
Concept explainers
Interpretation:
The pH of the solution, obtained by dissolving the product, of the given reaction, in water, at a temperature of 25°C is to becalculated.
Concept introduction:
Equilibrium constant isdefined as the ratio of concentration of products to the concentration of reactants and is expressed as follows:
Ka=[products][reactants].
Molarity is defined as the number of moles of solute dissolved in one litre of solution and is expressed as follows:
Molarity=Moles of soluteLiter of solution.

Answer to Problem 101AP
Solution: pH is 5.72.
Explanation of Solution
Given information: The white phosphorous burnt is 10.0 g.
The volume of solution is 0.500 L.
If the white phosphorus burns in excess of oxygen, the reaction is as follows:
P4(s)+5O2(g)→P4O10(s)
The product formed (P4O10) is dissolved in water and the reaction that takes place is as follows:
P4O10(s)+6H2O(l)→4H3PO4(aq).
The moles formed by H3PO4 are to be calculated as follows:
Moles=Given massMolar mass.
Substitute the values in the above equation:
MH3PO4=10.0 g P4×(1 mol P4123.9 g P4)(1 mol P4O101 mol P4)(4 mol H3Po41 mol P4O10)MH3PO4=0.323 mol H3Po4.
Thus, the moles of H3PO4 are 0.323 mol .
Molarity of the phosphoric acid solution is calculated as follows:
Molarity=Moles of soluteVolume of Solution (L).
Substitute 0.323 mol for the moles of solute and 0.500 L for the volume of solution in the above equation as follows:
Molarity=(0.323 mol0.500 L);Molarity=0.646 M.
The equilibrium expression for a phosphoric acid solution is as follows:
H3PO4(aq) + H2O(aq)⇌H3O+(aq) + H2PO−(aq)initial(M): 0.646 0 0change(M): -x +x +x_Equilibrium(M): 0.646 -x x x
The value of Ka for phosphoric acid is 7.5×10−3, taken from the reference table 16.8.
The [H3O+] is calculated with the help of the equilibrium constant as follows:
Ka=[H3O+][H2PO−4][H3PO4].
Substitute (0.646−x) for the concentration of phosphoric acid, (x) for the concentration of hydronium ions, (x) for the concentration of dihydrogen phosphate ions, and 7.5×10−3 for Ka in the above equation as follows:
7.5×10−3=(x)(x)(0.646−x)7.5×10−3=(x2)(0.646−x)x2=(7.5×10−3)(0.646−x).
Rearrange the above equation:
(x2+7.5×10−3x−4.85×10−3)=0.
This is a quadratic equation, by solving:
x=0.066 M.Therefore, [H3O+]=0.066 M.
The pH of the solution is to be calculated as follows:
pH=−log[H+].
Here, the concentration of hydronium ions ([H3O+]) is equal to the concentration of [H+].
pH =−log(0.066)pH=1.18.
The pH of the solution at 25°C is 1.18.
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Chapter 24 Solutions
Chemistry
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