Acid-base titrations and choice of indicator — NEET & JEE Questions

Question 1 · difficulty L1 · recall

What marks the end point of an acid-base titration when a suitable indicator is used?

• A. Formation of a metal precipitate in every titration
• B. Complete evaporation of the solution
• C. A persistent colour change near equivalence
• D. A sudden increase in the mass of the burette

Question 2 · difficulty L2 · understanding

Which indicator is generally suitable for titrating a weak acid with a strong base?

• A. Potassium permanganate because it is self-indicating
• B. Methyl orange only because the equivalence point is acidic
• C. Starch because iodine is produced
• D. Phenolphthalein

Question 3 · difficulty L2 · understanding

In the titration of oxalic acid against sodium hydroxide, the indicator used and its colour change at the end point are

• A. phenolphthalein; colourless to pink
• B. methyl orange; yellow to red
• C. phenolphthalein; pink to colourless
• D. methyl orange; colourless to pink

Question 4 · difficulty L2 · understanding

Methyl orange shows which colours in acidic and in basic solutions respectively, and in roughly which pH range does it change?

• A. Yellow in acid, red in base; about pH 8 to 10
• B. Red (pink) in acid, yellow in base; about pH 3.1 to 4.4
• C. Colourless in acid, pink in base; about pH 8.3 to 10
• D. Blue in acid, red in base; about pH 7

Question 5 · difficulty L3 · application

A burette is rinsed only with distilled water and then filled with the titrant without conditioning it. What error is most likely?

• A. Residual water dilutes the titrant
• B. The titrant becomes more concentrated than its stock solution
• C. The analyte is completely neutralised before titration begins
• D. The indicator changes into a primary standard

Question 6 · difficulty L3 · application

Hydrochloric acid is titrated against ammonium hydroxide. Which indicator should be chosen, and why?

• A. Phenolphthalein, because the equivalence point is basic
• B. Either indicator, because the pH changes sharply on both sides
• C. Methyl orange, because the salt formed (NH$_4$Cl) is acidic and the equivalence point lies below pH 7, within methyl orange's range
• D. No indicator, because the titration cannot be done

Question 7 · difficulty L3 · application

20.0 mL of 0.100 M NaOH is exactly neutralised by 25.0 mL of a hydrochloric acid solution. The molarity of the acid is

• A. 0.125 M
• B. 0.100 M
• C. 0.050 M
• D. 0.080 M

Question 8 · difficulty L3 · application

25.0 mL of 0.050 M oxalic acid (a dibasic acid) is titrated with 0.100 M NaOH. The volume of NaOH needed to reach the end point is

• A. 25.0 mL
• B. 12.5 mL
• C. 50.0 mL
• D. 6.25 mL

Question 9 · difficulty L4 · analysis

A student adds 20 mL of distilled water to the conical flask after pipetting a fixed amount of acid into it. How does this affect the titre in an ideal titration?

• A. The titre doubles because the acid volume doubled
• B. Essentially unchanged; acid moles are unchanged
• C. The titre becomes zero because the acid is diluted
• D. The required base volume decreases in exact proportion to dilution

Question 10 · difficulty L4 · application

In the laboratory, a standard solution of oxalic acid is prepared by weighing, but sodium hydroxide solution has to be standardised by titration. Which statement gives the correct reason and the correct mass of oxalic acid dihydrate (molar mass 126 g mol$^{-1}$) needed for 250 mL of 0.100 M solution?

• A. NaOH is a gas at room temperature; 12.6 g
• B. NaOH is hygroscopic and absorbs CO$_2$ from the air, so its weighed mass does not give its true amount, whereas oxalic acid dihydrate is a stable, pure, non-hygroscopic solid (a primary standard); 3.15 g
• C. Oxalic acid is hygroscopic; NaOH is a primary standard; 3.15 g
• D. Both can be weighed accurately; 1.26 g