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.