[Borek]

So, the equation is

Kc = 100 x/((1-x)(1.5-x)²) = 0.005

or

x/((1-x)(1.5-x)²) = 0.00005

This can be true only if x is much lower than 1 (check what happens if x gets close to 1). That means in turn that you can simplify substituting 1-x=1 and 1.5-x=1.5.

To be sure - substitute, solve for x, then check if calculated x is really much smaller than 1. If it is - your result should be OK. If it is not - you may have to solve cubic.

For pH calculation (which is nothing else but equilibrium calculation) there exist a so called 5% rule of thumb that says you can approximate x-y as x if y is smaller than 5% of x. Note, that after using this rule you should always check if the assumption (5%) is satisfied.

I'm not sure if your equilibrium constant is small enough for this approximation, as if K_c is very small, then the dissociation x will be very small too.

It doesn't matter whether K value is small or large, what matters is whether summed values are comparable or not.

[Astrokel]

Thanks Borek for the reply!