Question

Analysis of treated water from a drinking water supply that originates from runoff of a relatively clean watershed yielded the following species concentrations:

Species	Mass conc. (g m-3)
SiO2	8.3
Ca2+	39
Mg2+	9
CO32-	6.4
SO42-	22

pH = 9.0
Total dissolved solids (TDS) = 216 g m^-3

(a) Use electroneutrality to check if any major species are missing from the analysis.

(b) Determine the molar concentration of bicarbonate in this water and recheck the electroneutrality relationship.

(c) Determine the alkalinity of the sample.

(d) Check to see if the solids balance is satisfied, and briefly comment on your finding.

Answer 1

a) SiO2 is neutral and can be ignored

Ca2+ is 39/40 g/mol = 0.975 mol

Mg2+ is 9/24.3 g/mol = 0.37 mol

CO32- = 6.4/60 g/mol = 0.106 mol

SO42- = 22/96g/mol = 0.229 mol

Since cation are higher some anion is missing

b) So the anion can be bicarbonate which will be (0.975+0.37) * 2 -[ (0.106+0.229)*2] = 2.09 mol of HCO3-

So now electroneutrality will be balanced

c) pH is 9.0

So alkalinity which is defined as the amount of acid that the water can neutralize is

[H+] = 10^-pH = 10^-9 = 1 x 10^-9 M

To get tot neutral pH you will have H+ = 1 x 10^-7 M

So alkalinity = 1 x 10-7 - 1 x 10-9 = 9.9 x 10^-8 M