Hydrogen chloride, HCl, can be manufactured commercially by reacting H₂ and Cl₂ in a graphite-lined furnace. Suppose you are a chemical engineer working with this reaction. You will need to know the expected equilibrium composition in order to control the reaction. In an equilibrium mixture of HCl, Cl₂, and H₂, the partial pressure of H₂ is 4.2 mPa and that of Cl₂ is 8.3 mPa. What is the partial pressure of HCl at 500. K in bar, given K = 4.0 * 10¹⁸ for H₂(g) + Cl₂(g) ⇌ 2 HCl(g)?

ANTICIPATE Because the equilibrium constant is large, you should expect a high equilibrium partial pressure for the product, HCl.

PLAN At equilibrium, the partial pressures of the reactants and products (in bar) satisfy the expression for K. Therefore, rearrange the expression to give the one unknown concentration and substitute the data.