Question

Problem 2: Following analysis passes the acceptability of analysis (satisfies the electroneutrality). Find the concentration of Ca2+ mg/L.
Na⁺ = 12.8 mg/L Cl^-= 10.4 mg/L
Ca²⁺ = ??? HCO₃^- = 195.2 mg/L
Mg²⁺ = 15.5 mg/L SO₄ ^2- = 33.1 mg/L
K⁺ = 7.8 mg/L

Answer 1

For this first we have to know the moles or mmoles of each ions, so, we have to know their molar mass:

Species	Molar mass(g.mol-1)
Na+	23
Cl-	35.5
Ca2+	40
HCO3-	61
Mg2+	24.3
SO42-	96
K+	39.1

NOTE: Molar mass of atoms and corresponding ions are same, because mass of electron does not contribute significantly.

So, now I make a table for calculating mmoles of ions:

Species	Mass(mg)	mmoles (mg / molar mass)
Na+	12.8	0.556
Cl-	10.4	0.293
Ca2+	x (say)	x / 40
HCO3-	195.2	3.2
Mg2+	15.5	0.638
SO42-	33.1	0.345
K+	7.80	0.199

Since the condition of electroneutrality is,

total positive charge = total negative charge

So, concentration of Ca²⁺ is 43.04 mg.L^-1.