When you learn ionic equilibrium in physical chemistry, you need to know how to calculate pH value in a mixture of strong base and a weak base. Sodium hydroxide and ammonia solutions is a mixture of strong base and weak base. Dissociation of ammonia (NH3) and pH of the overall solution depends on concentration of sodium hydroxide (NaOH).
In this tutorial, we will discuss following sections.
Sodium hydroxide (NaOH) is a strong base because it dissociates completely to sodium ion and hydroxyl ions. Therefore we can write the chemical equation for reaction as following.
Because ammonia poorly dissociates in water, it is considered as a weak base. Therefore, ammonia dissociates reversibly and equilibrium point is shifted to left by the nature. That means, concentration of ammonia is very much higher than concentrations of ammonium ions and OH- ions.
Here, we are going to calculate pH of mixture of 0.1 mol dm-3 ammonia and 0.1 mol dm-3 NaOH solution. Dissociation constant (Kb) of ammonia is 1.8 * 10-5 mol dm-3. Following steps are important in calculation of pH of ammonia solution.
Because ammonia dissociation is reversible, its equilibrium point can be shifted to right or left according to the concentration of constituents, according to the Le Chatelier's principle. Because NaOH dissociates completely in water, it give high OH- concentration. Therefore, , equilibrium point of NH3 dissociation is shifted to left more to reduce OH- concentration for keep dissociation constant as a constant. That means, dissociation of ammonia is furthermore reduced.
Because NaOH dissociates completely, it gives 0.1 mol dm-3 OH- concentration. Then we can make a table like following for NH3.
Let's consider equilibrium concentration of OH-, 0.1 +x
We know x is a small value because NH3 poorly dissociates. We learned that in pH calculation of aqueous ammonia solution. In this case, due to presence of NaOH, dissociation of NH3 is more and more limited. Therefore, x is a very small value compared to 0.1
Let's write the dissociation constant equation and substitute above values to the equation.
OH-(aq) is given by water is neglected because dissociation of water is very low compared to the ammonia dissociation.
Because OH-(aq) concentration is known now, pOH value of ammonia solution can be calculated.
At 250C, summation of pH and pOH is 14. We use that relationship to determine pH value.
There is a 0.1 M NH3 solution (100cm3) and it shows 9.5 pH value. If I add 1 g of sodium metal to that solution, what will happen to pH value of that solution.
You are going to do a dangerous thing. Adding sodium metal to water is a dangerous reaction and could cause severe injuries. So be careful in such reactions.
Adding sodium to the water will form sodium hydroxide which is a strong base and increases hydroxyl ion concentration. Therefore, it will increase the pH of the solution. With that it reduces the dissociation of ammonia in water.