1. Organic chemistry is best described as the study of which type of substances at this level?
ⓐ. Covalent carbon compounds
ⓑ. Only minerals obtained from rocks
ⓒ. Only compounds that are gases at room temperature
ⓓ. Salts formed by complete transfer of electrons
Correct Answer: Covalent carbon compounds
Explanation: Organic chemistry mainly deals with carbon compounds, especially those in which carbon forms covalent bonds with carbon, hydrogen, oxygen, nitrogen, halogens, and other elements. The word organic does not mean that the compound must come only from a living organism. Methane, ethanol, and acetic acid are common examples of carbon compounds studied in organic chemistry. Ionic salts and minerals may contain carbon in some cases, but they are not the central starting point of introductory organic chemistry. The useful distinction is between studying carbon-based covalent structures and treating every carbon-containing substance as automatically belonging to the same type of chemistry.
2. A molecule written as \( \mathrm{CH_4} \) is included in the opening study of organic chemistry mainly because it shows carbon bonded to hydrogen by covalent bonds. What is the compound?
ⓐ. Sodium carbonate
ⓑ. Methane
ⓒ. Carbon dioxide
ⓓ. Calcium carbide
Correct Answer: Methane
Explanation: \( \mathrm{CH_4} \) is methane, the simplest hydrocarbon. It contains one carbon atom bonded to four hydrogen atoms. This makes it a useful first example for understanding organic compounds because it shows carbon forming covalent bonds. Sodium carbonate, carbon dioxide, and calcium carbide contain carbon, but they do not serve as the simple hydrocarbon model represented by \( \mathrm{CH_4} \). The formula \( \mathrm{CH_4} \) also prepares the idea that carbon can complete its valency by sharing electrons rather than by forming simple ions.
3. The formula \( \mathrm{C_2H_5OH} \) represents an organic compound because it contains a carbon skeleton and a characteristic \( \mathrm{-OH} \) group. Its usual name is:
ⓐ. Ethane
ⓑ. Ethene
ⓒ. Ethanoic acid
ⓓ. Ethanol
Correct Answer: Ethanol
Explanation: \( \mathrm{C_2H_5OH} \) is ethanol. The part \( \mathrm{C_2H_5} \) contains carbon and hydrogen, while the \( \mathrm{-OH} \) group shows that it belongs to the alcohol family. Ethane is \( \mathrm{C_2H_6} \), so it has no oxygen atom. Ethene contains a carbon-carbon double bond and has the formula \( \mathrm{C_2H_4} \). Ethanoic acid contains the \( \mathrm{-COOH} \) group, so it should not be confused with an alcohol just because both contain carbon, hydrogen, and oxygen.
4. A compound has the formula \( \mathrm{CH_3COOH} \). What does this formula show most clearly at the foundation level?
ⓐ. A covalent organic molecule containing carbon, hydrogen, and oxygen
ⓑ. It is a metal carbonate
ⓒ. It contains only carbon and hydrogen
ⓓ. It is an ionic compound made of \( \mathrm{CH_3^+} \) and \( \mathrm{COOH^-} \)
Correct Answer: A covalent organic molecule containing carbon, hydrogen, and oxygen
Explanation: \( \mathrm{CH_3COOH} \) is acetic acid, also called ethanoic acid in systematic naming. The formula contains carbon, hydrogen, and oxygen, and it is treated as an organic compound because the atoms are arranged in a covalent carbon-based structure. It is not a hydrocarbon because hydrocarbons contain only carbon and hydrogen. The formula should not be split into free ions \( \mathrm{CH_3^+} \) and \( \mathrm{COOH^-} \) just because it contains grouped parts. In organic formulas, grouped writing often shows connectivity and functional groups, not complete ionic separation.
5. A set of formulas is given below:
| Formula | Basic interpretation |
|---|
| P. \( \mathrm{CH_4} \) | One-carbon hydrocarbon |
| Q. \( \mathrm{C_2H_6} \) | Two-carbon hydrocarbon |
| R. \( \mathrm{C_2H_5OH} \) | Carbon compound containing oxygen |
| S. \( \mathrm{NaCl} \) | Carbon compound |
Which entry is not a suitable organic-chemistry foundation interpretation?
ⓐ. P
ⓑ. S
ⓒ. Q
ⓓ. R
Correct Answer: S
Explanation: \( \mathrm{NaCl} \) contains sodium and chlorine, but no carbon. It is an ionic salt, so calling it a carbon compound is not suitable. \( \mathrm{CH_4} \) and \( \mathrm{C_2H_6} \) are hydrocarbons because they contain only carbon and hydrogen. \( \mathrm{C_2H_5OH} \) is also an organic compound because it contains a carbon skeleton and oxygen in the alcohol group. This comparison helps separate organic examples from ordinary inorganic salts instead of judging by whether a formula simply looks familiar.
