## 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 0.1 0.01 0.001 pH value 0 1 2 3

Sodium hydroxide solution
 Molarity/mol dm-3 1 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.