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buffer solution
system that minimises pH changes when small amounts of an acid or a base are added
what are the 2 components in buffer solutions
• a weak acid - conjugate base
what do weak acids, HA remove?
added alkali
what do conjugate bases, A- remove?
added acid
what happens to the pH as the buffer works?
pH changes but only by a small amount
on addition of an acid, H+(aq)
• [H+(aq)] increases • H+(aq) ions react with the conjugate base, A-(aq) • equilibrium position shifts to the left, removing most of the H+(aq)
on addition of an alkali, OH-(aq)
• [OH-(aq)] increases • small concentration of H+(aq) ions react with OH-(aq) ions: H+(aq) + OH-(aq) → H₂O(l) • HA dissociate, shifting the equilibrium position to the right to restore most of H+(aq) ions
[H+(aq)] and [A-(aq)] for weak acids and buffers
for weak acids: [H+(aq)]= [A-(aq)] for buffers: [H+(aq)]≠ [A-(aq)]
calculate the pH when the buffer solution contains 0.100 moldm⁻³ CH₃COOH and 0.3 moldm⁻³ CH₃OONa. Ka (CH₃COOH)= 1.74X10⁻⁵ moldm⁻³
calculate [H+(aq)] from Ka, [HA(aq)], and [A-(aq)] [H+(aq)] = Ka X [CH₃COOH(aq)]/ [CH₃COO-(aq)] = 1.74X10⁻⁵ X0.1/0.3 = 5.80X10⁻⁶ moldm⁻³ find pH pH= -log[H+(aq)]=-log(5.80X10⁻⁶)= 5.24
what is the end point of a titration
• indicator contains equal concentrations of HA and A- - colour will be in between the 2 extreme colours
what is an indicator?
weak acid
what is the vertical section of a pH curve
pH increases on addition of a very small volume of base - acid and base concentration similar
equivalence point of the titration
volume of one solution that exactly reacts with the volume of the other solution