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SCAN0008

# SCAN0008 - Mass Balance Concept Species from polyprotic...

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Unformatted text preview: Mass Balance Concept Species from polyprotic dissociation are effectively solved by use of mass balance concepts. Example: BaSO4 dissolving in water yields: - EaSOAs) a Ba2*+ so}- - 503' + H20 a» H50; + 0H- - 21-120 a» H3U’ + OH' Mass Balance yields: - [Bar] = [\$03 ] + fl-[SO4'] Help us to derive equations for Actd fractions in solution Fractional Comnosition: Monogrotic Systems HA c3 H+ + A', KE1 = [H*][A'] /[HA] What are the concentrations of HA and A' as a function of pH? Mass balance: F = [HA] 4- [A'] (1) Note: [A'] = F —[HA] :> K, = ([H+](F-[HA])} /[HA] of [HA] ‘ [W]F / ([H‘l "-L K.) (2) Now: um = [HA]/F = fraction ofl-lA molecules in solution. Dividing (2) by F gives [HA1/F = [H“] / (ll-1"] + Ka). Similarly, (XA_= [A'J / F =KA/(lll‘1 + K“) A Calculation Example - The pH ofa 0.25 M HF acid (KAI = 7,2 x 10") is 1.87. What are the concentrations of H2 HF, F‘ in this solution? Soln; F = [HF] + [F'] = 0.25M; [H+] = 0,0134M; Ku = 7.2 x104 [HA] = F.[H*] / ([H“] + lg) = (0.25 x 0,0l34)/(0.0134 + 7.2 x 10") = 0.237 M. [A'] = F.Ka / ([H*] + Kg.) = 0.25 x 7,2 x 10" / (0.0134 + 7.2 x10'4)= 0.0127M. - Use your old way. Compare the answers. 6, 5; ‘2’} ...
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