AdministratorJune 26, 2021 at 10:37 pm1023 Replies201950 XPContributions: EnterMedSchool Donor Team Manager VolunteerReputation: Level 7
A few drops of universal indicator solution were added to exactly 50 mL of sodium hydroxide solution, NaOH, of concentration 1 mol / L. Exactly 50 mL of hydrochloric acid, HCl , of concentration 1 mol / L, was added drop by drop to the sodium hydroxide solution containing the universal indicator solution.
What colour changes would be observed as the hydrochloric acid was added?
A. red → orange → yellow → green
B. blue → red
C. red → orange → yellow → green → blue → violet
D. violet → blue → green
E. violet → blue → green → yellow → orange → red
AdministratorJuly 16, 2021 at 11:10 pm1025 Replies673326 XPContributions: Team ManagerReputation: Level 9
This is a weird one for me as well.. I would suggest that we should keep in mind that the starting colour will slowly change as we reach the equivalence point. We know that the moles of HCl added are the same as NaOH so the end of the titration is the equivalence point. I suppose that the colour change would have to make sense colour-spectrum-wise and this is why the answer is ‘D’.
MemberJuly 22, 2021 at 8:40 pm82 Replies70225 XPContributions: EnterMedSchool Donor Team Manager VolunteerReputation: Level 6
Hey, universal indicator is a mix of a couple common indicators and there is an actual scale for it, with purple being more alkaline, green being neutral, and red being the most acidic (google ‘universal indicator’ on images). We are starting with a strong base so we know it’ll be more purple (starting solution). You add acid to base in titrations (to avoid splashing) so it will gradually change from purple to neutral (green) since we are adding the same amount and same concentration of a strong acid and a strong base. This leaves us with the one and only option, D.
A) would be if adding an acid to a base
C) full spectrum of adding a base to an acid without stopping in the middle
D) correct (if you see the picture it will really click)
E) Full range going from base to an acid (no stopping at equivalence)
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