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-

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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


pH=pKa+log[HPO42-][H2PO4-]7.5=7.2+log([HPO42-][H2PO4-])

    7.5 =7.2+log([HPO42-][H2PO4-])-7.2    -7.20.3 =log([HPO42-][H2PO4-])

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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-

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