A chemistry graduate student is given of a hydrocyanic acid solution. Hydrocyanic acid is a weak acid with . What mass of should the student dissolve in the solution to turn it into a buffer with pH

Here is the full question:

A chemistry graduate student was given 500 mL of a 1.60 M hydrocyanic acid (HCN) solution. Hydrocyanic acid is a weak acid with . What mass of KCN should the student dissolve in the HCN solution to turn it into a buffer with pH  = 9.16?

36.85 g

Explanation:

From the given information above:

Recall that  number of moles of HCN = mass of HCN / molar mass of HCN= mass of HCN / molar mass of HCN

Therefore;

the number of moles of HCN =  

the number of moles of HCN =

the number of moles of HCN =  0.8

The acid dissociation constant =

Thus the pKa =

pKa = 9.31

However, the pH is given as:

KCN = 0.566 mol

The standard molar mass of KCN = 65.11 g/mol

Thus, the mass of KCN = number of moles of KCN × molar mass of KCN

the mass of KCN = 0.566 × 65.11

the mass of KCN that the student needs to dissolve in HCN is =  36.85 g

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