance = mass of substance molar mass

Mind bender: Calculate the number of atoms of oxygen in the atmosphere of the chemistry lab.

Questions p126, 129, 131

Avogadro’s Law

One mole of any gas at 1atm and 298K will occupy 24dm3

Questions p133, 135

Formula: Concentration (mol/dm3) = number of moles (mol) volume (dm3)

Concentration of Solutions

Questions p137

How to make a standard solution

Questions p139

TitrationsRead pp140-141 and answer the following questions…

What is a titration and what is it used for?

How do we know when a titration is complete?

What types of titration exist?

Define the following:Equivalence or end point

Meniscus

Titre

Concordant titres

Questions p141

What are indicators?

Why should you only add a few drops?

Draw methyl orange

Calculations

Questions p143

Percentage Yield

Define the terms:

Theoretical yield…

Actual Yield…

Formula: Percentage Yield =

Eg. What is the % yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2

4. When 5.00 g of KClO3 is heated it decomposes according to the equation: 2KClO3 → 2KCl + 3O2

a) Calculate the theoretical yield of oxygen. b) Give the % yield if 1.78 g of O2 is produced. c) How much O2 would be produced if the percentage yield

was 78.5%?

Questions p151

Atom Economy

Read pp153-153Why is atom economy important?

How do we calculate atom economy?

Atom Economy =

What is the main difference between atom economy and percentage yield?

Calculate the atom economy percentage for the Oswald Process (the desired product is nitric acid).

NH3 + 2O2 ! HNO3 + H2O

Questions p153