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Calculate the pH for
(a) 0.02 M HCl
(b) 0.10 M HCN
Solution:
pH means "power of hydrogen". Some chemists say that pH means "potential of hydrogen". It is a measurement of acidity or alkalinity of a solution and it has a scale from 0 to 14. If pH is ranging from 0 to 6, then the solution is acidic. If pH is 7, then the solution is neutral like water, for example. If pH is ranging from 8 to 14, then the solution is alkaline. The pH of any acidic solution can be calculated by using the equation,
where [H+] is the concentration of hydrogen ion in molarity.
(a) For 0.02 M HCl solution
Since HCl is a strong acid, then it is completely ionized into hydrogen ion and chloride ion as follows
In a liter of solution, 0.02 moles of HCl is ionized completely into 0.02 moles of H+ and 0.02 moles of Cl-. Hence, the concentration of [H+] is 0.02.
Therefore, the pH of 0.02 M HCl solution is
If the pH of an acid is closer to 0, then it is a strong acid.
(b) For 0.10 M HCN solution
Since HCN is a weak acid, then it is partially ionized into hydrogen ion and cyanide ion as follows
In a liter of solution at equilibrium,
where x is moles of HCN that partially ionized. The ionization constant for HCN is given by the equation
From Table of Ionization Constants of Acids and Bases, Ka = 4.0 x 10-10 for hydrocyanic acid. The amount of HCN that partially ionized is
Since the value of Ka is less than 1 x 10-3, then we can neglect x at the denominator as follows
Hence, the concentration of hydrogen ion is
Therefore, the pH of 0.10 M HCN solution is
If the pH of an acid is closer to 7, then it is a weak acid.
From Table of Ionization Constants of Acids and Bases, Ka = 4.0 x 10-10 for hydrocyanic acid. The amount of HCN that partially ionized is
Since the value of Ka is less than 1 x 10-3, then we can neglect x at the denominator as follows
Hence, the concentration of hydrogen ion is
Therefore, the pH of 0.10 M HCN solution is
If the pH of an acid is closer to 7, then it is a weak acid.