The number of hydroxide ions that are needed to completely neutralize 1.0 liter of 0.50M HCl is calculated as below
find the moles of H^+ used from HCl
moles = molarity x volume
= 0.50 x 1 =0.50 moles of H^+ used
write ionic equation for reaction
that is H ^+ + OH^- = H2O
by use of mole ratio between H^+ to OH^- which is 1:1 the moles of OH^- used = 0.50 x1/1 = 0.50 moles
By use of Avogadro law constant
1 mole = 6.02 x10^23 ions
what about 0.5 moles
= 0.5 moles/ 1 moles x 6.02 x10^23 = 3.01 x10^23 ions of OH^-