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Alkene Q undergoes an addition reaction with chlorine gas to form compound R - HSC - SSCE Chemistry - Question 28 - 2023 - Paper 1
Question 28
Alkene Q undergoes an addition reaction with chlorine gas to form compound R. (a) Describe a chemical test that could be done in a school laboratory to confirm that... show full transcript
Worked Solution & Example Answer:Alkene Q undergoes an addition reaction with chlorine gas to form compound R - HSC - SSCE Chemistry - Question 28 - 2023 - Paper 1
Step 1
Describe a chemical test that could be done in a school laboratory to confirm that Q is an alkene. Include expected observations in your answer.
Answer
One effective test to confirm the presence of an alkene is to add bromine water to a sample of alkene Q. When bromine water is added, the solution will initially appear brown due to the dissolved bromine. If Q is indeed an alkene, the solution will decolorize, indicating the alkene's reactivity through the addition reaction with bromine.
Step 2
Provide a structural formula for compound R. Support your answer with calculations.
Answer
From the given data, we know that compound R contains approximately 32% carbon by mass.
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To determine the molecular formula, we first calculate the number of carbon atoms. Given that the atomic mass of carbon is approximately 12 g/mol, we can set up the calculation:
% ext{ Carbon} = rac{ ext{mass of Carbon}}{ ext{molar mass of compound}} imes 100 ightarrow 32 = rac{36}{ ext{molar mass of R}} imes 100 ightarrow ext{molar mass of R} = 36 / 0.32 = 112.5 ext{ g/mol}
Thus, the total mass of R is approximately 114 g/mol. However, the molecular ion peak in the mass spectrum indicates a molecular ion at m/z = 114. Therefore, we have:
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Non-carbon mass = 114 - 36 = 78.
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This non-carbon mass must account for the chlorine atoms present.
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Given that the atomic mass of chlorine is approximately 35.5 g/mol:
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Therefore, exactly two Cl atoms are present in a molecule of compound R.
So, compound R has the molecular formula C₃H₆Cl₂ and can be structurally represented as follows:
H Cl
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H - C - C - C - Cl
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H H
This structural formula reflects the identified atoms and ratios present in compound R.