Addition of 25 cm3

of water to 100 cm3

of 0.100 mol dm-3 Ba(OH)2. Find pH. Dilution question.

Given concentration of OH- in original volume but want to find out concentration of OH- in

diluted volume.

– Concentration x old volume/ new volume = concentration of OH-. This is the quicker version

of what you would to work out concentration in buffer.

– Multiply by 2 since dibasic.

– Then sub into Kw formula to find concentration of H+.

Above only works with strong bases and strong acids. Same method with strong acids but no

need to sub into Kw formula, instead -log(H+).

Adding HCl to buffer solution.

– Find mol of the HCl added.

– Find mol of HA and A- in buffer solution.

– Add mol of HCl mol of HA.

– Subtract mol of HCl from mol of A-.

– Substitute new mol values of HA and A- into buffer formula. Don’t bother find concentration

as gives same value since working out ratios.

Adding Mg to buffer solution.

– Find mol of Mg added. Multiply by molar ratio- check equation.

– Find mol of HA and A- in buffer solution.

– Subtract mol of Mg from mol of HA.

– Add mol of Mg to A-.

– Sub in new values in buffer formula.

Calculating pH when strong acid and strong base mixed together and not a buffer.

– Calculate moles H+. Calculate moles OH-

– Calculate moles XS H+ or OH- since they react together to form water.

– Calculate XS [H+ ] or XS [OH- ] using total volume.

– Calculate pH. If excess [OH-] use Kw formula.

Calculating pH when strong acid mixed with strong acid.

– Calculate moles of H+ in both acids

– Add moles together.

– Find concentration of H+ using total volume. Convert to pH.