6. The term “molecular formula” for \( \mathrm{C_2H_6} \) tells us:
ⓐ. the exact three-dimensional shape of every bond
ⓑ. the complete order in which all atoms are connected
ⓒ. kinds and numbers of atoms in one molecule
ⓓ. the boiling point and melting point of the compound
Correct Answer: kinds and numbers of atoms in one molecule
Explanation: A molecular formula gives the actual number of atoms of each element in one molecule. For \( \mathrm{C_2H_6} \), it shows that the molecule contains \(2\) carbon atoms and \(6\) hydrogen atoms. It does not by itself show the complete arrangement of bonds between atoms. A structural formula is needed when the connectivity of atoms must be shown. This distinction becomes important later because different compounds can sometimes have the same molecular formula but different structures.
7. The formula \( \mathrm{C_2H_6} \) and a drawing showing \( \mathrm{H_3C-CH_3} \) do not give exactly the same kind of information. The drawing adds information about:
ⓐ. how the atoms are connected
ⓑ. the relative atomic masses of carbon and hydrogen
ⓒ. the colour of the compound
ⓓ. the percentage abundance of isotopes
Correct Answer: how the atoms are connected
Explanation: \( \mathrm{C_2H_6} \) tells the number of carbon and hydrogen atoms in ethane. A structural representation such as \( \mathrm{H_3C-CH_3} \) also indicates that the two carbon atoms are bonded to each other and the hydrogens are attached to carbon. The drawing therefore gives connectivity, not just composition. It does not give isotope abundance or physical appearance. Organic chemistry uses structural formulas frequently because many properties depend on atom arrangement, not only on the molecular formula.
8. A beginner says, “Every compound containing carbon must be treated in the same way as methane or ethanol.” The best correction is:
ⓐ. Carbon compounds are never organic
ⓑ. Bonding and structure matter in carbon compounds
ⓒ. Methane and ethanol are not carbon compounds
ⓓ. Only compounds obtained from plants contain carbon
Correct Answer: Bonding and structure matter in carbon compounds
Explanation: Methane and ethanol are typical organic compounds because they contain covalent carbon-based structures. However, not every carbon-containing substance is discussed in exactly the same way as ordinary organic molecules. Compounds such as carbonates, bicarbonates, and simple oxides of carbon are often treated separately in basic chemistry. The correction is not that carbon is absent from organic compounds, but that bonding pattern and structural behaviour are important. This avoids the over-simple idea that the mere presence of the symbol \( \mathrm{C} \) is enough to classify all carbon compounds identically.
9. Match the formula in Column I with the most suitable foundation description in Column II.
| Column I | Column II |
|---|
| P. \( \mathrm{CH_4} \) | 1. Organic acid example |
| Q. \( \mathrm{C_2H_6} \) | 2. Simplest hydrocarbon example |
| R. \( \mathrm{C_2H_5OH} \) | 3. Two-carbon hydrocarbon example |
| S. \( \mathrm{CH_3COOH} \) | 4. Alcohol example |
ⓐ. P-3, Q-2, R-4, S-1
ⓑ. P-2, Q-4, R-3, S-1
ⓒ. P-2, Q-3, R-4, S-1
ⓓ. P-1, Q-3, R-4, S-2
Correct Answer: P-2, Q-3, R-4, S-1
Explanation: \( \mathrm{CH_4} \) is methane, the simplest hydrocarbon. \( \mathrm{C_2H_6} \) is ethane, a two-carbon hydrocarbon. \( \mathrm{C_2H_5OH} \) is ethanol, so it is an alcohol example because of the \( \mathrm{-OH} \) group. \( \mathrm{CH_3COOH} \) is acetic acid, an organic acid example. The matching should be based on both the atoms present and the group written in the formula, not just on the number of carbon atoms.
10. In early organic chemistry, hydrocarbons are introduced before many functional derivatives. A hydrocarbon contains:
ⓐ. carbon, hydrogen, and oxygen only
ⓑ. any compound containing the symbol \( \mathrm{C} \)
ⓒ. only hydrogen and a metal
ⓓ. carbon and hydrogen only
Correct Answer: carbon and hydrogen only
Explanation: A hydrocarbon is a compound made only of carbon and hydrogen. Methane \( \mathrm{CH_4} \) and ethane \( \mathrm{C_2H_6} \) are simple hydrocarbons. Ethanol \( \mathrm{C_2H_5OH} \) is not a hydrocarbon because it also contains oxygen. Acetic acid \( \mathrm{CH_3COOH} \) is also not a hydrocarbon for the same reason. The word hydrocarbon should be read as “hydrogen plus carbon only,” not as any organic compound with a carbon chain.
