Nicotinic acid (niacin) is a monoprotic acid with the formula HC6H4NO2. A solution that is 0.012M in nicotinic acid has a pH of 3.39 at 25*C. What is the acid-ionization constant, Ka and pKa for this acid at 25*C?
is the ionization equation:
HC6H4NO2+ H20(l) <>C6H4NO^-(aq) + OH^-(aq)
it doesn't seem right. is it with H3O+ instead?
That is not the ionization reaction.
HC6H4NO2+ H20(l) <>C6H4NO^-(aq) + H3O^+(aq)
To determine the correct ionization equation for nicotinic acid (niacin), we need to consider the proton transfer in water.
The correct ionization equation for a monoprotic acid like nicotinic acid involves the transfer of a proton (H+) from the acid to water, forming the hydronium ion (H3O+).
So, the correct ionization equation is:
HC6H4NO2 + H2O ⇌ C6H4NO2- + H3O+
Now, let's proceed to determine the acid-ionization constant (Ka) and pKa for nicotinic acid.
The acid-ionization constant (Ka) can be calculated using the formula:
Ka = [C6H4NO2-] [H3O+] / [HC6H4NO2]
Given that the solution is 0.012M in nicotinic acid (HC6H4NO2) and has a pH of 3.39, we can determine the concentration of hydronium ion ([H3O+]) using the pH value. Remember that pH is defined as the negative logarithm (base 10) of the hydronium ion concentration.
pH = -log[H3O+]
Rearranging the equation to solve for [H3O+], we have:
[H3O+] = 10^(-pH)
Substituting the pH value of 3.39 into the equation, we get:
[H3O+] = 10^(-3.39)
Now, since nicotinic acid (HC6H4NO2) is a monoprotic acid, the concentration of [C6H4NO2-] is equal to the concentration of [H3O+], since they have a 1:1 stoichiometric ratio. Therefore, [C6H4NO2-] = [H3O+].
Substituting the concentrations into the Ka formula, we have:
Ka = [H3O+]^2 / [HC6H4NO2]
Ka = ([H3O+])^2 / (0.012M)
Now, substitute the calculated value of [H3O+] and solve for Ka.
Finally, to calculate pKa, we can use the formula:
pKa = -log(Ka)
Plug the calculated value of Ka into the equation to find pKa.
By following these steps, you can determine the acid-ionization constant (Ka) and pKa for nicotinic acid at 25°C.