The following represent the Ka values for a series of weak acids. Which would be the best choice to prepare a buffer at pH=4.00?
2.7 x 10^4
1.2 x 10^-4
4.3 x 10^-8
You want the acid with pKa closest to 4.00. Convert those Ka values to pKa and compare.
To determine which weak acid would be the best choice to prepare a buffer at pH=4.00, we need to find an acid whose Ka value is close to the desired pH.
First, let's understand what a buffer is. A buffer is a solution that resists changes in pH when small amounts of acid or base are added to it. To create a buffer solution at a specific pH, you typically use a weak acid and its conjugate base or a weak base and its conjugate acid.
In this case, we are looking for a weak acid to create a buffer at pH=4.00. The pH of a solution is calculated using the formula: pH = -log[H+], where [H+] represents the concentration of hydrogen ions.
Since we are looking for a buffer at pH=4.00, we can calculate [H+] by taking the antilog of -pH: [H+] = 10^(-pH).
For pH=4.00, [H+] = 10^(-4.00) = 1.0 x 10^(-4).
Now, let's compare this concentration of [H+] to the Ka values given for the three weak acids:
1) Ka = 2.7 x 10^4
2) Ka = 1.2 x 10^(-4)
3) Ka = 4.3 x 10^(-8)
We want the concentration of [H+] to be close to 1.0 x 10^(-4).
Among the three options, the closest Ka value to 1.0 x 10^(-4) is the second option, Ka = 1.2 x 10^(-4). Therefore, the best choice to prepare a buffer at pH=4.00 would be the weak acid with a Ka value of 1.2 x 10^(-4).