Stoichiometry
A sample of well water contains 140 gm/m³ Ca²⁺ ions and 345 gm/m³ Na⁺ ions. The hardness of the sample of water, expressed in terms of equivalent CaCO₃ in gm/m³ is (assuming atomic masses of Ca :40, Na : 23, C : 12, O : 16)

485
345
350
140

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