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:

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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.

↶ 7.3 Concentration of Acid and Alkali →

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