The pH of hydrocyanic acid before and after the addition of sodium cyanide is 0.371 and 9.121
Calculation of pH of 720:
Given: Concentration of hydrocyanic acid (HCN) = 0.425 M
To find: pH
Reaction:
[tex]HCN \,\,\,\,\,\rightarrow \,\,\,\,\,\,H^+ + CN^-[/tex]
0.425M 0.425M
[tex]pH= -[logH^+]\\pH=-[log\,0.425]\\pH= 0.371[/tex]
pH after adding 0.249 mol of sodium cyanide (NaCN)
Pka of HCN= 9.21
Concentration of sodium cyanide = [tex]\frac{0.249}{0.720}\,M[/tex]
Concentration of NaCN= 0.346 M
According to Henderson–Hasselbalch equation:
[tex]pH= p_k_a + log\frac{salt}{acid}[/tex]
Putting the values in the above equation,
[tex]pH= 9.21 + log\frac{0.346}{0.425} \\\\pH= 9.21 - 0.089\\pH= 9.121[/tex]
Note: Pka value of HCN is assumed as 9.21
Learn more about Henderson–Hasselbalch equation here:
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