The pH of a lake water is measured in the field to be pH-8.0. A 200 mL sample of

Question

The pH of a lake water is measured in the field to be pH-8.0. A 200 mL sample of the water required 10.0 mL of 0.10 M HCl to titrate it to the H2CO3equivalence point. a.) What is the alkalinity of the water? Express your answer as eq L and as mg L CaCO3 b.) What are the concentrations of total carbonate (CT and each of the carbonate species H2CO3, HCO3, and CO in the lake, assuming that carbonate species and hydroxide are the only contributors to alkalinity? Hint: Since pH is known, use the definition of alkalinity in terms of the alpha notation for the carbonate system and the associated equations for concentrations J

Explanation / Answer

a) moles of H2CO3 = moles of HCl = molarity of HCl x volume of HCl

                           = 0.10 M x 0.010 L

                           = 1 x 10^-4 mols

molarity of H2CO3 in solution = moles/total volume

                            = 1 x 10^-4/(0.2 + 0.01)

                            = 4.76 x 10^-4 M

alkalinity of water = 4.76 x 10^-4 moles/L x 100.09 g/mol

                            = 47.64 mg/L of CaCO3

b) Using equation,

[HCO3^2-]/total CO2(aq) = Ka1.Ka2/[H+]^2+[H+]Ka1+Ka1.Ka2

with Ka1 = 4.45 x 10^-7

Ka2 = 4.69 x 10^-11

pH = 8.0 = -log[H+]

[H+] = 1 x 10^-8 M

Feed values,

[HCO3-]/total CO2(aq) = 4.45x10^-7x4.69x10^-11/(1x10^-8)^2+(1x10^-8)x4.45x10^-7+4.45x10^-7x4.69x10^-11

                              = 4.59 x 10^-3

[CO3^2-] = 2.18 x 10^-6 M

[H2CO3] = 4.76 x 10^-4 M

[HCO3-] = 1 x 10^-8 M

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