Question

An aqueous solution contains 0.30 M hydrocyanic acid.

One Liter of this solution could be converted into a buffer by the addition of which of the following reagents?

(Assume that the volume remains constant as each substance is added.)

Question 15 options:(more than one choice)

	0.15 mol HClO4
	0.30 mol KClO4
	0.31 mol HClO4
	0.31 mol KCN
	0.149 mol NaOH

Answer 1

• For an efficient buffer, the realation between pH and pKa of weak acid are in relation,

• 1 L of 0.30 M HCN = (1x0.30) LxM HCN = 0.30 Lx(mol/L) = 0.30 mol HCN.
• An acidic buffer is consisted of weak acid and its conjugate base. So, for HCN the acid is itself and conjugate base will be any salt containing CN- ion. Thus, no acid can be added to generate buffer from HCN.
• So, first option is KCN obviously. Since, concentration is proportional to no. of moles. So, from Henderson-Hasselbalch equation we get,

So, for 0.31 mol of KCN the pH will be in the range of pKa + 1, thus that will be an effective buffer.

• Another thing is we can half neutralize the acid, to generate its salt. That thing is done with 0.149 mol NaOH. It neutralizes 0.149 mol of HCN to generate 0.149 mol NaCN and remaining moles of HCN is (0.30 - 0.149) mol = 0.151 mol. So again from HH equation we get,

Thus, this method will also generate an efficient buffer.

• Thus, options are - 0.31 mol KCN and 0.149 mol NaOH.