Concept explainers
Calculate the concentrations of all species in a
Interpretation:
The concentrations of all the species present in the solution, which contains phosphoric acid, is to be determined.
Concept introduction:
The first ionization of the polyprotic acid takes place as
The second ionization of the polyprotic acid takes place as
The third ionization of the polyprotic acid takes place as
Percent ionization is the percentage of acid that gets dissociated upon addition to water. It depends on the hydronium ion concentration.
Here,
Answer to Problem 152AP
Solution:
Explanation of Solution
Given information:
The concentration of phosphoric acid
Refer to Table
When phosphoric acid is dissolved in water, the dissociation takes place in three steps, as it is a triprotic acid. First, one proton is partially dissociated, since phosphoric acid is a weak acid. Thus, the pH of the solution is determined by the contribution made by all the three proton dissociation steps.
The reaction of the first proton dissociation of phosphoric acid is depicted as
Prepare an equilibrium table and represent each of the species in terms of
Now, substitute these concentrations in equation (1) as
Since the value of
Thus,
Calculate the percent dissociation from equation (4) as
Since the percent dissociation is more than
Since concentration cannot be negative,
Thus,
Also,
Now, the reaction of the second proton dissociation of phosphoric acid is depicted as
Prepare an equilibrium table and represent each of the species in terms of
Now, substitute these concentrations in equation (2) as
Since the value of
Thus,
Also,
Calculate the percent dissociation from equation (4) as
Since the percent dissociation is significantly less than
Now, the reaction of the third proton dissociation of phosphoric acid is depicted as
Prepare an equilibrium table and represent each of the species in terms of
Now, substitute these concentrations in equation (3) as
Since the value of
Thus,
Calculate the percent dissociation from equation (4) as
Since the percent dissociation is significantly less than
Also,
And,
Also,
Therefore, the concentration of all the species in the phosphoric acid solution are as follows:
Therefore, the concentration of all the species in the phosphoric acid solution are as follows:
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Chapter 16 Solutions
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