Question

Hi there, I did an experiment using the titration of EDTA to determine the hardness of water. 1.)How do I determine the amount of moles of EDTA? 2.)How do I find the average water hardness of the water I used in this experiment?

I started with 10 ml of water, added 5 drops of Ph 10 to it, and then EBT indicator. I then used the tirator to dispense EDTA into the beaker to find out if how much EDTA it would take to turn the solution in the beaker from purple pink to light grey. On average, 0.17 mL of EDTA was necessary for a change in color to occur.

Answer 1

1. If molarity of EDTA = 0.1M

0.1M means 0.1 mole per litre

Therefore, no of mole in 0.17 ml = (0.1mol/1000ml)×0.17ml = 0.000017 mole of EDTA

2. No of mole of EDTA = no of mole of CaCO3

Molar mass of CaCO3 = 100.08 g/mol

Mass of CaCO3 = 0.000017 × 100.08 = 0.0017g= 1.7 mg

Your water sample is 10ml

So, 1.7 mg in 10ml of sample

Therefore , in 1000ml the mass is 170mg

Therefore , water hardness = 170 mg/litre = 170 ppm

This is water hardness as CaCO3 but you determine separately Ca 2+ and Mg2+ ions , then water hardness is addition of concentrations of these two ions .