11. Carbon is placed at the centre of organic chemistry because its atomic number and valence-shell arrangement allow extensive covalent bonding. What is the atomic number of carbon?
ⓐ. \(4\)
ⓑ. \(8\)
ⓒ. \(12\)
ⓓ. \(6\)
Correct Answer: \(6\)
Explanation: The atomic number of carbon is \(6\). This means a neutral carbon atom has \(6\) protons and \(6\) electrons. Its electronic configuration is \(1s^2 2s^2 2p^2\), giving \(4\) electrons in the outermost shell. These valence electrons are the basis for carbon forming four covalent bonds in many organic compounds. The number \(4\) is related to tetravalency, while \(6\) is the atomic number.
12. The electronic configuration of carbon is:
ⓐ. \(1s^2 2s^2 2p^2\)
ⓑ. \(1s^2 2s^2 2p^4\)
ⓒ. \(1s^2 2s^2 2p^6\)
ⓓ. \(1s^2 2s^1 2p^3\)
Correct Answer: \(1s^2 2s^2 2p^2\)
Explanation: Carbon has atomic number \(6\), so its \(6\) electrons are filled as \(1s^2 2s^2 2p^2\). The first shell holds \(2\) electrons, and the remaining \(4\) electrons occupy the second shell. This second shell is the valence shell for carbon. The configuration explains why carbon commonly shares electrons to complete an octet. Confusing carbon with oxygen would lead to \(1s^2 2s^2 2p^4\), which belongs to an atom with \(8\) electrons.
13. A carbon atom in methane forms four \( \mathrm{C-H} \) covalent bonds. This shows that carbon is:
ⓐ. monovalent
ⓑ. divalent
ⓒ. trivalent
ⓓ. tetravalent
Correct Answer: tetravalent
Explanation: Tetravalent means having valency \(4\). In methane, \( \mathrm{CH_4} \), carbon is bonded to four hydrogen atoms. Each \( \mathrm{C-H} \) bond is a covalent bond formed by sharing an electron pair. This satisfies carbon’s tendency to complete its octet through sharing. The prefix “tetra” refers to four bonds or valency units here, not to four carbon atoms.
14. A short note says: “Carbon forms strong \( \mathrm{C-C} \), \( \mathrm{C-H} \), \( \mathrm{C-O} \), \( \mathrm{C-N} \), and \( \mathrm{C-X} \) bonds in many organic compounds.” Here \( \mathrm{X} \) most commonly represents:
ⓐ. a halogen atom
ⓑ. a noble gas atom
ⓒ. a metal cation
ⓓ. a free electron
Correct Answer: a halogen atom
Explanation: In organic chemistry, \( \mathrm{X} \) is commonly used to represent a halogen such as fluorine, chlorine, bromine, or iodine. A \( \mathrm{C-X} \) bond therefore means a carbon-halogen bond. Such bonds are found in haloalkanes and many other organic halogen compounds. Noble gases do not commonly form ordinary carbon bonds in the basic organic structures studied here. The symbol \( \mathrm{X} \) is a group symbol in this context, not an unknown charge or electron.
15. Carbon usually forms covalent bonds in organic compounds instead of simple \( \mathrm{C^{4+}} \) or \( \mathrm{C^{4-}} \) ions because:
ⓐ. carbon has a completely filled outer shell
ⓑ. losing or gaining four electrons is difficult
ⓒ. carbon has no valence electrons
ⓓ. carbon can form only ionic compounds with hydrogen
Correct Answer: losing or gaining four electrons is difficult
Explanation: Carbon has four valence electrons, so forming \( \mathrm{C^{4+}} \) would require removal of four electrons, and forming \( \mathrm{C^{4-}} \) would require addition of four electrons. Both processes are energetically unfavorable for ordinary organic compounds. Instead, carbon completes its octet mainly by sharing electrons and forming covalent bonds. This explains why bonds such as \( \mathrm{C-H} \), \( \mathrm{C-C} \), \( \mathrm{C-O} \), and \( \mathrm{C-N} \) are common in organic molecules. The key point is that tetravalency is usually expressed through four shared electron-pair bonds, not through simple four-unit ionic charge formation.
