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6.2. Equilibrium - II (icon Equilibrium) — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry6.2. Equilibrium - II (icon Equilibrium)1 Mark
MCQ
The titration curve of weak acid vs. strong base with phenolphthalein as indictor) is shown below. The $K _{\text {phenolphthalein }}=4 \times 10^{-10}$. Given: $\log 2=0.3$

The number of following statements which is/are correct about phenolphthalein is $.....$.

$A$. It can be used as an indicator for the titration of weak acid with weak base.

$B$. It begins to change colour at $pH =8.4$

$C$. It is a weak organic base

$D$. It is colourless in acidic medium

  • A
    $1$
  • B
    $2$
  • C
    $3$
  • D
    $4$
✓

Answer

$(B)$ $pk _{ In }=-\log \left(4 \times 10^{-10}\right)=9.4$

Indicator range

$\Rightarrow pk _{\text {In }} \pm 1$

i.e. $8.4$ to $10.4$

$(D)$ In acidic medium, phenolphthalein is in unionized form and is colourless.

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