So we just have ammonia here, so concentration of NH3. So let's go ahead and plug in what we have. For the concentration of ammonium, we have 0. So we put 0. For the concentration of hydroxide, we have x.
So we go ahead and put an x in here. And then that's all over the concentration of ammonia at equilibrium, and we go over here, and for ammonia at equilibrium it's 0. So we write over here 0. And we can plug in the Kb value, 1. So let's go ahead and plug in the Kb. So if we say that x is extremely small number, then we don't have to worry about it when we're adding it to 0. And so we just say this is equal to 0. So this is times x. And make sure you understand this x is this x. And then once again, 0.
So now, we would have this. And we need to solve for x. So let's go ahead and do that. Let's get out the calculator here. We need to solve for x. And that gives us what x is equal to. And so x is equal to 7. So x is equal to 7. So this would be the concentration of hydroxide, so 7.
Our problem wanted us to calculate the pH. So if we know the concentration of hydroxide ions, we can find the pOH by taking the negative log of the concentration of hydroxide. So negative log of the concentration of hydroxide ions will give us the pOH. So let's do that. So we have the negative log of 7. And this gives us the pOH, where we round to 5. So the pOH is 5. Let's get a little bit more room here. So pOH is equal to 5. And then we're home free because pH plus pOH is equal to So we just plug in our pOH, solve for the pH.
And so we've calculated the final pH of our solution. The amount of NaCl that could dissolve to reach the saturation point would be lowered. This phenomenon is the common ion effect and plays important roles in pharmaceutical and environmental areas. For example, when calcium fluoride dissolves into calcium and fluoride ions, the solubility product expression is:. This expression must always hold, even if some ionic species come from other sources. A buffer solution is composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.
Addition of excess ions will alter the pH of the buffer solution. Therefore, the common ion effect takes a role in pH regulation.
In the case of an an acidic buffer, the hydrogen ion concentration decreases, and the resulting solution is less acidic than a solution containing the pure weak acid. The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium.
The common ion effect suppresses the ionization of a weak base by adding more of an ion that is a product of this equilibrium. Consider the common ion effect of OH - on the ionization of ammonia. Adding the common ion of hydroxide shifts the reaction towards the left to decrease the stress in accordance with Le Cha telier's Principle , forming more reactants.
This decreases the reaction quotient, because the reaction is being pushed towards the left to reach equilibrium. The reaction is put out of balance, or equilibrium. At first, when more hydroxide is added, the quotient is greater than the equilibrium constant.
Adding a common ion decreases solubility, as the reaction shifts toward the left to relieve the stress of the excess product. Adding a common ion to a dissociation reaction causes the equilibrium to shift left, toward the reactants, causing precipitation. What happens to the solubility of PbCl 2 s when 0. Because K sp for the reaction is 1. The molarity of Cl - added would be 0. The equilibrium constant remains the same because of the increased concentration of the chloride ion. To simplify the reaction, it can be assumed that [Cl - ] is approximately 0.
The exceptions generally involve the formation of complex ions, which is discussed later. The following examples show how the concentration of the common ion is calculated. John poured If a common ion is added to a weak acid or weak base equilibrium, then the equilibrium will shift towards the reactants, in this case the weak acid or base.
Consider the lead II ion concentration in this saturated solution of PbCl 2. The balanced reaction is. The concentration of lead II ions in the solution is 1. Consider what happens if sodium chloride is added to this saturated solution. Sodium chloride shares an ion with lead II chloride. The chloride ion is common to both of them; this is the origin of the term "common ion effect".
What happens to that equilibrium if extra chloride ions are added? Of course, the concentration of lead II ions in the solution is so small that only a tiny proportion of the extra chloride ions can be converted into solid lead II chloride. The lead II chloride becomes even less soluble , and the concentration of lead II ions in the solution decreases. This type of response occurs with any sparingly soluble substance: it is less soluble in a solution which contains any ion which it has in common.
This is the common ion effect. If an attempt is made to dissolve some lead II chloride in some 0. As before, define s to be the concentration of the lead II ions. The calculations are different from before.
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