Chapter 15, Problem 16E

In equilibrium problems involving equilibrium constants that are small to the initial concentrations of reactants, we often assume that the quantity x (which represents how far the reaction proceeds toward products) is small. When this assumption is made, we can ignore the quantity x when it is subtracted from a large number but not when it is multiplied by a large number. In other words, $\text{2}.\text{5}–\text{x}\approx \text{2}.\text{5}$ , but $\text{2}.\text{5x}\ne \text{2}.\text{5}$ . Explain why we can a small x in the first case but not in the second.