Question

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2. For the following, assume pKi = 6.35 and pK2 10.33. (a) The alkalinity and initial pH of a water sample are measured to be 0.01 eq/L and 6.35. What are the bicarbonate and carbonate concentrations in mol/L? Answer: 0.01 Mand 0 (b) The alkalinity and initial pH of a water sample are measured to be 500 mg/L as CaCO3 and 10.33. What are the bicarbonate and carbonate concentrations in mg/L? Answer: 199 and 196 mg/L as HCOS and COs respectively. (Same molar concentrations but different mass concentrations.) (c) What is the alkalinity of carbonated water whose pH is about 3? Answer: 0 (mention why)

Answer 1

Part (a).

The initial pH of the water sample is 6.35, which is the same as pK1 (6.35), hence the bicarbonate concentration will be the same as the alkalinity of the water sample, whereas the carbonate ion is not at all present at this pH.

Therefore, alkalinity = [HCO₃^-] = 0.01 eq/L or 0.01 mol/L

The charge on bicarbonate is -1, hence, eq/L is the same as mol/L.

And [CO3^2-] = 0 M

Part (c).

The pH of the water is 3, which indicates a strongly acidic nature of water, hence the alkalinity of carbonated water is zero.