Chemistry Revision notes of Balancing of a Chemical Equation,Limiting reagent, Strength of solution

Balancing of a Chemical Equation

According to Law of conservation of mass, the mass and the number of atoms present in the reactant(s) should be equal to the mass and number of atoms present in product(s). To balance a chemical equation, following steps are followed: • Check the number of atoms of each element present on RHS and LHS of a equation whether they are equivalent or not. • If not, multiply the element by a number to the corresponding element. • Continue multiplying until all numbers become equivalent to the corresponding element. e.g. `Na_2O + HCl → NaCl+H_2O` Balancing of Equation

Element	LHS	RHS
Na	2	`1 xx 2 = 2`
O	1	1
H	`1xx2 =2`	2
Cl	`1xx2= 2`	`1xx2 = 2`

The balanced chemical equation is `Na_2O+2HCl → 2NaCl + H_2O` A balanced chemical equation can provide the following information regarding the chemical reaction: • The number of atoms and molecules taking part in the reaction and the corresponding masses in atomic mass unit (amu or u). • The number of moles taking part in the reaction, with the corresponding masses in grams or in other convenient units. • Relationship between the volume of the reactants and the products if all of them are in the gaseous state.

Element	LHS	RHS
Na	2	`1 xx 2 = 2`
O	1	1
H	`1xx2 =2`	2
Cl	`1xx2= 2`	`1xx2 = 2`

Limiting Reagent

In a chemical reaction, the substance that is consumed completely is called limiting reagent because it limits the amount of product. The other reactant present in excess is called excess reagent. e.g. `2H_2 (g) + O_2 (g) → 2H_2O (g)`

Mole before reaction	10	7	0
Mole after reaction	0	2	10

Therefore, `H_2` is limiting reagent and `O_2` is excess reagent .

Mole before reaction	10	7	0
Mole after reaction	0	2	10

Therefore, `H_2` is limiting reagent and `O_2` is excess reagent .

Strength of a Solution

The most common unit is molarity and normality.

(i) Normality(N) : It is the number of gram equivalents of solute dissolved per litre of the solution. It is denoted by N.

`text(Normality ) (N) = text(Gram equivalent of solute)/text{Volume of solution ( in L)}`

e.g. 0.50 g equiv `L^(-1)` (or 0.50 N) solution of `H_2SO_4` means that 0.50 g equiv. of `H_2SO_4` is dissolved in 1
litre of solution.

(ii) Molarity(M) : It is the number of moles of solute present per litre of the solution. It is denoted by M.

`text(Molarity) (M) = text(Number of moles of solute)/text{Volume of solution (in L)}`

e.g. 0.25 mol `L^(-)` (or 0.25 M) solution of NaOH means that 0.25 mol of NaOH is dissolved in 1 L of solution.

(iii) Molality (m) : It is the number of moles of the solute dissolved in 1 kg of solvent. It is denoted by `m`.

`text(Molality) (m) = text(Number of moles of solute)/text{Mass of solvent (in kg)}`

e.g. `1.00 mol kg^(-1)` (or 1.00 m) solution of `KCl` means that `1 mol (74.5) g) KCl` is dissolved in 1 kg of water.

(iv) Formality (F) : It is the number of gram formula weight of solute dissolved per litre of solution. When formula weight equals to the atomic weight, then formality equals to molarity.

`text(Formality) (F) = text(Gram formula weight of solute )/text(Volume in litre)`

Note : Molality is independent of temperature whereas molarity, normality and formality change with temperature. This is because volume depends upon temperature and the mass do not.