UPCAT Chemistry — Stoichiometry & Chemical ReactionsFlash Cards

What is stoichiometry?

Stoichiometry is the study of the quantitative relationships between chemical formulas and chemical equations. It allows us to calculate the amounts of reactants needed or products formed in chemical reactions using mole ratios from balanced equations.

Balance this synthesis reaction: Zn + I₂ → ZnI₂

Zn + I₂ → ZnI₂ This equation is already balanced. Each side has 1 Zn atom and 2 I atoms. This is a synthesis (combination) reaction where two elements combine to form one compound.

Calculate the number of moles in 36g of water (H₂O). Molar mass of H₂O = 18 g/mol

Formula: n = mass/molar mass Step 1: n = 36g ÷ 18 g/mol Step 2: n = 2 mol Answer: 36g of water contains 2 moles

What type of reaction is: 2Al₂O₃ → 4Al + 3O₂?

This is a decomposition reaction. Pattern: AB → A + B. One compound (aluminum oxide) breaks down into two or more simpler substances (aluminum metal and oxygen gas). Decomposition reactions often require heat or electricity.

Calculate the molar mass of calcium carbonate (CaCO₃)

Molar mass = sum of atomic masses Ca: 40.08 g/mol × 1 = 40.08 g/mol C: 12.01 g/mol × 1 = 12.01 g/mol O: 16.00 g/mol × 3 = 48.00 g/mol Total molar mass = 40.08 + 12.01 + 48.00 = 100.09 g/mol

Balance this combustion reaction: C₆H₆ + O₂ → CO₂ + H₂O

2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O Steps: Balance C first (6×2=12), then H (6×2=6), finally O (24+6=30, so 15 O₂). Combustion reactions produce CO₂ and H₂O from hydrocarbons.

What is Avogadro's number and what does it represent?

Avogadro's number is 6.022 × 10²³. It represents the number of particles (atoms, molecules, ions, or formula units) in exactly one mole of any substance. Named after Amadeo Avogadro, it's the bridge between atomic scale and macroscopic measurements.

Identify the reaction type: Br₂ + 2KI → 2KBr + I₂

This is a single replacement (single displacement) reaction. Pattern: AB + X → AX + B. Bromine (more reactive) displaces iodine from potassium iodide. The activity series determines which elements can replace others.

Calculate the number of molecules in 0.5 moles of CO₂

Formula: Number of molecules = moles × Avogadro's number Step 1: Molecules = 0.5 mol × 6.022 × 10²³ molecules/mol Step 2: Molecules = 3.011 × 10²³ molecules Answer: 0.5 moles of CO₂ contains 3.011 × 10²³ molecules

What is the Law of Conservation of Mass?

The Law of Conservation of Mass states that mass cannot be created or destroyed in chemical reactions. The total mass of reactants equals the total mass of products. This principle is why chemical equations must be balanced - the same number of each type of atom must appear on both sides.

Balance this double displacement reaction: K₂Cr₂O₇ + Ca(OH)₂ → CaCr₂O₇ + KOH

K₂Cr₂O₇ + Ca(OH)₂ → CaCr₂O₇ + 2KOH Double displacement pattern: AB + XY → AY + XB. K⁺ and Ca²⁺ ions switch places. Balance by ensuring equal numbers of each ion on both sides.

Calculate the mass of 2.5 moles of sodium chloride (NaCl). Molar mass = 58.5 g/mol

Formula: mass = moles × molar mass Step 1: mass = 2.5 mol × 58.5 g/mol Step 2: mass = 146.25 g Answer: 2.5 moles of NaCl has a mass of 146.25 g

What is the difference between theoretical yield and actual yield?

Theoretical yield is the maximum amount of product that can be formed based on stoichiometric calculations (assumes 100% reaction). Actual yield is the amount of product actually obtained in the laboratory. Percent yield = (actual/theoretical) × 100%. Actual yield is usually less than theoretical due to side reactions, incomplete reactions, or product loss.

Balance this acid-base reaction: Sr(OH)₂ + H₂SO₄ → SrSO₄ + H₂O

Sr(OH)₂ + H₂SO₄ → SrSO₄ + 2H₂O Acid-base pattern: Acid + Base → Salt + Water. Balance OH⁻ and H⁺ ions to form water molecules. Each Sr(OH)₂ provides 2 OH⁻, so 2 H₂O molecules are formed.

Calculate the percent composition of oxygen in water (H₂O). Molar mass = 18 g/mol

Formula: % element = (mass of element/molar mass of compound) × 100% Mass of O in H₂O = 16.00 g/mol Step 1: % O = (16.00/18.00) × 100% Step 2: % O = 88.89% Answer: Oxygen makes up 88.89% of water's mass

What is a limiting reactant?

A limiting reactant (or limiting reagent) is the reactant that is completely consumed first in a chemical reaction, thus limiting the amount of product that can be formed. The other reactant(s) are in excess. To find the limiting reactant, calculate moles of each reactant and compare their mole ratios to the balanced equation.

In the reaction 2H₂ + O₂ → 2H₂O, how many moles of water can be produced from 3 moles of H₂?

Using mole ratio from balanced equation: 2 mol H₂ : 2 mol H₂O = 1:1 ratio Step 1: 3 mol H₂ × (2 mol H₂O/2 mol H₂) Step 2: 3 mol H₂ × 1 = 3 mol H₂O Answer: 3 moles of water can be produced

What is molarity and how is it calculated?

Molarity (M) is a measure of solution concentration defined as moles of solute per litre of solution. Formula: M = moles of solute / liters of solution Units: mol/L or M Example: 0.5 mol NaCl in 2 L solution = 0.25 M NaCl solution

Balance this equation and identify the reaction type: CH₄ + O₂ → CO₂ + H₂O

CH₄ + 2O₂ → CO₂ + 2H₂O This is a combustion reaction. Methane (hydrocarbon) reacts with oxygen to produce carbon dioxide and water. All combustion reactions are exothermic and follow the pattern: Hydrocarbon + O₂ → CO₂ + H₂O

Calculate the number of atoms in 0.25 moles of aluminum (Al)

Formula: Number of atoms = moles × Avogadro's number Step 1: Atoms = 0.25 mol × 6.022 × 10²³ atoms/mol Step 2: Atoms = 1.506 × 10²³ atoms Answer: 0.25 moles of aluminum contains 1.506 × 10²³ atoms