What is the pH of a 1M HCN solution , #Ka=10^{-10}#?

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Answer 1 · 2018-07-11T11:48:41

#pH=5#

HCN #rightleftharpoons# #H^+# + #CN^-#
#K_a# = [#H^+#] [#CN^-#] #//[HCN]#=#10^-10#

HCN
#Initial_(HCN)=1M#
#Delta_(HCN)=-xM#
#Equilibrium_(HCN)=(1-x)M#

#H^+#
#Initial_(H^+)=0M#
#Delta_(H^+)=+xM#
#Equilibrium_(H^+)=xM#

#CN^-#
#Initial_(CN^-)=0M#
#Delta_(CN^-)=+xM#
#Equilibrium_(CN^-)=xM#

#K_a# = [#x#] [#x#] #//[1-x]#=#10^-10#**
#=x^2 //1 =10^-10#
Here the x value can be ignored, and therefore x becomes 0 in #[1-x]#

#x=sqrt(10^-10)M#

#pH = -logx#
#pH = -logsqrt(10^-10)#
#pH=5#