PMC National MDCAT Chemistry: Organic Chemistry Basics MCQs

PMC National MDCAT Chemistry: Organic Chemistry Basics MCQs — sample questions

1. Question 1

   Q1. A 0.44 g sample of an organic compound containing only carbon, hydrogen, and oxygen is burnt, yielding 0.88 g of CO2. What is the percentage of carbon?

   • A) 54.54%
   • B) 27.27%
   • C) 12.00%
   • D) 44.00%

   Answer: 54.54%

   Explanation: Calculation uses mass of CO2 and sample. 27.27% is wrong because it ignores the 12/44 ratio of carbon in CO2.

2. Question 2

   Q2. A chemist analyzes a hydrocarbon and finds it contains 85.7% carbon and 14.3% hydrogen by mass. What is its empirical formula?

   • A) CH
   • B) CH2
   • C) C2H4
   • D) CH4

   Answer: CH2

   Explanation: Atomic ratios are 1:2. C2H4 is the molecular formula, not the empirical formula which must be the simplest ratio.

3. Question 3

   Q3. During a laboratory experiment, a student synthesizes an alkane with a molecular mass of 72 amu. How many structural isomers can this alkane form?

   • A) 1
   • B) 2
   • C) 3
   • D) 5

   Answer: 3

   Explanation: Pentane (C5H12) has mass 72 and forms three isomers; 2 is wrong as it ignores the branched neopentane structure.

4. Question 4

   Q4. A fuel sample is prepared by mixing 70% iso-octane and 30% n-heptane by volume. What is the octane number assigned to this specific fuel blend?

   • A) 100
   • B) 0
   • C) 30
   • D) 70

   Answer: 70

   Explanation: Octane number equals the percentage of iso-octane. 30 is wrong because it represents the n-heptane percentage, not the rating.

5. Question 5

   Q5. In a molecule of 1,3-butadiene (CH2=CH-CH=CH2), what is the total number of sigma bonds and pi bonds present respectively?

   • A) 9 sigma, 2 pi
   • B) 7 sigma, 2 pi
   • C) 11 sigma, 2 pi
   • D) 9 sigma, 1 pi

   Answer: 9 sigma, 2 pi

   Explanation: Includes six C-H and three C-C sigma bonds. 7 sigma is wrong because it forgets C-H bonds.