Hw 12.55

[904940852]

Why is it than in a) the x^2/(.2-x)=ka becomes (.2)ka=x^2 but in b) x^2/(.2-x)=ka becomes
(.2-x)ka=x^2

In one case, the x in (.2-x) is assumed to be zero and in the other, it’s assumed to have a value. Why is that?

[Yang Chen 2E]

I think it has to do with the first being a weak acid, but the second one still counts as a weak acid. I'm not sure when we can omit or need to retain x.

[Jana Sun 1I]

Generally, to determine whether or not to omit the x in the concentration of the reactant depends on x. If the calculated value of x is less than 5% of the initial concentration of that product, you make generally omit x. However, I find this to be kind of confusing as well. You don't really know if x is less than 5% of the initial value until you actually figure out what x is. But in order to figure out what x is, you have to determine whether or not you can omit x in the calculated concentration of the reactant. It seems almost like a catch 22 situation. Is there another, similarly accurate way we can figure out whether or not to omit x in the calculation?