Question

75cc of water sample require 10.2cc of 0.01M EDTA for titration in presence of eriochrome black T indicator. Calculate the total hardness.

Answer 1

The hardness of water is mainly due to the dissolved calcium salts, while magnesium is also presented in the drinking water. The interaction between EDTA and calcium cations:

$Ca^{2+}+EDTA^{4-} \rightarrow Ca(EDTA)^{2-};$

$C_{total}=\frac{C(EDTA)*V_{av}}{V_{w}}=\frac{0.01M*10.2mL}{75mL}=1.36 mmol/L;$

This could be converted:

$TotalHardness=1.36mmol/L*40mg/mmol=54.4mg/L;$

or $54.4 \mu g/mL.$