--- title: "Leaving Cert Chemistry — Example Quiz" tags: [chemistry, leaving-cert, science, second-level] time_limit: 30 allow_overrun: true random_order: false --- ##### Question q001 type: single_choice correct: [b] An atom of an element has 11 protons, 12 neutrons and 11 electrons. What is its **mass number**? A. 11 B. 23 C. 22 D. 34 ###### Explanation Mass number is protons plus neutrons: 11 + 12 = 23. Electrons are not counted, as their mass is negligible. The *atomic* number is 11, which identifies the element as sodium. --- ##### Question q002 type: single_choice correct: [c] Which type of bonding occurs between a metal and a non-metal? A. Covalent B. Metallic C. Ionic D. Hydrogen ###### Explanation A metal loses electrons to form a positive ion and a non-metal gains them to form a negative ion; the resulting electrostatic attraction is an ionic bond. Covalent bonding involves shared electrons between non-metals, and metallic bonding occurs between metal atoms. --- ##### Question q003 type: multi_choice correct: [b, c] Which TWO of the following are properties typical of **ionic compounds**? (Select two.) A. Low melting point B. Conduct electricity when molten C. Generally soluble in water D. Conduct electricity when solid ###### Explanation Ionic compounds conduct only when the ions are free to move — molten or in solution, not in the solid lattice. They are generally water-soluble and have **high** melting points because of the strong electrostatic forces throughout the lattice. --- ##### Question q004 type: single_choice correct: [c] How many moles are present in 36 g of water? (Relative molecular mass of H₂O = 18) A. 0.5 mol B. 1 mol C. 2 mol D. 18 mol ###### Explanation Moles = mass ÷ relative molecular mass = 36 ÷ 18 = 2 mol. Each mole of water contains 6.02 × 10²³ molecules, so this sample contains roughly 1.2 × 10²⁴ molecules. --- ##### Question q005 type: true_false correct: [a] Going down a group in the periodic table, atomic radius increases. ###### Explanation True. Each successive element down a group has an additional occupied energy level, so the outer electrons sit further from the nucleus. Increased shielding by inner electrons also reduces the effective nuclear pull on them. --- ##### Question q006 type: single_choice correct: [b] A solution has a pH of 3. It is best described as: A. Weakly basic B. Acidic C. Neutral D. Strongly basic ###### Explanation pH below 7 is acidic, and 3 is strongly so. The scale is logarithmic, so pH 3 has ten times the hydrogen ion concentration of pH 4 and a hundred times that of pH 5. --- ##### Question q007 type: multi_choice correct: [a, c, d] Which THREE of the following will increase the **rate** of a chemical reaction? (Select three.) A. Increasing the temperature B. Decreasing the surface area of a solid reactant C. Adding a catalyst D. Increasing the concentration of a reactant ###### Explanation Higher temperature gives particles more kinetic energy, so collisions are more frequent and more of them exceed the activation energy. A catalyst lowers activation energy. Higher concentration means more frequent collisions. **Decreasing** surface area slows a reaction down — increasing it speeds it up. --- ##### Question q008 type: single_choice correct: [d] What is the general formula of an **alkane**? A. CₙH₂ₙ B. CₙH₂ₙ₋₂ C. CₙHₙ D. CₙH₂ₙ₊₂ ###### Explanation Alkanes are saturated hydrocarbons with the formula CₙH₂ₙ₊₂ — methane is CH₄, ethane C₂H₆. CₙH₂ₙ is the alkene general formula, and CₙH₂ₙ₋₂ is that of the alkynes. --- ##### Question q009 type: single_choice correct: [b] Which particle determines the **identity** of an element? A. Neutron B. Proton C. Electron D. Positron ###### Explanation The number of protons is the atomic number, and it defines the element. Changing the neutron count gives an isotope of the same element; changing the electron count gives an ion of the same element. Change the proton count and it is a different element entirely. --- ##### Question q010 type: sequence correct: [c, a, d, b] Place the following steps of a **volumetric analysis** (titration) in the correct order. A. Fill the burette with the solution of known concentration B. Record the burette