A 0.25 mol/L solution of benzoic acid, KC7H5O2 and antiseptic also used as a food preservative, has a pH of 2.40. Calculate the Ka of benzoic acid at SATP
My work:
C6H5COOH --> C6H5COO^- + H^+
Ka = (C6H5COO-)(H+)/ C6H5COOH
I know my next steps are to use pH = log(H+) to calculate H+
and then
substitute into Ka expression and determine Ka
How to do this?
Thanks in advance
No.
(0.00398)^2/(0.25-0.00398) =
6.44 x 10^-5
pH = 2.40
pH = -log(H+)
-log(H^+) = 2.40
log(H^+) = -2.40
plug -2.40 into your calculator and hit the 10x key to obtain 0.00398. check my work.
0.003898-- this is H+?
Ka = ( )(0.00398)/( )
What would be the concentration of C6H5COO- ?
Look at the equation.
C6H5COOH ==> C6H5COO^- + H^+
For every 1 mol C6H5COOH that ionizes, you get 1 mol C6H5COO^- and 1 mol H^+. So they are the same.
I get 4.0 x 10^-3 but im incorrect
To calculate the Ka of benzoic acid at SATP, you can follow these steps:
Step 1: Convert the pH to [H+]
To determine the concentration of H+ ions from the pH, you can use the equation: [H+] = 10^(-pH). In this case, the pH is 2.40. Therefore, [H+] = 10^(-2.40).
Step 2: Determine the concentration of benzoic acid
The concentration of benzoic acid can be determined using the given information that the solution is 0.25 mol/L.
Step 3: Determine the concentration of benzoate ion
Since benzoic acid is a weak acid, it partially dissociates into its conjugate base, the benzoate ion. Given that the initial concentration of benzoic acid is 0.25 mol/L, and the reaction is: C6H5COOH → C6H5COO^- + H+, we know that the concentration of benzoate ion ([C6H5COO^-]) is initially zero and increases by the same amount that [H+] decreases.
Step 4: Calculate the Ka value
The Ka expression for the dissociation of benzoic acid is: Ka = ([C6H5COO^-])([H+]) / [C6H5COOH].
Substitute the values obtained:
Ka = ([C6H5COO^-])([H+]) / [C6H5COOH]
Ka = ([H+])([H+]) / [C6H5COOH]
Ka = (10^(-2.40))(10^(-2.40)) / (0.25)
To calculate the final value, you need to substitute the values into the expression and calculate it using a calculator.