Calculate pH of Ammonia Solution | Examples | Online Calculator
Ammonia is very much soluble in water and forms a weak basic aqueous solution. Ammonia poorly dissociates to ammonium ions and hydroxyl ions. Because, ammonia is a weak base, equilibrium concentration of ammonia is higher than equilibrium concentration of ammonium ion and hydroxyl ions. Otherwise, we can say, equilibrium point of the reaction is shifted to the left by nature.
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
In this tutorial, we will discuss following sections.
- Dissociation of ammonia
- Calculate pH of ammonia by using dissociation constant (Kb) value of ammonia
- pH values of common aqueous ammonia solutions
- Online calculator to find pH of ammonia solutions
Dissociation of ammonia in water
Ammonia dissociates poorly in water to ammonium ions and hydronium ion. This reaction is reversible and equilibrium point is shifted to left side (In strong bases such as NaOH, equilibrium point is shifted to the right side). That means, concentration of ammonia is very much higher than concentrations of ammonium ions and OH- ions.
Calculate pH of ammonia by using dissociation constant (Kb) value of ammonia
Here, we are going to calculate pH of 0.1 mol dm-3 aqueous ammonia 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.
- First, pOH is found and next, pH is found as steps in the calculations. See the below example.
- Dissociation of water is negligible compared to the dissociation of ammonia. Therefore, hydroxyl ion concentration received by water is neglected.
Ammonia is a weak base. Therefore, dissociated concentration is very small compared to the initial concentration of ammonia. Therefore, we make an assumption of equilibrium concentration of ammonia is same as the initial concentration of ammonia. Then, we can substitute the equilibrium concentration of ammonia (NH3), ammonium ion (NH4+) and hydroxyl ion (OH-) to the equation.
Now, we know the concentration of OH- ions.
- x = [OH-(aq)] = [NH4+(aq)] = 1.34 * 10-3 mol dm-3
Assumptions made in calculations
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.
- pOH = -log[OH-(aq)]
- pOH = -log[1.34 * 10-3]
- pOH = 2.88
At 250C, summation of pH and pOH is 14. We use that relationship to determine pH value.
- pH + pOH = 14
- pH + 2.88 = 14
- pOH = 11.12
When ammonia solution is diluted by ten times, it's pH value is reduced by 0.5. As an example, 0.1 mol dm-3 ammonia solution is diluted to 0.01 mol dm-3, pH value is reduced from 11.13 to 10.63.
Questions
What will happen to the pH value of open bottle of ammonia solution to the atmosphere?
Question: I have made 0.1 mol dm-3 ammonia solution in my lab. I went out for a some reason and forgot to close the lid. I came back after 10 minutes and check my pH value. pH value was reduced than initial value? What will be the reason for that?
Answer:
When a gaseous compounds is dissolved in a closed container, that system comes to an equilibrium after some time. But, if system is open, there cannot be an equilibrium. Here also, that is the case.
Ammonia exist as a gaseous compound in room temperature. If you have opened the lid of aqueous ammonia solution bottle, ammonia molecules will start to come to the atmosphere. It reduced the concentration of ammonia in the solution and hydroxyl ion concentration as well. That's why pH value is reduced with time.
