Question

An unknown weak acid with a concentration of 0.079 M has a pH of 1.80. What is the Ka of the weak acid

Answer 1

The Ka of the unknown weak acid is 0.016 M.

Step-by-step explanation:

The Ka of a weak acid can be calculated using the pH and concentration of the acid. First, convert the pH to [H+]:

[H+] = 10^(-pH) = 10^(-1.80) = 0.016 M

Next, use the equation for the ionization of the weak acid to set up an expression for Ka:

Ka = [A-][H+]/[HA]

Since the concentration of the unknown weak acid is 0.079 M and the concentration of [H+] is 0.016 M, we can substitute these values into the equation:

0.079 = [A-] * 0.016 / 0.079

Simplifying the equation, we can solve for [A-]:

[A-] = 0.016 * 0.079 / 0.079 = 0.016 M

Therefore, the Ka of the unknown weak acid is 0.016 M.

Answer 2

3.18x10⁻³

Step-by-step explanation:

The equilibrium of a weak acid can be written as:

• HA ⇔H⁺ + A⁻

And Ka can be expressed as:

• Ka=
  `([H^+][A^-])/([HA])`

Keep in mind that [H⁺] = [A⁻].

We can calculate [H⁺] from the pH:

• pH = 1.80 = -log[H⁺]

• [H⁺]=
  `10^(-pH)`=0.0158

Now we compute the calculated [H⁺] (and [A⁻]) with the [HA] given by the problem:

Ka =
`((0.0158)^2)/(0.079)` = 3.18x10⁻³