Question

Select the correct alternative from the given choices. When one mole each of \(\mathrm{CO}\) and \(\mathrm{O}_{2}\) are made to react at STP, the total number of moles at the end of the reaction is (a) \(1.5\) moles (b) 1 mole (c) 4 moles (d) 2 moles

Short answer

Answer: 1.5 moles

Step-by-step solution

Write the balanced chemical equation for the reaction

For the reaction between carbon monoxide (CO) and oxygen gas (O2), the balanced chemical equation is: 2CO + O2 → 2CO2

Determine the limiting reactant

For every 2 moles of CO reacting, 1 mole of O2 is required. Since there is only 1 mole of CO and 1 mole of O2, the CO will be the limiting reactant. It means that not all the available O2 will react.

Stoichiometry calculations

Perform the stoichiometry calculations based on the balanced chemical equation and the limiting reactant CO: 1 mole of CO reacts with (1/2) mole of O2 to produce 1 mole of CO2.

Find the remaining moles of the unreacted compound

Since only (1/2) mole of O2 reacted, there is still (1/2) mole of O2 left unreacted after the reaction.

Calculate the total number of moles of all compounds at the end of the reaction

All of the CO is reacted, yielding 1 mole of CO2. There is still (1/2) mole of O2 remaining. The total number of moles at the end of the reaction will be the sum of CO2 and O2 moles: 1 + (1/2) = 1.5 moles The correct answer is choice (a) 1.5 moles.

Key concepts

Chemical Reactions

At the heart of chemistry lie chemical reactions, which are processes that involve the transformation of one or more substances into different substances. These reactions can be represented by chemical equations that show the reactants, the substances that undergo change, on the left side, and the products, the new substances formed, on the right side.

Understanding a chemical reaction includes grasping the concept of a mole, which is a unit that represents a very large number of atoms or molecules, specifically Avogadro's number (\(6.022 \times 10^{23}\) particles). It is essential in stoichiometry to use moles as a counting unit because it allows chemists to predict the amounts of each reactant needed and product formed in a reaction.

For example, in the reaction between carbon monoxide and oxygen gas, we saw that one mole of CO reacts with oxygen to form carbon dioxide, illustrating the changes these substances undergo during the reaction.

Limiting Reactant

In any chemical reaction, the limiting reactant is the substance that runs out first and thus determines the amount of product that can be formed. It is the reactant that is present in the smallest stoichiometric amount, based on the balanced equation and the initial quantities of reactants.

In the given exercise, carbon monoxide (CO) and oxygen gas (\(O_2\)) were the reactants. By comparing the mole ratio of the reactants to those required by the balanced chemical equation, we can identify the limiting reactant. Since one mole of CO was available while the equation indicated that two moles are needed to fully react with one mole of oxygen, CO was the limiting reactant. This concept is particularly important because it allows us to calculate the exact amount of product(s) that will be formed and determine the reactants' quantities left unreacted, if any.

Balanced Chemical Equation

A balanced chemical equation is key to stoichiometry calculations because it reflects the conservation of mass, indicating that the number of atoms of each element is the same on both sides of the equation. Each chemical equation must be balanced, which means it obeys the law of conservation of mass.

For example, the reaction between CO and \(O_2\) given by the equation 2CO + \(O_2 \to 2CO_2\) shows a balanced process where two molecules of carbon monoxide react with one molecule of oxygen gas to produce two molecules of carbon dioxide. Regardless of the quantities of reactants we start with, the balanced equation guides us in determining the stoichiometric relationships and is essential when performing stoichiometry calculations to predict the amount of each substance involved in the reaction.