Concentration of Acids

and Alkalis

Definition:

- Quantity of solute in a given volume of solution whichis usually 1 dm3 (text book)

- The mass (in grams) or the number of moles of solutedissolved in a solvent to form 1.0 dm3 (1ooo cm3 ) ofsolution (reference book)

Hence, the concentration of a solution can be defined in two ways:

Concentration = Mass of solute dissolved (g)

(g/dm3) Volume of solution (dm3)

Concentration = Number of moles of solute (mol)

(mol/dm3) Volume of solution (dm3)

Number of moles of solute that are present in 1 dm3

of solution

Molarity = Concentration (g/ dm3)

(mol/dm3) Molar mass (g / mol)

= Number of moles of solute (mol)

Volume of solution (dm³)

However, the concentration unit that is widely used bychemists is molarity (mol/dm³) or molarconcentration (M)

The two units of concentration can be inter-convertedas below:

Molarity(mol/dm³)

Concnetration(g/dm³)

Relationship between molarity and concentration

X Molar mass

÷ Molar mass

Molarity = Number of moles of solute (mol)

Volume of solution (dm³)

From the above equation, we can calculate the number ofmoles of solute in a given volume of solution.

Number of moles of solute = Molarity x volume ofsolution

In short,

n= Number of moles of solute (mol)

M= Molarity of solution (mol dm¯³)

V= Volume of solution (dm³)

n = MV

5 g of copper (II) sulphate dissolved in water to form 500 cm³ solution. Calculate the concentration of copper (II) sulphate solution in g/dm³.

What is the mass of sodium carbonate required to dissolve in water to prepare a 200 cm³ solution and contains 50 g dm¯³.

Calculate the number of moles of ammonia in 150 cm³ of 2 mol dm¯³ aqueous ammonia.

A 250 cm³ solution contains 0.4 moles of nitric acid. Calculate the molarity of the nitric acid.

4.0 g of sodium carbonate powder, Na₂CO₃, is dissolved in water and made up to 250 cm³. what is the molarity of the sodium carbonate solution?

[r.a.m.: C=12, O=16, Na=23]

Dilute hydrochloric acid used in school laboratories usually has a concentration of 2.0 mol dm¯³. calculate the mass of hydrogen chloride in 250 cm³ of the hydrochloric acid?

[r.a.m.: H=1, Cl=35.5]

Standard solution – a solution in which itsconcentration is accurately known (tx book) / asolution with a known concentration (ref book)

Volumetric flask (standard flask) – apparatus with aknown volume such as 100 cm³, 200 cm³, 250 cm³, 500cm³ and 1000 cm³.

- Used to prepare standard solution.

- Can measure the volume of a liquid accurately, up toone decimal point.

Steps involved in the preparation of a standardsolution:

a) Calculate the mass (m g) of the chemical required toprepare v cm³ of solution where v is the volume of thevolumetric flask.

b) Weight out the exact mass (m g) of the chemicalaccurately in a weighting bottle using an electronicbalance.

c) Dissolve m g of the chemical in a small amount ofdistilled water.

d) Transfer the dissolved chemical into the volumetricflask.

e) Add enough water until the graduation mark.

Dilution method

Dilution – process of diluting a concentrated solutionby adding a solvent such as water to obtain a moredilute solution.

When a solution is diluted, the volume of solventincreases but the number of moles of solute remains constant.

Hence, the concentration of the solution decreases.

If a solution with volume of V₁ cm³ and molarity of M₁mol dm¯³ is diluted to become V₂ cm³, the newconcentration of the diluted solution, M₂ mol dm¯³can be determined by:

Number of moles of = M₁V₁

solute before dilution 1000

Number of moles of = M₂V₂

solute after dilution 1000

However, the number of moles of solute beforedilution is the same as the number of moles of soluteafter dilution.

M₁V₁ = M₂V₂

------- ------- OR M₁V₁ =M₂V₂

1000 1000

M₁ - molarity of solution before water is added

V₁ - volume of solution before water is added

M₂ - molarity of solution after water is added

V₂ - volume of solution after water is added

pH value of an acid or an alkali depends on 2 factors:

a) degree of dissociation

b) molarity or concentration

At the same concentration, the pH value of an acid oran alkali depends on the degree of dissociation.

a) Degree of dissociation of an acidincreases, the pH value of the acid isdecreases

b) Degree of dissociation of an alkaliincreases, the pH value of the alkali isincreases

For an acid or alkali, its pH value depends on themolarity of the solution.

a) Molarity of an acid increases, the pHvalue of the acid is decreases

b) Molarity of an alkali increases, the pHvalue of the alkali is increases