How many moles of HCl will be required to preapare one litre of a buffer solution (containing NaCN and HCN) of pH 8.5 using 0.01 gram formula mass of NaCN .` K_a` for HCN `= 4.1 xx 10^(-10)? `

Explanation

HCN for buffer will be formed by the reaction
`NaCN + HCl rarr NaCl + KCN`
mmol of NaCN present initially =` 0.01/49 xx 1000 = 0.2`
let x mmol of HCl is added to that x m mol of NaCN will be neutralized forming x mmol of HCN.
`pH = pK_(a) + log[NaCN]/[HCN]`
`8.5 = -log(4.1 xx 10^(-10))+ log 0.2-x/x implies x = 0.177 mmol`
`HClO_(4)` is strongest acid.
As its conjugate base is most stable