Alkalinity as CaCO3 from Bicarbonate
Mediumalkalinitywater chemistry
A water sample contains 122 mg/L of bicarbonate ion, HCO₃⁻, which has an equivalent weight of 61. Expressed as calcium carbonate, with an equivalent weight of 50, the alkalinity is most nearly:
- A50 mg/L
- B100 mg/L
- C122 mg/L
- D150 mg/L
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Answer: B. 100 mg/L
Converting to a CaCO₃ basis multiplies by the ratio of equivalent weights: (122 mg/L)(50/61) = 100 mg/L as CaCO₃. Inverting the ratio to 61/50 gives 149 mg/L; the conversion always scales down when moving from bicarbonate to calcium carbonate because CaCO₃ has the smaller equivalent weight.
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