What is the pKa of a compound used to prepare a buffer of pH 10.65 using 3 times as much conjugate base than the weak acid?
The solubility of sodium sulphate (Na2SO4) in water is 9.4 g/100 mL. Calculate the Ksp for sodium sulphate.
ratio of conjugate base / acid = 3/1
pH = pKa + log [conjugate base / acid]
10.65 = pKa + log [3]
pKa = 10.17
b)
solubility S = 9.4 g / 100 mL
moles = 9.4 / 142 = 0.0662
Molarity = 0.0662 / 0.1 = 0.662 M
Na2SO4 -------------------> 2 Na+ + SO42-
2 S S
Ksp = [Na+]^2[SO42-]
= (2S)^2 x S
= 4 S^3
= 4 x 0.662^3
= 1.16
Ksp of Na2SO4 = 1.16
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