Organic Chemistry Practice Questions Set 2

  1. Which chemical formula is both an empirical formula and a molecular formula?
  1. CH4
  2. C2H6
  3. CH3COOH
  4. CH3CH2COOCH3
Show Answer
The correct answer is A!

A formula is both empirical and molecular when it cannot be reduced (the GCD of subscripts is 1). CH4 is already simplest.

C2H6 reduces to CH3, acetic acid’s molecular formula is C2H4O2, and the ester likewise reduces.

 

  1. Which element is present in all organic compounds?
  1. Hydrogen
  2. Nitrogen
  3. Carbon
  4. Oxygen
Show Answer
The correct answer is C!

By definition, organic compounds contain carbon. Many also contain hydrogen, oxygen, nitrogen, etc., but carbon is universal.

 

  1. What products are obtained when CH4(g) burns completely in excess oxygen?
  1. CO and H2O
  2. CO and C
  3. CO2 and H2O
  4. CO2 and CO
Show Answer
The correct answer is C!

Complete combustion of a hydrocarbon yields CO2 and H2O. For methane:

\(\text{CH}_4 + 2 \text{O}_2 \rightarrow \text{CO}_2 + 2 \text{H}_2\text{O}\)

 

  1. Which hydrocarbon is an alkene (assume acyclic)?
  1. C2H2
  2. C3H6
  3. C4H10
  4. C4H12
Show Answer
The correct answer is B!

Acyclic alkenes follow C\(n\)H2\(n\). C3H6 fits. C2H2 is an alkyne (C\(n\)H2\(n\)2), C4H10 is an alkane (C\(n\)H2\(n\)+2), and C4H12 is not a valid hydrocarbon formula.

 

  1. Which formula represents butane?
  1. CH3CH3
  2. CH3CH2CH3
  3. CH3CH2CH2CH3
  4. CH3CH2CH2CH2CH3
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The correct answer is C!

Butane has four carbons in a chain: CH3–CH2–CH2–CH3. The other choices are ethane, propane, and pentane.

 

  1. A hydrocarbon is considered saturated if it contains
  1. Single covalent bonds only
  2. A double covalent bond only
  3. A triple covalent bond
  4. Single and double covalent bonds
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The correct answer is A!

Saturated hydrocarbons (alkanes) contain only single C–C bonds. Multiple bonds make a molecule unsaturated.

 

  1. As a chemical bond forms between two hydrogen atoms, energy is released and the stability of the system
  1. Increases
  2. Decreases
  3. Remains the same
Show Answer
The correct answer is A!

Bond formation lowers potential energy, and lower energy corresponds to greater stability.

 

  1. In a chemical reaction, as a species is oxidized, its oxidation number
  1. Decreases
  2. Increases
  3. Remains the same
Show Answer
The correct answer is B!

Oxidation is an increase in oxidation number (loss of electrons), and reduction is a decrease (gain of electrons).

 

  1. A hydrocarbon with no ring structure and no double or triple bonds is called an ______.
  1. Alkene
  2. Alkyne
  3. Alkane
  4. Alkali
Show Answer
The correct answer is C!

Acyclic hydrocarbons with only single bonds are alkanes (saturated). Alkenes have C=C,. while alkynes have C≡C.

 

  1. A hydrocarbon with double bonds and no triple bonds is called an ______.
  1. Alkene
  2. Alkyne
  3. Alkane
  4. Alkali
Show Answer
The correct answer is A!

Alkenes contain at least one C=C and no C≡C, alkynes contain C≡C, and alkanes contain only single bonds.

 

  1. A hydrocarbon with triple bonds is called an ______.
  1. Alkene
  2. Alkyne
  3. Alkane
  4. Alkali
Show Answer
The correct answer is B!

Alkynes contain a carbon–carbon triple bond (C≡C). “Alkali” refers to basic substances or Group 1 metals, not hydrocarbons.

 

  1. Which of the following is the molecular formula for ethylene?
  1. C2H2
  2. C2H4
  3. C2H6
  4. C2H8
Show Answer
The correct answer is B!

Ethylene (ethene) is C2H4, an alkene (C\(n\)H2\(n\) for \(n = 2\)).

 

  1. Which of the following is the molecular formula for ethane?
  1. C2H2
  2. C2H4
  3. C2H6
  4. C2H8
Show Answer
The correct answer is C!

Ethane is the C2 alkane: C2H6 (C\(n\)H2\(n\)+2 for \(n = 2\)).

 

  1. Which of the following is the molecular formula for acetylene?
  1. C2H2
  2. C2H4
  3. C2H6
  4. C2H8
Show Answer
The correct answer is A!

Acetylene (ethyne) is C2H2, an alkyne (C\(n\)H2\(n\)−2 for \(n = 2\)).

 

  1. Which of the following is the molecular formula for butane?
  1. C2H
  2. C2H6
  3. C4H8
  4. C4H10
Show Answer
The correct answer is D!

Butane is the C4 alkane: C4H10 (C\(n\)H2\(n\)+2 for \(n = 4\)). C4H8 would be an alkene or cycloalkane.

 

  1. Which of the following is the molecular formula for hexane?
  1. C5H11
  2. C5H12
  3. C6H12
  4. C6H14
Show Answer
The correct answer is D!

Hexane is the C6 alkane: C6H14 (CnH2\(n\)+2 for \(n = 6\)). C6H12 would be an alkene or cycloalkane.

 

  1. Which of the following is the molecular formula for pentane?
  1. C5H11
  2. C5H12
  3. C6H12
  4. C6H14
Show Answer
The correct answer is B!

Pentane is the C5 alkane: C5H12 (C\(n\)H2\(n\)+2 for \(n = 5\)).