# Problem: The dissociation of H2PO4- to HPO42- has a pKa of 7.2. For a 0.2 M phosphate buffer at pH 7.5, calculate the following values.a. the ratio of HPO42- to H2PO4b. the concentrations of HPO42- and H2PO4-

###### FREE Expert Solution

We are asked to calculate the ratio of HPO42- to H2PO4 of the given buffer system and the concentrations of HPO42- and H2PO4-.

For Buffer 2: H2PO4-   HPO42- + H+, pKa = 7.2

▪ Weak acid:            H2PO4- (donates a proton, H+)

▪ Conjugate base:    HPO42(base formed after the weak acid donates or loses a proton, H+)

Using the Henderson-Hasselbalch equation for this buffer, we can determine the conj. base/ weak acid ratio at pH = 7.5

$\mathbf{pH}\mathbf{=}{\mathbf{pK}}_{\mathbf{a}}\mathbf{+}\mathbf{log}\left(\frac{\left[{{\mathrm{HPO}}_{4}}^{2-}\right]}{\left[{H}_{2}{{\mathrm{PO}}_{4}}^{-}\right]}\right)\phantom{\rule{0ex}{0ex}}\mathbf{7}\mathbf{.}\mathbf{5}\mathbf{=}\mathbf{7}\mathbf{.}\mathbf{2}\mathbf{+}\mathbf{log}\mathbf{\left(}\frac{\left[{{\mathrm{HPO}}_{4}}^{2-}\right]}{\left[{H}_{2}{{\mathrm{PO}}_{4}}^{-}\right]}\mathbf{\right)}$

84% (471 ratings) ###### Problem Details

The dissociation of H2PO4- to HPO42- has a pKa of 7.2. For a 0.2 M phosphate buffer at pH 7.5, calculate the following values.

a. the ratio of HPO42- to H2PO4

b. the concentrations of HPO42- and H2PO4-