Sample 1Medium
A burette holds sodium hydroxide solution, and a conical flask beneath it holds 10.0 mL of 0.10 M oxalic acid solution with two drops of phenolphthalein. The solution in the flask turns from colourless to a permanent pale pink after 20.0 mL of the NaOH solution has been added. What is the molarity of the NaOH solution?
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The answer is A.
H2C2O4 + 2NaOH → Na2C2O4 + 2H2O. Moles of oxalic acid = 0.10 × 0.0100 = 1.0 × 10⁻³ mol, so NaOH = 2.0 × 10⁻³ mol in 0.0200 L, giving 0.10 M.
- B.
This treats oxalic acid as monobasic; each mole needs two moles of NaOH.
- C.
This inverts the volume ratio; the larger volume of NaOH means it is the more dilute solution.
- D.
This both treats oxalic acid as monobasic and halves the result again.
NCERT: Class 11 Chemistry Laboratory Manual