The Dissociation Constant for an Acid with a PK_a Value

The dissociation constant for an acid with a pK_a value of 6.0 is

A) 1 × 10 ^{− 6}
B) − 1 × 10⁶
C) 1 × 10⁶
D) − 1 × 10 ^{− 6}