Page 29 - Q & A STPM 2022 Chemistry

Chemistry Term 3 STPM Chapter 14 Introduction to Organic Chemistry
Secondly, determine the mole ratio of C : H : O
C : H : O = 3.6 : 0.8 : 1.6 Exam Tips
12 16 Exam Tips
= 0.3 : 0.8 : 0.1 First, determine the mass of
= 3 : 8 : 1 C, H and O in 6.0 g of Y.
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Empirical formula of Y is C 3 H 8 O.

Section C Essay Questions

Question 4
An organic compound, X, has the following composition by mass:

C, 35.0%; H, 6.6%; Br, 58.4%
When 1.36 g of X is vaporised, it occupies a volume of 389.2 cm at 200°C and
3
101 kPa.
(a) Determine the empirical and molecular formula of X.
(b) Draw and name all possible isomers for X and determine which of them
(if any) is/are chiral.

Answer:
(a) C : H : Br = 35 : 6.6 : 58.4
12 80
= 2.9 : 6.6 : 0.73
= 4 : 9 : 1
Empirical formula of X is C 4 H 9 Br.

Using pV = nRT,
(101 × 10 )(389.2 × 10 ) = n × 8.31 × (200 + 273)
3
–6
n = 0.010
Common Errors
Relative molecular mass of X = 1.36 When using the equation
0.010 pV = nRT, students
= 136 forget to convert the
temperature from °C to
3
Let the molecular formula of X be (C 4 H 9 Br) n , K, volume from cm to
3
[(12 x 4) + 9 + 80]n = 136 m and pressure from Term
kPa to Pa.
136n = 136
n = 1 3
The molecular formula of X is C 4 H 9 Br.

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14 Q&A STPM Chem T3 Layout.indd 345 4/19/19 5:29 PM