Dissociation Constant

[Jeannie Kennedy 3F]

Can someone help explain why we can approximate x when Ka is less than 10^-4?

[Gabrielle Arreglo 3F]

When Ka is less than 10^-4, that means there is very little reactant being produced and so very little change in reactant/product, making the x very small. This x will be small compared to the concentration, so you can just ignore the x in the terms that have (concentration +- x). For example, let's say the concentration of oxygen is 0.5 and it increases by x, which is 1*10^-5. 0.5 is basically the same is 0.50001 so you can ignore the x and just use 0.5 as the equilibrium concentration of oxygen during your calculations.