According to the constant equation. we can get an equation for the H concentration H]=Ka×[HAA Take the negative logarithm of both sides of the equation. That is logHt--log(Kax HAVAJ -log Ka- log Hava The left side equals the pH. The pKa of a weak acid is defined in a manner similar to pH and poh pKa=-log KaAccording to the constant equation, we can get an equation for the H+ concentration. [H+ ] = Ka  [HA]/[A- ] Take the negative logarithm of both sides of the equation. That is, -log[H+ ] = -log{Ka  [HA]/[A- ]} = -log Ka – log{ [HA]/[A- ]} The left side equals the pH. The pKa of a weak acid is defined in a manner similar to pH and pOH. pKa = -log Ka