Question

Calculate the hardness using the following concentrations:

Na⁺=248mg/L

K⁺=5.5mg/L

Ca²⁺=54mg/L

Mg²⁺=26mg/L

Answer 1

Answer :-

Hardness of a water sample is due to multivalent cations.It is mainly due to calcium and magnesium ion content. Ca2+ and Mg2+ hardness is usually reported as parts per million (ppm) of calcium carbonate (by weight).

Here, in this question mg2+ and ca2+ will give hardness.

( i ) Hardness due to ca2+ ion = gram equivalent of ca2+ × 50

Gram equivalent of ca2+

= concentration of ca2+ in sample/ gram equivalent weight of ca2+

Gram equivalent weight of ca2+ = moleculer weight of ca / valancy on it

= 40/2 =20 grams

Gram eqivalent of ca2+ ions = 54/20   

= 2.7 grams equivalent

For converting gram equivalent as coco₃ (in ppm or mg/l), it is multiplied by 50.

Hardness due to ca2+ ions = 2.7×50 = 135 mg/l as caco₃

(ii) Hardness due to mg2+ ions =gram equivalent of mg2+ ×50

Gram eq. Of mg2+ = concⁿ of mg2+ in sample / gram equivalent wt. Of mg2+

Gram equivalent wt. of mg2+ = molecular wt./valancy

= 24/2 =12 grams

Gram eq. Of  mg2+ ions = 26/12 = 2.166

Hardness due to mg2+ ions = 2.166×50

= 108.33 grams as caco₃

Total hardness = 135 + 108.33

= 243.33 mg/l as caco₃