Question

If you make up a solution of 100ml of 0.1m hepes in the basic form, what will the ph be?

Answer 1

The $pka$value of HEPES is 7.5 and it is a weak acid. Now, the value of $pkb$ is calculated by below formula:

$pk_{a}+pk_{b}=14$

$pk_{b}=14-pk_{a}$

=$14-7.5$

= $6.5$

Now, calculate $kb$:

$pkb =-logkb$

$6.5 =- log kb$

$kb = 3.16\times 10^{-7}$

As it is given that HEPES in the basic form having concentration $0.1 M$. The hydroxide ion concentration is calculated by below formula for weak base:

$[OH^{-}] =\sqrt{k^{b}\times c}$

Substitute the value of $k^{b}$ and c in above formula:

$[OH^{-}] =\sqrt{3.16\times 10^{-7}\times 0.1 M}$

= $1.78\times 10^{-4}$

Now, calculate the value of $pOH$ from the above value.

$pOH = -log[OH]$

Put the concentration of $OH$ in above formula:

$pOH  = -log (1.78\times 10^{-4})$

= $3.75$

In last step, calculate the $pH$:

$pH+pOH = 14$

$pH = 14-pOH$

Substitute the value of $pOH$

$pH =14-3.75$

= $10.25$

Thus, the value of $pH$ = $10.25$