16. The small size of carbon is important in organic chemistry because it helps carbon form:
ⓐ. weak and very long bonds only with metals
ⓑ. only single bonds and never multiple bonds
ⓒ. strong \( \mathrm{C-C} \) and carbon-nonmetal bonds
ⓓ. unstable ionic bonds with all elements
Correct Answer: strong \( \mathrm{C-C} \) and carbon-nonmetal bonds
Explanation: Carbon is a small second-period atom, so its valence orbitals can overlap effectively with orbitals of other small atoms. Good overlap leads to strong covalent bonds such as \( \mathrm{C-C} \), \( \mathrm{C-H} \), \( \mathrm{C-O} \), and \( \mathrm{C-N} \). This bond strength is one reason for the stability and variety of organic compounds. The small size of carbon does not mean that it can form only single bonds; carbon can also form double and triple bonds. The same size factor also supports catenation, where carbon atoms bond repeatedly to one another.
17. A note compares three properties of carbon:
| Property | Consequence in organic compounds |
|---|
| P. Four valence electrons | Can form four covalent bonds |
| Q. Small atomic size | Can form strong covalent bonds |
| R. Intermediate electronegativity | Can bond with both less and more electronegative elements |
What is the best conclusion from the table?
ⓐ. Carbon forms organic compounds only because it is a metal
ⓑ. carbon bonding depends on several atomic properties
ⓒ. Carbon cannot form bonds with oxygen or nitrogen
ⓓ. Carbon forms organic compounds only by gaining four electrons
Correct Answer: carbon bonding depends on several atomic properties
Explanation: The table links carbon’s organic chemistry to more than one atomic property. Four valence electrons explain tetravalency, small size helps strong covalent bonding, and intermediate electronegativity allows carbon to bond with several elements. Organic diversity is therefore not due to a single isolated fact. Carbon can bond with oxygen and nitrogen, as seen in compounds containing \( \mathrm{C-O} \), \( \mathrm{C=O} \), or \( \mathrm{C-N} \) bonds. The combined effect of valence, size, and electronegativity makes carbon unusually suitable for building many stable molecular structures.
18. In \( \mathrm{CH_4} \), carbon completes its octet by:
ⓐ. gaining four electrons completely from hydrogen atoms
ⓑ. losing four electrons completely to hydrogen atoms
ⓒ. forming a double bond with each hydrogen atom
ⓓ. sharing four electron pairs with four hydrogen atoms
Correct Answer: sharing four electron pairs with four hydrogen atoms
Explanation: Methane contains four \( \mathrm{C-H} \) covalent bonds. Each covalent bond represents a shared electron pair between carbon and hydrogen. Through these four shared pairs, carbon effectively has eight electrons in its valence shell. Hydrogen completes its duet through one shared pair with carbon. The octet of carbon in methane is achieved by sharing, not by complete transfer of electrons.
19. A carbon atom is attached to two oxygen atoms in \( \mathrm{CO_2} \), but it still has valency \(4\). This is because:
ⓐ. valency is always equal to the number of atoms attached
ⓑ. oxygen does not form covalent bonds with carbon
ⓒ. each \( \mathrm{C=O} \) double bond counts as two valencies
ⓓ. carbon becomes divalent whenever oxygen is present
Correct Answer: each \( \mathrm{C=O} \) double bond counts as two valencies
Explanation: Valency counts the combining capacity of an atom, not simply the number of atoms directly attached to it. In \( \mathrm{CO_2} \), carbon is bonded to two oxygen atoms, but each \( \mathrm{C=O} \) bond is a double bond. A double bond represents two shared electron pairs, so two double bonds use four valency units of carbon. This is why carbon remains tetravalent even though only two atoms are attached to it. The distinction between number of attached atoms and total bond order is essential in organic structures.
20. The formula \( \mathrm{C_2H_4} \) represents ethene. If the two carbon atoms are joined by a double bond, the bonding around the carbon skeleton is best described as:
ⓐ. one \( \mathrm{C-C} \) single bond and six \( \mathrm{C-H} \) bonds
ⓑ. two separate \( \mathrm{C-C} \) single bonds and four \( \mathrm{C-H} \) bonds
ⓒ. one \( \mathrm{C=C} \) double bond and four \( \mathrm{C-H} \) bonds
ⓓ. one \( \mathrm{C\equiv C} \) triple bond and two \( \mathrm{C-H} \) bonds
Correct Answer: one \( \mathrm{C=C} \) double bond and four \( \mathrm{C-H} \) bonds
Explanation: Ethene has the formula \( \mathrm{C_2H_4} \), so the two carbon atoms together are bonded to four hydrogen atoms. The remaining valency of each carbon is satisfied by a double bond between the two carbon atoms. Therefore, the carbon skeleton contains one \( \mathrm{C=C} \) bond and four \( \mathrm{C-H} \) bonds. The single-bond alternative describes ethane-like bonding and gives too many hydrogens for ethene. The formula and the bond order must agree with carbon’s valency of \(4\).
