"K_b =\\frac{K_w}{K_a}"
"K_{b1}=\\frac{1\\times10^{-14}}{5.1\\times10^{-12}}= 1.96\\times10^{-3}"
"K_{b2} = \\frac{1\\times10^{-14}}{1.7\\times10^{-7}}=5.88\\times10^{-8}"
"K_{b3}=\\frac{1\\times10^{-14}}{5.5\\times10^{-3}}=1.82\\times10^{-12}"
1) "PO_4^{3-} + H_2O \\leftrightarrow HPO_4^{2-}+OH^-"
I 0.8 0 0
C -x +x +x
E 0.8-x x x
"K_{b1} = \\frac{[HPO4^{2-}][Oh^-]}{PO4^{3-}}=\\frac{x^2}{0.8-x}=1.96\\times10^{-3}"
"x=0.039"
"[OH^-]= 0.039M"
"[HPO_4^{3-}]=0.039M"
2) "HPO_4^{3-}+H_2O \\leftrightarrow H_2PO_4^{2-}+ OH^-"
I 0.039 0 0
C -x +x +x
E 0.039-x x x
"K_{b2}=\\frac{[H_2PO_4^{2-}][OH^-]}{[HPO_4^{3-}]}=\\frac{x^2}{0.039-x}=5.88\\times10^{-8}"
"x= 0.00024"
"[OH^-] = 0.00024M"
"[H_2PO_4^{2-}] = 0.00024 M"
3) "H_2PO_2^{2-} + H_2O \\leftrightarrow H_3PO_4 + OH^-"
I 0.00024 0 0
C -x +x +x
E 0.00024-x x x
"K_{b3} = \\frac{[H_3PO_4][OH^-]}{[H_2PO_4^{2-}]}=\\frac{x^2}{0.00024-x}=1.82\\times10^{-12}"
"x= 2.1\\times 10^{-8}"
"[OH^-] = 2.1\\times10^{-8}M"
"[OH^-]_{total} = 0.039+0.00024+2.1\\times10^{-8} = 0.039"
"pOH = -log_{10}(0.039) = 1.4"
"pH = 14-pOH = 14-1.4 = 12.6"
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