
Concept explainers
A 10.0-g sample of white phosphorus was burned in an excess of oxygen. The product was dissolved in enough water to make 500.0 mL of solution. Calculate the

Interpretation:
The pH of the solution formed by the combustion of phosphorus and consequential dissolution in water is to be determined.
Concept introduction:
The first ionization of a poly-protic 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,
pH of the solution is calculated as
Molarity
Answer to Problem 140AP
Solution:
The pH of the solution is
Explanation of Solution
Given information:
White phosphorous is burned in oxygen according to the reaction:
According to the stoichiometry,
So,
Molar mass of
Number of moles of
Now,
According to the stoichiometry, 1 mole of
Use equation (4) to calculate molarity of the solution as
Refer to table
When phosphoric acid is dissolved in water, dissociation takes place in three steps as it is a triprotic acid. First, one proton is partially dissociated, becausephosphoric 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 represented 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 (2) as
Since the percent dissociation is more than
Since concentration cannot be negative, so
Thus,
The second and third ionization constants for phosphoric acid have a low value. Thus, it will have negligible contribution in the hydronium ion concentration.
Now, use equation (3) to calculate pH of the solution as
The pH of the solution is calculated to be
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Chapter 16 Solutions
EBK CHEMISTRY
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