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How many grams of methylacetylene (propyne) CH\(_3\) — C Ξ CH will completely discharge the...

Chemistry
JAMB 1982

How many grams of methylacetylene (propyne) CH\(_3\) — C Ξ CH will completely discharge the colour of 8g of bromine?
(Br = 80, C = 12, H = 1)

  • A. 0.5
  • B. 1.0
  • C. 2.0
  • D. 3.0
  • E. 4.0
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Correct Answer: Option B
Explanation

  CH\(_3\) - C≡CH     +  2Br\(_2\)  → CH\(_3\)-CBr\(_2\) -CH(Br\(_2\))

      1 mole          :    2 moles

Recall that n = \(\frac{m}{M}\) 

Molar mass of Br\(_2\) = 80 x 2 = 160 g/mol

                 n = \(\frac{8}{160}\)

                 n = 0.05 mole

If      1 mole of CH\(_3\) - C≡CH reacts with 2 moles of Br\(_2\) 

then   X mole of CH\(_3\) - C≡CH will react with 0.05 mole of Br\(_2\) 

        X = \(\frac{0.05}{2}\)

        X = 0.025 mole of CH\(_3\) - C≡CH

Also, recall that n = \(\frac{m}{M}\) 

Molar mass of CH\(_3\) - C≡CH = C3H4 = (12 X 3) + (1 X 4) = 40 g/mol

n = \(\frac{m}{M}\)

n = 0.025 x 40 = 1.0 g  


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