Final answer:
To find the mass of oxygen needed to react with 18g of carbon to produce 72g of carbon dioxide, subtract the mass of carbon from the mass of CO₂. The calculation shows that 54g of oxygen is required for the reaction.
Step-by-step explanation:
In the reaction C + O₂ = CO₂, 18g of carbon react with oxygen to produce 72g of carbon dioxide. To determine the mass of oxygen needed in the reaction, we use the law of conservation of mass which states that matter is neither created nor destroyed in a chemical reaction. Therefore, the mass of the reactants must equal the mass of the products. Starting with 18g of carbon and 72g of carbon dioxide produced, let's calculate the mass of oxygen that has combined with carbon.
The molar mass of carbon (C) is approximately 12 g/mol, and the molar mass of carbon dioxide (CO₂) is approximately 44 g/mol. Using the stoichiometry of the balanced equation, for every 1 mole of carbon that reacts, 1 mole of CO₂ is produced. Thus, 18g of carbon equals 18g / 12g/mol = 1.5 moles of carbon, which would produce 1.5 moles of CO₂. The mass of 1.5 moles of CO₂ is 1.5 moles * 44 g/mol = 66g of CO₂.
Since the total mass of CO₂ produced was 72g, the mass of oxygen that combined with carbon can be found by subtracting the mass of carbon from the mass of CO₂ (72g - 18g = 54g). Therefore, 54g of oxygen was needed to react with 18g of carbon to produce 72g of CO₂.