Calculate the mass of $\text{CO}_2$ produced when $116$ g of $n$-butane ($\text{C}_4\text{H}_{10}$, molar mass $58$ g/mol) is completely combusted in excess oxygen.

Stoichiometry is the quantitative study of the relationships between reactants and products in chemical reactions. It is entirely based on the law of conservation of mass (matter is neither created nor destroyed in a chemical reaction) and the mole concept. The mole (mol) is the SI unit for amount of substance, defined as the amount of substance containing exactly 6.022 × 10^23 (Avogadro's number, N_A) elementary entities. One mole of any substance has a mass equal to its molar mass (molecular weight) in grams: 1 mol C = 12.011 g, 1 mol H = 1.008 g, 1 mol O = 15.999 g, 1 mol CO2 = 44.01 g, 1 mol H2O = 18.015 g. The fundamental stoichiometry calculation: (1) Write and balance the chemical equation. (2) Convert given masses to moles: n = m/M_r. (3) Use mole ratios from balanced equation to find moles of desired substances. (4) Convert moles to masses (or volumes for gases at STP: 1 mol gas at STP = 22.4 L). The coefficients in a balanced chemical equation represent the exact mole ratios in which reactants combine and products form. For combustion of n-butane: C4H10 + 13/2 O2 → 4CO2 + 5H2O means that 1 mol C4H10 reacts with 6.5 mol O2 to produce 4 mol CO2 and 5 mol H2O. If we have 2 mol C4H10: 2 × 4 = 8 mol CO2 are produced, with mass = 8 × 44.01 = 352 g.

In real chemical reactions, reactants are rarely provided in exact stoichiometric ratios — one reactant is usually present in excess and another is the limiting reagent (or limiting reactant) that is completely consumed first and determines the maximum amount of product that can form. Identifying the limiting reagent: (1) Calculate moles of each reactant. (2) Divide by the stoichiometric coefficient of each reactant. (3) The reactant with the smallest ratio is the limiting reagent — it is "used up" first. Alternatively: assume each reactant is limiting in turn, calculate moles of product formed, and the reaction that gives the LEAST product identifies the actual limiting reagent. Example: if 116 g n-butane (2 mol) reacts with 208 g O2 (6.5 mol), the stoichiometry requires 2 mol C4H10 × 6.5 = 13 mol O2. But only 6.5 mol O2 is available → O2 is limiting. Moles C4H10 consumed = 6.5/6.5 = 1 mol → only 1 mol × 4 = 4 mol CO2 = 176 g produced (not 352 g). Theoretical yield: the maximum amount of product calculated from stoichiometry assuming complete reaction of the limiting reagent. Actual yield: the amount of product actually obtained experimentally. Percentage yield = (actual yield / theoretical yield) × 100%. Percentage yield < 100% due to: reaction not going to completion, side reactions, product lost during isolation/purification.

Combustion analysis is an experimental technique for determining the elemental composition of organic compounds by combustion and measurement of the CO2 and H2O produced. Procedure: a known mass of the organic compound is burned in excess O2 in a combustion tube. The gases pass through: (1) MgClO4 (magnesium perchlorate, anhydrous) or P4O10 to absorb H2O. (2) NaOH or ascarite to absorb CO2. The increase in mass of each absorber gives the mass of H2O and CO2 produced. Calculation: mass of CO2 → moles CO2 → moles C (1 mol CO2 = 1 mol C). Mass of H2O → moles H2O → moles H (1 mol H2O = 2 mol H). If the compound also contains O: mass of O = mass of compound - mass of C - mass of H. Empirical formula: ratio of C:H:O in smallest whole numbers. Molecular formula: integer multiple of empirical formula, determined by measuring the molar mass (by mass spectrometry or colligative property methods). This technique was pioneered by Justus von Liebig (1831) and remains the primary method for determining organic compound composition, now usually coupled with mass spectrometry.

