Question

Find the pH and the concentrations of all species in the following solutions, each of 0.02 M concentration. Solve numerically and show all calculations. Assume a closed system.

a. HNO3
b. HCl
c. NH4ΝΟ3
d. H2O2

Answer 1

a] Dissociation reaction:

HNO3 <===> H⁺ + NO3^-

This implies that 1 mol of HNO3 is equivalent to 1 mole of H⁺ ion in the solution.

pH = -log₁₀(H⁺)

For 0.02 M HNO3, concentration of H⁺ ions = 0.02 M

pH =  -log₁₀(0.02)

.: pH = 1.7........................(<7 implies acidic)

b) Similarly,

HCl <==> H⁺ + Cl^-

pH =  -log₁₀(0.02)

.: pH = 1.7

c) NH4NO3 <==> NH4⁺ + NO3^- ............(1)

NH4⁺ + H2O <==> NH3 + H3O⁺ .........(2)

NH4⁺ formed from reaction 1 = 0.02 M

Let x be the amount of NH4⁺ reacted in reaction 2

NH3 formed = x

H3O⁺ formed = x

NH4⁺ reamining = 0.02 - x

Equilibrium constant K for this reaction is 5.6 x 10^-10

.: x = 3.34 x 10^-6 M

pH = -log₁₀[H3O⁺]

pH = -log₁₀[3.34 x 10^-6]

.: pH = 5.47