reading at the end point C. Rinse the pipette with the solution it will contain D. Add indicator to the conical flask and titrate ###### Explanation Apparatus is rinsed with the solution it will hold, so residual water cannot dilute it. The burette is then filled with the standard solution. Indicator is added to the measured aliquot in the conical flask, the titration is carried out, and the reading at the end point is recorded. --- ##### Question q011 type: single_choice correct: [a] What is the oxidation number of hydrogen in most of its compounds? A. +1 B. −1 C. 0 D. +2 ###### Explanation Hydrogen is +1 in most compounds. The exception is in metal hydrides such as sodium hydride (NaH), where the more electronegative hydrogen takes −1. Uncombined H₂ has an oxidation number of 0. --- ##### Question q012 type: true_false correct: [b] A catalyst increases the yield of product in a reversible reaction at equilibrium. ###### Explanation False. A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached **sooner** but its position is unchanged — the yield is the same. Only changes in concentration, pressure or temperature shift the position of equilibrium. --- ##### Question q013 type: single_choice correct: [c] Which of the following is an example of an **exothermic** process? A. Melting ice B. Photosynthesis C. Combustion of methane D. Thermal decomposition of limestone ###### Explanation Combustion releases heat to the surroundings, so it is exothermic. Melting, photosynthesis and thermal decomposition all absorb energy and are endothermic. --- ##### Question q014 type: single_choice correct: [b] Isotopes of the same element differ in their number of: A. Protons B. Neutrons C. Electrons D. Protons and electrons ###### Explanation Isotopes have the same atomic number but different mass numbers, which means the same protons and a different number of neutrons. Chlorine-35 and chlorine-37 both have 17 protons; they differ by two neutrons. --- ##### Question q015 type: single_choice correct: [d] Which gas is produced when a dilute acid reacts with a **metal carbonate**? A. Hydrogen B. Oxygen C. Chlorine D. Carbon dioxide ###### Explanation Acid plus carbonate gives a salt, water and carbon dioxide, which turns limewater milky. Hydrogen is the gas produced when a dilute acid reacts with a reactive **metal**, which is the distinction being tested here. --- ##### Question q016 type: multi_choice correct: [a, d] Which TWO of the following are **noble gases**? (Select two.) A. Argon B. Chlorine C. Nitrogen D. Neon ###### Explanation Argon and neon are in group 0 (group 18), with full outer energy levels, which is why they are so unreactive. Chlorine is a halogen in group 7, and nitrogen is in group 5. --- ##### Question q017 type: single_choice correct: [b] In the electrolysis of molten sodium chloride, which product forms at the **cathode**? A. Chlorine gas B. Sodium metal C. Hydrogen gas D. Oxygen gas ###### Explanation The cathode is negative, so it attracts positive ions. Sodium ions are reduced there by gaining an electron to form sodium metal. Chloride ions are oxidised at the anode to give chlorine gas. --- ##### Question q018 type: true_false correct: [a] The relative atomic mass of chlorine is 35.5 because naturally occurring chlorine is a mixture of isotopes. ###### Explanation True. Chlorine occurs as roughly 75% chlorine-35 and 25% chlorine-37. The relative atomic mass is the weighted average of the isotopic masses, which lands at about 35.5 — no individual chlorine atom has that mass. --- ##### Question q019 type: single_choice correct: [c] Which functional group characterises an **alcohol**? A. −COOH B. −CHO C. −OH D. −NH₂ ###### Explanation The hydroxyl group −OH defines an alcohol, as in ethanol (C₂H₅OH). −COOH is a carboxylic acid, −CHO an aldehyde, and −NH₂ an amine. --- ##### Question q020 type: single_choice correct: [b] What colour does litmus turn in a basic solution? A. Red B. Blue C. Green D. Colourless ###### Explanation Litmus is red in acid and blue in base. A useful memory aid is that both "base" and "blue" begin with B. Universal indicator gives a fuller range, running through green at neutral to purple in strong base. ---