Consider the following equilibrium process at \(700^{\circ} \mathrm{C}:\) $$2 \mathrm{H}_{2}(g)+\mathrm{S}_{2}(g) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{~S}(g)$$ Analysis shows that there are 2.50 moles of \(\mathrm{H}_{2}\), \(1.35 \times 10^{-5}\) mole of \(\mathrm{S}_{2}\), and 8.70 moles of \(\mathrm{H}_{2} \mathrm{~S}\) present in a 12.0-L flask. Calculate the equilibrium constant \(K_{\mathrm{c}}\) for the reaction.

Chemical equilibrium is a fundamental concept in chemistry, representing a state in which the rates of the forward and reverse reactions are equal, leading to no net change in the concentrations of reactants and products over time. It's important to note that reaching equilibrium doesn't mean the reactants and products are present in equal amounts, but rather that their ratios are constant.

When a chemical system is at equilibrium, it obeys the law of mass action which states that at a given temperature, a chemical system may reach a state where a certain ratio of reactant and product concentrations has a constant value, known as the equilibrium constant \( K_c \). This constant helps chemists understand the extent of a reaction—whether it favors the formation of products, is balanced, or favors the reactants. Learning how to calculate \( K_c \) offers insight into the reaction's behavior under specific conditions, which is crucial for fields such as pharmaceuticals, environmental science, and industrial chemistry.

Reaction stoichiometry involves the quantitative relationships between the amounts of reactants used and products formed in a chemical reaction. It's derived from the balanced chemical equation, providing a clear ratio of how many moles of each substance react or are produced.

To work with stoichiometry in an equilibrium scenario, you should ensure that the balanced equation is correctly interpreted: coefficients become exponents in the equilibrium expression, and each substance's molarity is raised to the power of the respective coefficient. Grasping this concept is imperative as it forms a critical link between the balanced chemical equation and the calculation of the equilibrium constant \( K_c \). For instance, in the reaction \( 2 H_2(g) + S_2(g) \rightleftharpoons 2 H_2S(g) \) the stoichiometry dictates that two moles of hydrogen react with one mole of sulfur to produce two moles of hydrogen sulfide.

Molarity concentration, commonly referred to as molarity, is a measure of the concentration of a solute in a solution. It is expressed as the number of moles of solute per liter of solution (mol/L or M). In the context of equilibrium, understanding molarity is critical because the equilibrium constant \( K_c \) is calculated using the molarities of the reactants and products.

To find molarity, you divide the number of moles of the substance by the volume of the solution it's in. For instance, if you have a 12.0-L container with 2.50 moles of \( H_2 \), the molarity would be \( \frac{2.50 \text{ moles}}{12.0 \text{ L}} = 0.208 \text{ M} \). It is essential for students to understand that this concentration means there's 0.208 moles of \( H_2 \) for every 1 liter of space in the flask, and such precise measurements determine the equilibrium state of a chemical reaction.