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In the above experiment (Fig. 1) the litmus paper will initially…

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In the above experiment (Fig. 1) the  litmus paper will initially

In the above experiment (Fig. 1) the litmus paper will initially

  • A. be bleached
  • B. turn green
  • C. turn red
  • D. turn blue

QUICK ANSWER…

D

SOLUTION 

When heat is applied to solid ammonium chloride, sublimation occurs, and two gasses – ammonia and hydrogen chloride will be evolved.

Now, the relative molecular mass of ammonia = (N) + 3(H) = 14 + 3(1) = 14 + 3 = 17g/mol

The relative molecular mass of hydrogen chloride = (H) + (CL) = 1 + 35.5 = 36.5g/mol

This means ammonia is lighter, and so will diffuse faster and reach the damp litmus faster than the hydrogen chloride.

Ammonia gas is alkaline in nature, in fact, when this gas comes in contact with a damp neutral litmus pater, the moisture around the litmus will react with the ammonia gas to form ammonium hydroxide which is alkaline.

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Now, the color of litmus paper in alkaline solution is blue, so the litmus will initially turn blue, and then later turn red when hydrogen chloride shows up.

So, option D is the right answer.

Now for the right answer to the above question:

  1. Option A is incorrect. chlorine is the known bleacher of litmus, and not ammonia and hydrogen chloride.
  2. Option B is incorrect. litmus paper is never green in any medium.
  3. C is incorrect. it will turn red in the long run, and not initially.
  4. D is the correct answer. The lighter ammonia reaches the litmus first and turns it blue.

QUICK TIPS…

You may please note these/this:

  • Chlorine is the notoriously known bleacher of litmus; it turns it white.
  • When ammonium chloride is heated, two gasses will be liberated.
  • The lighter gas will diffuse quicker.

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/ culled from 2018 JAMB-UTME Chemistry question 3 /

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