If the pKa value for a given acid is low, what does that say about the acid (is it weak or strong) ?

If the pKa of the acid is low (negative), then the acid is strong.

Explanation:

Ka, the acid ionization constant, measures the strength of an acid in a solution. Stronger acids have higher Ka values.

We defined: pKa = - log[Ka]

This function is a decreasing function, meaning that pKa will be getting smaller and smaller, while increasing Ka (high values of Ka will have negative pKa values). Therefore, stronger acids (high values of Ka), will have low (negative) pKa values.