Map Sapling Learning Phosphoric acid is a triprotic acid with the following pKG

Question

Map Sapling Learning Phosphoric acid is a triprotic acid with the following pKG values: pk.1 2.148 p 7.198 pK.3 12.375 You wish to prepare 1.000 L o 0100 phosphate buffer at pH7.700. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and NazHPO4, in a 1.000 volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? Number Number mass Na, HPO L g mass NaH,PO. t other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? heck all that apply) H3PO4 and NaH2PO4 H3PO4 and NazHPO4 HaPO4 and Na3PO4 NaHzPO4 and NasPO4 Na2HPO4 and NasPO4 Exit Next

Explanation / Answer

pH of buffer is given by Hendersen equation

pH = pKa + log [conjugte base]/[acid]

Here pH = 7.7 and pKa = pKa2 = 7.198

thus log [conjugate base] /[acid] = 3.18

that is the ratio is 3.18 and thus [conjugate base ] = 3.18x0.01 =0.0318 M

and [acid] = 1.00x0.01 = 0.01M

Thus mass of conjugate base (Na2HPO4) = 0.0318x 142g/mol=

= 4.5156 g

and mass of NaH2PO4(acid) = 0.01 x120g/mol

= 1.2 g

Q2) The other combination we can use is

Na2HPO4 and Na3PO4

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