Question

question 3
why is hydrogen cyanide(hcn) a brønsted acid?
hcn donates a proton (h+ ion) to another species.
hcn accepts a proton (h+ ion) from another species.
hcn undergoes oxidation in a reaction.
hcn forms a complex with a metal ion.
question 4
which concentrations are included in the ka expression for a weak acid, ha?
h+ only
h+ and a- only
h+, a-, and ha only
h+, a-, ha, and h2o

Explanation:

Question 3

According to the Brønsted - Lowry acid - base theory, an acid is a proton ($H^{+}$) donor. For $HCN$, in a reaction, it can donate a proton to another species. For example, in the reaction $HCN + H_{2}O
ightleftharpoons CN^{-}+H_{3}O^{+}$, $HCN$ donates a proton to $H_{2}O$.

For a weak acid $HA$, its dissociation reaction is $HA
ightleftharpoons H^{+}+A^{-}$. The acid - dissociation constant $K_{a}=\frac{[H^{+}][A^{-}]}{[HA]}$. Water is in large excess (if it is an aqueous solution) and its concentration is considered constant and is not included in the $K_{a}$ expression.

Answer:

HCN donates a proton ($H^{+}$ ion) to another species.

Question 4