Homework Soursewhat would be the best choice to use to buffer a solution pH
what would be the best choice to use to buffer a solution pH=10.76? bases 1.8x10^-6, 4.6x10^-5, 3.2x10^-3, 5.7x10^-4
Solution
If you have to make up a buffer solution of a certain pH, it\'s fairly simple to decide on a weak acid to use to make the buffer. Consider the equation for the [H+] of a buffer
[H+] = Ka*(nHA/nA-)
There are two ways to adjust the pH of a buffer solution:
Pick a weak acid with a Ka close to the [H+] you want.
Adjust the HA to A- ratio to get the exact pH
or remember that, -log Ka=pKa and pH=-log[H+], so if you want to make a buffer for pH 4.8 you need to pick a weak acid with pKa about equal to the pH you want. In your case a) acetic acid (pka=4.76 would do the job)
Example: If you want to make a buffer of pH 4.5, how would you do it?
Solution: First, you need to pick a weak acid with a Ka about equal to the [H+] that you want. pH 4.5 gives a desired [H+] of 3.2*10-5 M. If you look over a table of weak acids, you see that acetic acid, Ka = 1.8*10-5 is a good choice.
Next, you need to adjust the ratio of acetic acid to acetate ion so as to get the exact pH. Use the above equation:
nHA=moles of weak acid
nA-=moles of conjugate base
Ka*(nHA/nA-)
3.2*10-5 = 1.8*10-5*(nHA/nA-)
nHA/nA- = 1.8
Thus, if you add acetic acid and sodium acetate to water in a ratio of 1.8 moles acid to 1.0 moles sodium acetate, you will get a buffer of pH 4.5.
[H+] = Ka*(nHA/nA-)
There are two ways to adjust the pH of a buffer solution:
Pick a weak acid with a Ka close to the [H+] you want.
Adjust the HA to A- ratio to get the exact pH
or remember that, -log Ka=pKa and pH=-log[H+], so if you want to make a buffer for pH 4.8 you need to pick a weak acid with pKa about equal to the pH you want. In your case a) acetic acid (pka=4.76 would do the job)
Example: If you want to make a buffer of pH 4.5, how would you do it?
Solution: First, you need to pick a weak acid with a Ka about equal to the [H+] that you want. pH 4.5 gives a desired [H+] of 3.2*10-5 M. If you look over a table of weak acids, you see that acetic acid, Ka = 1.8*10-5 is a good choice.
Next, you need to adjust the ratio of acetic acid to acetate ion so as to get the exact pH. Use the above equation:
nHA=moles of weak acid
nA-=moles of conjugate base
Ka*(nHA/nA-)
3.2*10-5 = 1.8*10-5*(nHA/nA-)
nHA/nA- = 1.8
Thus, if you add acetic acid and sodium acetate to water in a ratio of 1.8 moles acid to 1.0 moles sodium acetate, you will get a buffer of pH 4.5.
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