For each [OH−], determine the pH and state whether the solution is acidic or basic.
(a) [OH−] = 1.8 × 10−5 M
(b) [OH−] = 8.9 × 10−12 M
(c) [OH−] = 3.1 × 10−2 M
(d) [OH−] = 1.96 × 10−9 M
Solution: Here, we are going to determine the pH of the given solutions.
Step1:
pH is the negative logarithm to base 10 of hydrogen ion concentration whereas pOH is the negative logarithm to base 10 of hydroxyl ion concentration, i.e.,
pOH = - log[OH- ] and pH = - log[H+ ]
And pH + pOH =14
We can distinguish acidic, neutral and basic aqueous solutions by the relative values of their pH
Acidic: pH < 7
Neutral: pH = 7
Basic: pH > 7
Step2:
- Given, [OH- ] = 1.8 x 10-5 M
Therefore pOH = - log[OH- ]
= - log (1.8 x 10-5)
= - log 1.8 - log 10-5
= -0.255 + 5
= 4.745
Therefore, pH = 14 - pOH
= 14 - 4.745
= 9.255
Thus, pH = 9.255
Since, pH > 7, therefore, the solution is basic.
b) Given, [OH- ] = 8.9 x 10-12 M
Therefore pOH = - log[OH- ]
= - log (8.9 x 10-12)
= - log 8.9 - log 10-12
= - 0.949 + 12
= 11.051
Therefore, pH = 14 - pOH
= 14 - 11.051
= 2.949
Thus, pH = 2.949
Since, pH < 7, therefore, the solution is acidic.
c) Given, [OH- ] = 3.1 x 10-2 M
Therefore pOH = - log[OH- ]
= - log (3.1 x 10-2)
= - log 3.1 - log 10-2
= - 0.491 + 2
= 1.509
Therefore, pH = 14 - pOH
= 14 - 1.509
= 12.491
Thus, pH = 12.491
Since, pH > 7, therefore, the solution is basic.
d) Given, [OH- ] = 1.96 x 10-9 M
Therefore pOH = - log[OH- ]
= - log (1.96 x 10-9)
= - log 1.96 - log 10-9
= -0.292 + 9
= 8.708
Therefore, pH = 14 - pOH
= 14 - 8.708
= 5.292
Thus, pH = 5.292
Since, pH < 7, therefore, the solution is acidic.
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