Post-UTME Chemistry
Past Questions

AI Explanation

**The reasoning** The **atomic number** is the number of protons in an atom's nucleus — and this is what defines an element. Carbon, by definition, is the element with **6 protons**. Therefore, carbon's atomic number is **6**. This never changes. Whether it's carbon in your pencil, in a diamond, or in your body, it always has 6 protons. That's what makes it carbon! **Why the wrong options tempt you** - **A) 4** — You might confuse this with beryllium's atomic number or carbon's common valency (it forms 4 bonds). - **C) 8** — This is oxygen's atomic number. Easy to mix up if you're rushing through the periodic table. - **D) 12** — This is carbon's most common **mass number** (6 protons + 6 neutrons = 12). Many students confuse atomic number with mass number! **Quick takeaway** Atomic number = number of protons = the element's identity; carbon will *always* be 6 because it always has 6 protons. Mass number includes neutrons too, so don't mix them up!

AI Explanation

**The reasoning** Methane is the simplest **hydrocarbon** — a compound made of only carbon (C) and hydrogen (H). The name itself gives you a clue: "meth-" means one carbon atom, and "-ane" tells you it's a saturated hydrocarbon (all single bonds). Carbon has 4 valence electrons and needs 4 more to be stable. Each hydrogen has 1 electron and needs 1 more. So one carbon atom bonds with **four hydrogen atoms**: CH₄. This tetrahedral molecule is what we burn in cooking gas! **Why the wrong options tempt you** - **CO₂** is carbon dioxide — what you breathe out. It has carbon, but paired with oxygen, not hydrogen. - **H₂O** is water — hydrogen is there, but with oxygen instead of carbon. - **NH₃** is ammonia — contains hydrogen, but the central atom is nitrogen, not carbon. All these contain hydrogen or carbon, so they might look familiar if you're rushing! **Quick takeaway** Remember: **Meth = 1 carbon, -ane = hydrocarbon, so methane = CH₄** — one carbon holding hands with four hydrogens, like cooking gas in your kitchen.

AI Explanation

## The reasoning Common salt is the everyday substance we sprinkle on our jollof rice and use to season our soup! Its chemical name is **sodium chloride**, and its formula is **NaCl** — one sodium atom (Na) bonded to one chlorine atom (Cl). This compound forms when sodium (a soft, reactive metal) transfers one electron to chlorine (a greenish gas that needs one electron). They create an **ionic bond**, producing the white crystals we know as table salt. ## Why the wrong options tempt you **KCl** (potassium chloride) — This looks similar because potassium is in the same group as sodium. It's actually used as a salt substitute, but it's not our everyday salt. **CaCl₂** (calcium chloride) — The "Cl" might confuse you, but calcium needs to bond with *two* chlorine atoms. This is used for drying things, not seasoning food. **MgCl₂** (magnesium chloride) — Another compound with chlorine, but magnesium also bonds with two chlorine atoms. Not table salt! ## Quick takeaway **Common salt = sodium chloride = NaCl** — remember "Na" for sodium (from its Latin name *Natrium*) plus one chlorine makes the salt on your table!

AI Explanation

**The reasoning** Noble gases are elements in **Group 18** (the last column) of the periodic table. They have a full outer electron shell, which makes them extremely stable and unreactive — they rarely form compounds with other elements. The noble gases are: Helium (He), Neon (Ne), **Argon (Ar)**, Krypton (Kr), Xenon (Xe), and Radon (Rn). Argon is the third noble gas. It has 18 electrons with a complete outer shell (2, 8, 8 configuration), so it doesn't need to gain or lose electrons. That's why it's chemically inert. **Why the wrong options tempt you** - **Oxygen (O₂)** — Very common gas in air (~21%), so students think "common gas = noble gas." But oxygen is highly reactive, not noble! - **Hydrogen (H₂)** — Lightest element, also a gas. But it's extremely reactive (burns easily), opposite of noble. - **Chlorine (Cl₂)** — A gas at room temperature, but it's a halogen (Group 17), very reactive and dangerous. **Quick takeaway** Noble gases are the "loners" of chemistry — they sit in Group 18, have full electron shells, and refuse to react with almost anything; memorize: He, Ne, Ar, Kr, Xe, Rn.

AI Explanation

**The reasoning** Alcohols are organic compounds where a hydroxyl group (—OH) is directly attached to a carbon atom. Think of common alcohols you know: ethanol (the type in drinks), methanol (in methylated spirit), propanol. They all have that —OH group bonded to a carbon chain. This —OH is what makes them alcohols and gives them their characteristic properties — like being able to dissolve in water and having those boiling points you studied. **Why the wrong options tempt you** - **—COOH** looks similar and also contains OH, but this is a *carboxylic acid* group (like in vinegar/ethanoic acid). The C=O makes it acidic, not alcoholic. - **—CHO** is an *aldehyde* group (like in methanal/formaldehyde). Yes, it has oxygen, but no OH. - **—NH₂** is an *amine* group (contains nitrogen, not the oxygen you need for alcohols). The trap? All these groups contain common elements, but each defines a completely different family of organic compounds. **Quick takeaway** **Alcohol = —OH attached to carbon.** Remember: "OH" literally sounds like alcohol's effect — "Ohh!" 😄