Thursday, 22 January 2015

7.3.3 - Laboratory Activity : Relationship between the pH value and molarity of acidic solutions and an alkali


Laboratory Activity 7.3.3:
Relationship between the pH value and molarity of acidic solutions and an alkali
Aim:What is the relationship between the pH values and molarities of acidic and alkaline solution
Problem statement: What is the relationship between the pH values and molarities of acidic and alkaline solution?
Hypothesis:
» The higher the molarity of an acidic solution, the lower is its pH value.
» The higher the molarity of an alkaline solution, the higher is its pH value.
Variable:
» Fixed variable : Types of acid and alkali
» Manipulated variable : Molarity of acidic and alkaline solutions
» Responding variable : pH value

Material:
» Hydrochloric acid (0.001M, 0.01M, 0.1M, 1M)
» Sodium hydroxide (0.001M, 0.01M, 0.1M, 1M)

Apparatus:
» Beaker
» Measuring cylinder
» pH meter
Procedure:

1. 20cm3 hydrochloric acid, 1M concentration is put into a beaker. The pH meter is placed in the solution and the reading is recorded.
2. Step 1 is repeated with 0.1 M, 0.01 and 0.001 M solutions of hydrochloric acid.
3. 20cm3 sodium hydroxide solution, 1M concentration is put into a beaker. The pH meter is placed in the solution and the reading is recorded.
4. Step 3 is repeated with 0.1 M, 0.01 and 0.001 M solutions of sodium hydroxide.
Results:

Hydrochloric acid
Molarity/mol dm-3 1.0 0.1 0.01 0.001
pH value 0 1 2 3

Sodium hydroxide solution
Molarity/mol dm-3 1.0 0.1 0.01 0.001
pH value 14 13 12 11
Discussion:

The pH value of the acid decreases as its molarity increases. The concentration of H+ ion is higher at higher molarity. The higher the H+ ion, the lower is the pH value.

The pH value of the alkaline increases as its molarity increases. The concentration of OH- ion is higher while the concentration of H+ ion is lower. The lower the H+ ion, the higher is the pH value.
Conclusion:

The pH value of an acid decreases as the molarity of the acid increases.

The pH value of an alkali increases as the molarity of the alkali increases.

The hypothesis is accepted.


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