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100 mL sample of hard water is titrated ...

100 mL sample of hard water is titrated with 500 " mL of " 0.001 M EDTA solution at `pH=10`, using eriochrome black-T indicator to reach equivalence point. An equal another amount of hard water sample is boiled for 30 min. After filtration and cooling, the same sample is titrated with 200 " mL of " 0.011 M EDTA solution at `pH=10` using Mg-EDTA complex solution and erichrome black-T indicator to reach equivalence point.
(i). Calculate the total hardness of water sample (temporary`+`permanent) in ppm of `CaCO_3`.

Text Solution

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(a) Total hardness (first titration):
`M_(1)V_(1)(EDTA)=M_(2)V_(2)`(Total `Ca^(2+)` and `Mg^(2+)` in temporary and permanent hardness
`500xx0.001=M_(2)xx100`
`M_(2)=0.005` (Total `Ca^(2+)` and `Mg^(2+)`)
Total hardness in `p p m of CaCO_(3)=(0.005xx100xx10^(6))/(10^(3))`
`=500 p p m`
(b) Permanent hardness (second titration):
`M_(1)V_(1)(EDTA)=M_(2)V_(2)`(Total `Ca^(2+)` and `Mg^(2+)` due to permanent hardness)
`200xx0.01=M_(2)xx100`
`M_(2)=0.002`
(Total `Ca^(2+)` and `Mg^(2+)` in permanet hardness permanent hardness in `p p m of CaCO_(3)`
`=(0.002xx100xx10^(6))/(10^(3))=200 p p m`
(c) Temprorary hardness in p p m of `CaCO_(3))=500-200=300 p p m`
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