Question 8 of 10 Sapling Learning this questionhasbenaustomizedby chengpeng chen

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Question 8 of 10 Sapling Learning this questionhasbenaustomizedby chengpeng chen at Saint Louis University Phosphoric acid is a triprotic acid with the following pKa values: pK.I 2.148 pK.2 7.198 pK., 12.375 You wish to prepare 1.000 L of a 0.0100M phosphate buffer at pH 7.460. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and NazHPO4, 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,PO mass Na, HPO o previous check Answer 0 Next AExt Hint

Explanation / Answer

pH = pKa + log[Na2HPO4]/[NaH2PO4]

7.460 = 7.198 + log[Na2HPO4]/[NaH2PO4]

0.262 = log[Na2HPO4]/[NaH2PO4]

[Na2HPO4]/[NaH2PO4]= 10^0.262

[Na2HPO4]/[NaH2PO4] =1.83

[Na2HPO4] =   [NaH2PO4] * 1.83
now will need 1.83* the amount of Na2HPO4 as NaH2PO4
x / y = 1.83
x + y = 0.01

1.83 y + y = 0.01
2.83 y = 0.01

Y=0.0035
y = 0.0035 M = [NaH2PO4]
x = 0.0065 M = [Na2HPO4]

0.0035 moles NaH2PO4 x 120g/mole = 0.420 g
0.00 65 moles Na2HPO4 x 139g/mole = 0.9035 g

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