6. Which of the following is not true for a solution at 25° C that has a hydroxi
ID: 1034331 • Letter: 6
Question
6. Which of the following is not true for a solution at 25° C that has a hydroxide (OH) concentration of 2.5 x 106 M A. The Kw is independent of what the solution contains. B. Kw-1 x 10-14 C. [OH]-2.5 x 106 M D. The solution is acidic. 7. HA and HB are both weak acids in water, and HA is a stronger acid than HB. Which of the following statements is correct? A. A- is a stronger base than B, which is a stronger base than H:0, which is a stronger base than Cl. B. B is a stronger base than A, which is a stronger base than H20, which is a stronger base than CF. C. B is a stronger base than A", which is a stronger base than CI, which is a stronger base than H2O D. Cl is a stronger base than A, which is a stronger base than B, which is a stronger base than H20.Explanation / Answer
6) Answer is D
pH = 14 - pOH = 14 - (- log [OH-]) = 14 - (-log 2.5 x 10-6)) = 14 - (6 - log 2.5) = 14 -6 + 0.3979 = 8.3979 (= 8.4 approx.)
(Since log10 10x = x , log10 2.5 = 0.3979)
We know that if pH is below 7 then solution will be acidic and if pH is above 7 solution is basic. So the given solution is basic nature.
We know that ionization constant of water ,Kw does not change with concentration of protons or hydroxyl ions (Le chatelier principle). It depends on the temperature and pressure (A is true). And the value of Kw is 1 x 10-14 (B is true). It was given that OH- ion concentration is 2.5 x 10-6 (So C also true).
7) According to conjugate acid-base theory, Conjugate of a weak acid is a strong base and conjugate of a strong acid is a weak base. Since HA is a stronger acid than HB, Conjugate of HB (= B-) is a stronger base than conjugate of HA (= A-)
H2O is conjugate base of strong acid H3O+
Cl- is conjugate base of strong acid HCl
But HCl is a stronger acid than H3O+
So H2O is stronger base than Cl-
And HA and HB are weaker acids than HCl and H3O+
A-and B-are stronger bases than Cl-and H2O
From the above results we can conclude that
B-is stronger base than A-, which is stronger base than H2O, which is a stronger base than Cl-
So Answer is B
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