A forensic chemist needed to determine the conventration of `HCN` in the blood of a suspected homicle victim and decided to titrate a diluted sample fo the blood with iodine, using the reaction,
`HCN_((aq.)) + I_(3)^(-) rarr ICN_((aq.)) + 2I_((aq.))^(-) + H_((aq.))^(+)`
A diluted blood sample of volume `15mL` was titrated to the stoihometric point with `5.21mL` of an `I_(3)^(-)` aolurion. The molarity of `I_(3)^(-)` in the solution was determined by titrating it against arsenci `(III)` oxide, which in solution forms arenious acid, `H_93)As)_(3)`. It was found that `10.42 mL` of the tri-iodide solution was needed to reach teh stoichmetric point with a `10mL` sample of `0.1235 M H_(3) AsO_(3)` in the reaction.
`H_(3)AsO_(3(aq)) +v I_(3(aq))^(-) + H_(2)O_((l)) rarr H_(3)AsO_(4(aq)) + 3I_((aq.))^(-) + 2H_((aq.))^(+)`
(a) What is the normally of tri-iodie ions in the initlal solution?
(b) What is the molar concentration of `HCN` in the blood sample?

Answer

(a) `0.1185N`, (b) `0.412M`