Phenol, commonly called carbolic acid, is a weak organic acid. If you dissolve 0.892 g of the acid in enough water to make 357 mL of solution, what is the equilibrium hydronium ion concentration? And what is the pH o the solution?
First we must know the concentration of phenol solution.
We have, No. of moles = Mass / Molar mass
No. of moles of phenol = 0.892 g / 94.11 g/mol = 0.009478 mol
Volume of solution = 357 ml = 0.357 L
We have, Molarity = No. of moles of solute / volume of solution in L
[C6H5OH] = 0.009478 mol / 0.357 L = 0.02655 M
Consider dissociation of phenol in water.
C6H5OH (aq) + H2O C6H5O - (aq) + H3O + (aq)
Equilibrium constant expression for above reaction is K a = [H3O +] [C6H5O - ] / [C6H5OH] = 1.01 10 -10
Let's use ICE table to find out equilibrium concentrations of C6H5O - , H3O + and C6H5OH .
Concentration (M) | C6H5OH | H3O + | C6H5O - |
Initial | 0.02655 | ||
Change | -X | +X | +X |
Equilibrium | 0.02655 -X | X | X |
Putting above values in Ka formula we get
Ka = (X) (X) / 0.02655 - X = 1.01 10 -10
Phenol is a weak acid, hence we can assume X is very small as compared 0.02655. So we can write 0.02655 -X 0.02655.
Therefore, K a = X 2 / 0.02655 = 1.01 10 -10
X 2 = 1.01 10 -10 (0.02655)
X 2 = 2.682 10 -12
X = 1.638 10 -06 M = [H3O +] = [C6H5O - ]
We have, pH = -log [H3O +] = - log 1.638 10 -06 = 5.78
ANSWER : Equilibrium hydronium ion concentration = 1.64 10 -06 M
pH of phenol solution = 5.78
Phenol, commonly called carbolic acid, is a weak organic acid. If you dissolve 0.892 g of...
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