The relationship between the dissociation constants of a conjugate acid-base pair can be expressed quantitatively. For a weak acid, HA, its acid dissociation constant, Ka, is expressed as the hydronium ion concentration times the A ion concentration divided by the concentration of HA. The base dissociation constant, Kb, for its conjugate base, A ion, is expressed as the HA concentration times the hydroxide ion concentration divided by the concentration of the A ion. If the expressions for Ka and Kb are multiplied, the resulting equation is the equilibrium expression for Kw. This equation shows that the Ka for a weak acid and the Kb for its conjugate base are inversely proportional. A stronger acid with a higher Ka always has a proportionately weaker conjugate base with a lower Kb and vice versa. As the value of KW is constant, this equation can be used to calculate Ka or Kb of a conjugate acid-base pair if either one of them is known. For example, if the Kb for a base is 1 × 10−6, then the Ka for its conjugate acid can be calculated to be 1 × 10−8. The relationship between Ka, Kb, and KW can also be expressed in terms of their negative logarithms, pKa, pKb, and pKW. For the equation, Ka times Kb is equal to Kw, when the negative logarithm of both sides is taken, the resulting equation is pKa plus pKb is equal to pKW, which is 14 at 25 °C. pKa and pKb can also be used to compare the strengths of weak acids and bases. The lower the value of the pKa, the stronger the acid. In the same manner, the lower the value of the pKb, the stronger the base. For example, an acid with a pKa of 2.1 is stronger than an acid with a pKa of 4.6.