Hydrocarbons are organic compounds containing only carbon and hydrogen. Classification: Alkanes (saturated hydrocarbons, CnH2n+2): single bonds only, sp3 hybridisation, tetrahedral geometry. Methane (CH4), ethane (C2H6), propane (C3H8), n-butane (C4H10), isobutane/2-methylpropane (C4H10), pentane/isopentane/neopentane (C5H12), hexane (C6H14), heptane (C7H16), octane (C8H18). Properties: non-polar, immiscible with water, insoluble in water, soluble in organic solvents, increasing bp and mp with increasing chain length, branched isomers have lower bp than straight chain (less surface area = weaker London forces). Reactions: free radical halogenation (with Cl2 or Br2 in light/heat), combustion. Alkenes (CnH2n): one C=C, sp2 hybridisation, planar around double bond. Show addition reactions (addition of H2, HX, X2, HOX, H2O across double bond) and some substitution (allylic). Alkynes (CnH2n-2): one C≡C, sp hybridisation, linear around triple bond. More reactive than alkenes, can add two equivalents of reagent. Terminal alkynes (R-C≡C-H) have acidic H (pKa ~25), can form acetylide anions with strong bases. Aromatic compounds (CnHn for benzene and derivatives): contain benzene ring, undergo electrophilic aromatic substitution (EAS) rather than addition, maintaining aromaticity.

Petroleum (crude oil) is a complex mixture of hydrocarbons — primarily alkanes, cycloalkanes (naphthenes), and aromatics — formed by the anaerobic decomposition of marine organisms over millions of years under heat and pressure. The composition of crude oil varies widely by source: North Sea crude is relatively light (high-API gravity, more valuable); Venezuelan heavy crude is viscous and sulfur-rich; Arabian light crude has a well-balanced composition. Petroleum refining involves: Fractional distillation: crude oil is heated to ~400°C and fed into a distillation column, where different fractions condense at different heights based on boiling point range. Fractions (from bottom to top, decreasing bp): heavy fuel oil/residue (>370°C), lubricating oils/waxes (300-370°C), diesel/gas oil (200-300°C), kerosene/jet fuel (150-200°C), naphtha/gasoline (bp 40-180°C), liquefied petroleum gas LPG (propane/butane, bp <40°C), gases (methane, ethane, propylene, butylenes). Cracking: large, high-boiling hydrocarbons (C12-C70) are broken into smaller, more valuable fuel-range molecules (C5-C10). Thermal cracking: high temperature (450-750°C) and pressure, free radical mechanism, gives mixture of products. Catalytic cracking (FCC, fluid catalytic cracking): zeolite catalysts at 450-520°C, carbocation mechanism, gives higher quality gasoline with more branched and aromatic structures. Reforming: naphtha (C7-C10 alkanes) is converted to aromatic compounds (benzene, toluene, xylenes — BTX, valuable chemicals and octane boosters) over Pt/Re/Al2O3 catalysts.

The combustion of hydrocarbon fuels (natural gas, petroleum, coal) releases CO2, the primary anthropogenic greenhouse gas responsible for climate change. Complete combustion: C + O2 → CO2, CxHy + O2 → CO2 + H2O. Carbon dioxide as a greenhouse gas: CO2 absorbs infrared radiation emitted by the Earth's surface (in the 14-16 micron wavelength range, corresponding to the bending and stretching vibrations of the C=O bonds) and re-emits it in all directions, including back toward the Earth. This reduces the rate of cooling of the Earth and elevates the equilibrium surface temperature. The current atmospheric CO2 concentration (425 ppm in 2024) is approximately 50% higher than the pre-industrial level (280 ppm), primarily due to burning of fossil fuels and deforestation. Global average surface temperature has increased by approximately 1.1-1.2°C since the pre-industrial period, with well-documented effects: rising sea levels (thermal expansion + ice melt), more frequent and intense heat waves, altered precipitation patterns, ocean acidification (CO2 + H2O ⇌ H2CO3 → H+ + HCO3- → marine organisms with calcium carbonate shells face dissolution). Solutions: renewable energy (solar, wind, hydroelectric, nuclear), carbon capture and storage (CCS), improved energy efficiency, electrification of transport, sustainable agriculture, reforestation. The Paris Agreement (2015) set the goal of limiting warming to 1.5-2.0°C above pre-industrial levels, requiring net-zero CO2 emissions by approximately 2050 for the 1.5°C target.