IMAT 2019 Q49 : Universal indicator

Practice Question Solving
1

A few
solution, NaOH, of concentration 1 mol / L. Exactly 50 mL of hydrochloric acid, HCl , of
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drops of universal indicator solution were added to exactly 50 mL of sodium hydroxide
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

What do we need to know about universal indicators ?

2

Hi!
Here we need to remember the color range for universal indicator. For information: Red to orange to yellow are the more acidic colors in order from most acidic to least acidic, violet to blue to turquoise are the more basic colors in order from most basic to least basic, and green is a more neutral color.

NaOH is basic so we should expect the solution to start from a violet or blue color as these are alkaline colors for universal indicator (this means A and C cannot be true).

We then add HCl which should lower the pH, but we are also told we have the same concentration of NaCl and HCl so these should cancel each other out (as they are a strong acid and a strong base respectively) which should yield a neutral solution. That is the solution should stay at green and not go into the acidic yellow to red range. Thus, the answer is D.
Hope it helps!

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3

Thank you. I get it now.

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