7.3 COCENTRATION OF ACID AND ALKALI

NURUL ASHIKIN BT ABD RAHMAN : UPSI

a)State the meaning of concentration and

b) State the meaning of molarity correctly.

c) State the relationship between the number of moles with molarity and volume of a solution.

At the end of this lesson, students should be able to:

Solvent...? Solute...?Solution...?

Solution = Solvent + Solute

ConcentratioN• The quantity of solute in a given volume of

solution. Which is usually a dm3.

• Unit: g dm-3

• 1 dm3 = 1L= 1000 cm3 = 1000 mL

33

( )( )

( )

mass of solute gConcentration g dm

volume of solution dm

33

( )( )

( )

number of moles of solute molMolarity mol dm

volume of solution dm

MolaritY• The number of moles of solute that are present in

1 dm3 of solution.

• Unit : mol dm-3

Molar mass

Relationship between Molarity and concentration

Molarity (mol dm-3)

Concentration (g dm-3)

Molar mass

( ) ( )n molarity M volume of solution V

Relationship between moles with molarity and volume

Number of moles of solute,

Calculate:

a) The concentration of solutionin g dm-3 when 6.00 g of copper (II) sulphate is dissolved in water to form 500 cm3 solution.

b) The molarity of solution when 0.8 mol of sodium hydroxide is dissolvedin distilled water to form 250 cm3 of solution.

EXAMPLE 1

a) Mass of copper (II) sulphate = 6.00 gvolume of solution = 500 cm3 = 0.5 dm3

Concentration = 6.00 g/0.5 dm3

= 12 g dm-3

SOLUTION 1

b) Number of moles of NaOH = 0.8 molvolume solution = 250 cm3 = 0.25 dm3

molarity of NaOH = 0.8 mol/ 0.25 dm3

= 3.2 mol dm-3

Calculate:

What is the mass of potassium hydroxide required to dissolve in water to prepare a 200 cm3 solution that contains 30 g dm-3?

EXAMPLE 2

Answer: 6 g

Calculate:

0.2 mole of sodium carbonate is dissolved in water to produced a solution with concentration of 0.5 mol dm-3. What is the volume of sodium carbonate solution?

EXAMPLE 3

Answer: 400 cm3

Calculate:

A copper (II) sulphate solution has a concentration of 40.0 g dm-3. What is the molarity of this solution in mole dm-3?

[relative atomic mass: O,16; S,32; Cu,64]

EXAMPLE 4

Answer: 0.25 mol dm-3 of CuSO4

Calculate:

The concentration of dilute hydrochloric acid in a reagent bottle is 2.0 mol dm-3

a) What is the concentration of this solution in g dm-3?

b) Find the mass of hydrogen chloride in 250 cm3 of the hydrochloric acid?

[relative atomic mass: H,1;Cl,35.5]

EXAMPLE 5

EXAMPLE 6

Calculate the number of moles of sodium hydroxide in 25.0 cm3 of 1.5 mol dm-3 aqueous sodium hydroxide.

answer: 0.0375 mol

EXAMPLE 7

A 0.5 mol dm-3 potassium hydroxide solution contains 7.0 g potassium hydroxide. Calculate the volume of this solution.[relative atomic mass: H,1; O,16; K,39]

answer: 0.25 dm3 / 250cm3

a)Describe methods for preparing standard solutions.

b)Describe the preparation of a solution with a specified concentration using dilution method.

c) Relate pH value with molarity of acid and alkali.

d)Solve numerical problems involving molarity of acids and alkalis.

At the end of this lesson, students should be able to:

Standard Solution

A standard solution is a solution where its concentration is accurately known.

Prepare 100 cm3 of sodium hydroxide solution with a concentration of 0.5 mol dm-3

1. Molecular mass? [Na,23;O,16;H,1]2. Mass of NaOH?3. Prepare solution.

Standard SolutionPrepare 100 cm3 of sodium hydroxide solution with a concentration of 0.5 mol dm-3

Solution:Volume (V) = 100cm3 =0.1 dm3

Molarity (M)= 0.5 mol dm-3

Mass = ?

Molar mass NaOH:= 23+16+1 = 40 g mol-

Mole = mass/Molar mass

Mole (n) = Molarity (M) X Volume (V)

n = 0.5 mol dm-3 X 0.1 dm3

n = 0.05 mol

n = mass/molar mass

mass = n x molar mass

mass = 0.05 mol x 40 g mol-

mass = 2.0 g

Prepare The Solution

2.0 g of NaOH is weighed NaOH poured into a beakerContaining distilled water

Preparation Of a Solution by Dilution Method

process of diluting a concentrated solution by adding water to get a more dilute solution.

Dilution

M1 V1 = M2V2

M1: molarity of the solution before water is addedV1 : Volume of the solution before water is addedM2: molarity of the solution after water is addedV2 : Volume of the solution after water is added

Example :

90 cm3 of distilled water is added to 10 cm3 of a 1.0 mol dm-3 solution of HCl. Determine the molarity of the diluted solution

0.1 mol dm-3

Prepare 500 cm3 of a 0.5 mol dm-3 of Hydrochloric acid solution from 2.5 mol dm-3 of HCl solution.

1. Calculate the volume of solution before dilution? (V1)

2. The solution before dilution poured into 500 ml volumetric flask.

3. Water is added until reached graduation mark.

4. Close with stopper, then shake it.

Relate pH values with the molarity of acids and alkali

The higher the molarity of acid, the lower the pH

value.

The higher the molarity of an

alkali, the higher the pH value.

Example :

500cm3 of a solution that contains 2.0 mol sodium hydroxide is added to 1500cm3 of a solution that contains 4.0 mol sodium hydroxide. Calculate the molarity of the sodium hydroxide solution produced.

The End...