A certain buffer is made by mixing a weak acid HA and its conjugate base A–. When HA is present at a concentration of 0.5 mM and A– is present at a concentration of 0.1 mM, the buffer has a pH of 6.16. Calculate the ratio of the concentrations of HA and A– when the buffer has a pH of 7.02.

Respuesta :

Answer:The ratio of the concentrations of [tex]HA[/tex]  and [tex]A^-[/tex] when the buffer has a pH of 7.02 is 0.69

Explanation:

The dissociation constant for formic acid = [tex]K_a=1.8\times 10^{-4}[/tex]

Concentration of HA= 0.5 mM

Concentration of [tex]A^-[/tex]= 0.1 mM

pH = 6.16

First we have to calculate the value of [tex]K_a[/tex].

Using Henderson Hesselbach equation :

[tex]pH=pK_a+\log \frac{[A^-}{[HA]}[/tex]

Now put all the given values in this expression, we get:

[tex]6.16=pK_a+\log (\frac{[0.1]}{0.5})[/tex]

[tex]pK_a=6.86[/tex]

To calculate the ratio of the concentrations of [tex]HA[/tex]  and [tex]A^-[/tex] when the buffer has a pH of 7.02.

Using Henderson Hesselbach equation :

[tex]7.02=6.86+\log \frac{[A^-]}{[HA]}[/tex]

[tex]7.02=6.86+\log \frac{[A^-]}{[HA]}[/tex]

[tex]\frac{[A^-]}{[HA]}=1.44[/tex]

[tex]\frac{[HA]}{[A^-]}=0.69[/tex]

Thus the ratio of the concentrations of [tex]HA[/tex]  and [tex]A^-[/tex] when the buffer has a pH of 7.02 is 0.69

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