5.1.3~ acids, bases and buffers

• pH of a buffer: weak acid + strong base

  1. Work out the moles of acid and base
  2. identify if the base or acid is in excess
  3. limiting reagent tells us the mole of the salt [A$^-$]
  4. if the base is in excess work out the moles unreacted and the new concentration [$OH^-$]=$\frac{moles\ unreacted}{new\ volume\ }$
  5. $\left[H^+\right]$= $\frac{Kw}{\left[OH^-\right]}$
• What do you then do if the acid is in excess

  [HA]=$\frac{moles\ of\ HA\ unreacted\ }{new\ volume\ }$

  [A$^-$]=$\frac{moles\ of\ salt\ formed\ \left[A^-\right]}{new\ volume}$

  then [$H^+$]=$\frac{Ka\left[HA\right]}{\left[A^-\right]}$

  then -log[$H^+$]
• pH of a buffer: strong acid + strong base

  1. work out the moles of the acid and the base
  2. identify if it is the acid or base that is in excess and work out the left over moles/moles unreacted
  3. if the base is in excess work out the new conc
  4. new concentration=$\frac{moles\ unreacted}{total\ volume\ }$
  5. then $\left[H^+\right]$=$\frac{Kw}{\left[OH^-\right]}$
• What to you do when if the acid is in excess

  new concentration=$\frac{moles\ unreacted}{total\ volume}$

  then -log$\left[H^+\right]$
• calculating the pH of water

  in pure water $\left[H^+\right]=$$\left[OH^-\right]$

  therefore Kw=$\left[H^+\right]^2$

  therefore $\left[H^+\right]$= $\sqrt{Kw}$
• What is the effect of temperature on the pH of water and the neutrality of water

  as the temperature increase the equilibrium moves to the right to lower the temperature

  therefore $\left[H^+\right]and\left[OH^-\right]$ increase

  therefore Kw increases and the pH also increase however the water is still neutral as $\left[H^+\right]=$$\left[OH^-\right]$
• How do you calculate the pH of a strong base

  find $\left[OH^-\right]$ concentration

  then substitute into equation $\left[H^+\right]$=$\frac{Kw}{\left[OH^-\right]}$

  whereby the value of Kw at room temperature is $1\times10^{-14}$
• How do you calculate the pH of a diluted strong acid

  $\left[H^+\right]$=$\left[H^+\right]_{old}$$\times$$\frac{old\ volume}{new\ volume}$
• What is the relationship between the strength of an acid and its Ka and pKa value

  the bigger the value of Ka the stronger the acid

  the bigger the value of pKa the weaker the acid