Question

How many moles of $NaOH$ must be removed from one litre of aqueous solution to change its pH from 12 to 11?

• A. 0.009
• B. 0.01
• C. 0.02
• D. 0.1

Answer 1

Answer: (A)

0.009

Given Initial $pH=12$

Therefore $pOH=14-12pOH=-\log \left[{OH}^{-}\right]=2{\left[{OH}^{-}\right]}_{initial}={10}^{-2}$

Final $pH=11$

Therefore $pOH=14-11pOH=-\log \left[{OH}^{-}\right]=3{\left[{OH}^{-}\right]}_{final}={10}^{-3}$

Since the volume is one litre molarity and number of moles are equal.

Therefore number of moles of $\left[{OH}^{-}\right]$ removed $={\left[{OH}^{-}\right]}_{final}-{\left[{OH}^{-}\right]}_{initial}={10}^{-2}-{10}^{-3}$

$=0.009mol$