Stoichiometry & The Mole Concept

Writing Ionic Equations:

An ionic equation is a simplified chemical equation that shows the reactions of ionic compounds in water.

Ionic compounds are those compounds, which are soluble in water.

Inorder to write an ionic equation:

• Write the balanced chemical equation of the reaction. Include the state symbols.
• Identify ionic compounds that are soluble in water. These compounds become ions in H20. Rewrite the chemical equation in terms of ions.
• Cancel out the spectator ions (Common).
• Write the ionic equation.

Examples:

Relative Atomic Mass (Ar):

The mass of an atom compared with the Carbon – 12 atom is called its relative atomic mass.

The mass of one mole of atoms is its “relative  atomic mass” in grams.

Relative Molecular Mass (Mr):

The mass of a substance made of molecules is known as Relative Molecular Mass.

Hydrogen has (1*2) =   2      and H₂0 has relative molecular mass of (1*2) + 16 = 18

Relative Formula Mass (Mr):

The mass of a substance made of ions is known as relative formula mass.  Ammonia (NH₃) has Mr of (1*14) + (3*1) =  58.5.

The Mole:

A mole of a substance is the amount that contains the same number of units as the number of Carbon atoms in 12 grams of carbon-12.

Avogradoe’s Number:

Number of Particles in one mole = 6.02 * 10²³

Percentage Compostition of Compounds:

Percentage by mass of an element in a compound

Example:

Percentage of Hydrogen in Hydrogen Per Oxide:

Water in Coppoer(II) Sulphate:

Emperical Formula:

Emperical Formula of a compound shows:

• Types of elements present in it.
• Simplest Ratio of the different types of atoms in it.

Inorder to Find the Emperical Formula of a compound:

• Write down the each percentage/ mass separately.
• Divide each by their Mr.
• Now divide all of them with one which has the lowest ratio.
• If any one of the answer is in the decimal form, multiply both with any number (lowest) to get make it a whole number

Examples:

A  compound has 40% Carbon 6.6% Hydrogen & 53.3% Oxygen. Calculate the Emperical Formula of the compound.

C                              H                                O

40                            6.6                            53.3

40/12                      6.6/1                          53.3/16

3.33                         6.6                              3.33

3.33/3.33               6.6/3.33                     3.33/3.33

1                              2                                 1

Emperical Formula: CH₂O

E.g 2  O.72g of Mg combines with 0.28g of Nitrogen. Find its Emperical Formula.

Mg                               N

0.72                           0.28

0.72/24                      0.28/14

0.03                           0.02

0.03/0.02                  0.02/0.02

1.5                              1

1.5 * 2                        1 * 2

3                                 2

Emperical Formula :Mg₃N₂

Molecular Formulae:

The molecular formula shows the actual number of atoms that combine to form a molecule.

To Find the molecular Formula:

• Calculatefor the compound. This gives the number n.
• Multiply the numbers in the empirical formula n.

E.g. Emperical Formula = HO

Relative Molecular Mass = 34

(H = 1 , O = 16) . Find Molecular Formula?

34/17 =  2

HO * 2  = H₂0₂

Calculating The Volume of  Gas:

1 Mole of every substance occupies 24dm³.

Concentration:1 dm³ = 1000cm³

The Concentration of a solution, is the amount of solute in grams or noles, that is dissolved in 1 dm³ of solution.

Percentage yield:

Yeild is the amount of product, obtained from a reaction.

Actual Yeild:

It is the amount collected at the end of a chemical reaction. The actual yield is always less than the theoretical yield. It is also known as the Practical Yeild.

Theoratical Yeid:

It is the calculated yield of the amount of prosuct by using stoichiometry. In this Yeild 100% reactants are converted to products, with no losses.

Percentage Purity:

Percentage Purity indicates the amount od pure substance present in a sample of chemical substance.