In a dry test tube, 2 mL of HCl (0.1 N) is taken. To this, 4 mL of NaOH (0.1 N) is added. The solution is stirred well, followed by the addition of 2 drops of phenolphthalein. What will be the colour of the solution?
Pink
Milliequivalents of HCl = 2 mL x 0.1 N = 0.2 meq, and milliequivalents of NaOH = 4 mL x 0.1 N = 0.4 meq. Since NaOH is present in excess (0.4 meq versus 0.2 meq of HCl), after complete neutralisation 0.2 meq of NaOH remains unreacted, making the resulting solution basic. Phenolphthalein stays colourless in acidic and neutral solutions but turns pink in basic solutions with a pH above about 8.2. Hence, the solution will turn pink.
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