Phosphoric acid is a triprotic acid with the following pKa values: pKal=2.148 pK

Question

Phosphoric acid is a triprotic acid with the following pKa values: pKal=2.148 pKa,-7.198 pKa,-12.375 You wish to prepare 1.000 L of a 0.0300 M phosphate buffer at pH 7.560. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? Number Number mass NaH,Po1.87 mass Na,HPo-4.88 What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply). H3PO4 and NaH2PO4 H3PO4 and Na2HPO4 H3PO4 and Na3PO4 NaH2PO and NayPO4 Na2HPO4 and Na3P0 Incorrect

Explanation / Answer

the buffer equation

pH = pKa + log(HPO4-2 /H2PO4)

7.56 = 7.198 + log(Na2HPO4 / NaH2PO4)

10^(7.56 -7.198 ) = (Na2HPO4 / NaH2PO4)

(Na2HPO4 / NaH2PO4) =2.301

Na2HPO4= 2.301*NaH2PO4

total molarity = 1*0.03 = 0.03 M

NaH2PO4+ Na2HPO4 = 0.03

2.301*NaH2PO4 + NaH2PO4 = 0.03

NaH2PO4 = (0.03)/(2.301+1) = 0.00908 M

Na2HPO4=  2.301*NaH2PO4 = 2.301*0.00908 = 0.02089

if 1 L , then

mol of NaH2PO4 = 0.00908 *119.9770 = 1.089 g

mol of Na2HPO4= 0.02089*141.9588 = 2.9655 g

Q2.

combinations which will allow HPO4-2 and H2PO4- in solution

a)

will not give HPO4-2

b)

will give HPO4-2 and H2PO4-

c)

H3PO4 + Na3PO4 --> proper mix is required, but HPO4-2 and H2PO4- is achievable

d)

will not do

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