Final answer:
To find the acid-dissociation constant (Ka) for a weak acid given its pH, convert the pH to hydronium ion concentration, assume equimolar concentrations of H+ and A-, and apply the equation Ka = [H+][A-]/[HA] with the initial concentration of the acid.
Step-by-step explanation:
To calculate the acid-dissociation constant (Ka) for a weak acid (HA) from its pH, you first convert the pH to the hydronium ion concentration [H+]. The pH of 2.98 implies an [H+] of 10-2.98 M. Assuming that the weak acid dissociates only slightly, which is typical for weak acids, the concentration of A- (the conjugate base) will be equal to the [H+] and the concentration of the undissociated HA will approximately be the initial concentration.
The expression for Ka is: Ka = [H+][A-]/[HA]. We plug in the values: Ka = (10-2.98)2/0.190 to get the Ka value.