Barbituric acid, HC 4 H 3 N 2 O 3 , is used to prepare barbiturates, a class of drugs used as sedatives. Its K a is 9.8 × 10 − 5 . Calculate [H + ] in solutions prepared by adding enough water to the following to make 1.45 L. (a) 0.344 mol (b) 28.9 g
Barbituric acid, HC4H3N2O3, is used to prepare barbiturates, a class of drugs used as sedatives. Its Ka is
9.8
×
10
−
5
. Calculate [H+] in solutions prepared by adding enough water to the following to make 1.45 L.
(a) 0.344 mol
(b) 28.9 g
Expert Solution
Interpretation Introduction
(a)
Interpretation:
The concentration of hydrogen ion in the given solution should be calculated.
Concept introduction:
The dissociation reaction of a weak acid is represented as follows:
HB⇄H++B−
The expression for the acid dissociation constant will be as follows:
Ka=[H+][B−][HB]
Here, [H+] is equilibrium concentration of hydrogen ion, [B−] is equilibrium concentration of conjugate base and [HB] is the equilibrium concentration of weak acid.
Answer to Problem 51QAP
4.84×10−3 M
Explanation of Solution
The acid dissociation constant of barbituric acid is 9.8×10−5. The volume of solution is 1.45 L and number of moles of the acid is 0.344 mol.
The molarity of HC4H3N2O3 can be calculated as follows:
M=nV(L)
Putting the values,
M=0.344 mol1.45 L=0.24 M
Thus, the concentration of HC4H3N2O3 is 0.24 M.
The concentration all the species can be calculated using the ICE table as follows:
Since, the value of x cannot be negative thus, the value of x will be 4.84×10−3.
From the ICE table, it is equal to the concentration of hydrogen ion thus,
[H+]=4.84×10−3
Therefore, the concentration of hydrogen ion is 4.84×10−3 M.
Expert Solution
Interpretation Introduction
(b)
Interpretation:
The concentration of hydrogen ion in the given solution should be calculated.
Concept introduction:
The dissociation reaction of a weak acid is represented as follows:
HB⇄H++B−
The expression for the acid dissociation constant will be as follows:
Ka=[H+][B−][HB]
Here, [H+] is equilibrium concentration of hydrogen ion, [B−] is equilibrium concentration of conjugate base and [HB] is the equilibrium concentration of weak acid.
Answer to Problem 51QAP
3.86×10−3 M
Explanation of Solution
The mass of barbituric acid is given as 28.9 g. The molecular formula of the barbituric acid is HC4H3N2O3 thus, its molar mass is 128.1 g/mol.
Now, from mass and molar mass of the HC4H3N2O3, its number of moles can be calculated as follows:
n=mM=28.9 g128.1 g/mol=0.2256 mol
Now, the molarity of HC4H3N2O3 can be calculated as follows:
M=nV(L)
Putting the values,
M=0.2256 mol1.45 L=0.156 M
Thus, the concentration of HC4H3N2O3 is 0.156 M.
The concentration all the species can be calculated using the ICE table as follows:
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