The correct option is D 4.0×10−10
Given,
pH of hydrocyanic acid is 5.2
pH=−log H+=5.2
log[H+]=−5.2
[H+]=Antilog(−5.2)=6.31×10−6 M
HCN ⇌ H+ + CN−
0.1−6.31×10−6 6.31×10−6M 6.31×10−6 M
=0.1 M (6.31×10−6 is too smaller than 0.1, so we can neglect it)
Ka=[H+][CN−][HCN]
=(6.31×10−6M)×(6.31×10−6)M0.1 M
=3.9816×10−10=4.0×10−10