The number of moles of excess hydroxide ions at the end of the reaction is 0.123 mol/L.
The equation of the reaction is;
KOH(aq) + HNO3(aq) ------->KNO3(aq) + H2O(l)
Number of moles of KOH = 53.0/1000 L × 0.320 M = 0.01696 moles
Number of moles of HNO3 = 28/1000 L× 0.250 M = 0.007 moles
We can see that the acid, HNO3 is the limiting reactant so 0.007 moles of acid reacts with 0.007 moles base.
Number of moles of excess base = 0.00996 moles
Total volume of solution = 53.0 mL + 28.0 mL = 81 mL or 0.0081 L
Number of moles of excess base = 0.00996 moles/0.0081 L = 0.123 mol/L
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