Top 10 viva/interview questions on Hardness of wastewater test

Water that has high mineral content is known as Hard water. Hard water contains bicarbonate, chlorides and sulphates of calcium and magnesium. When treated hard water with soap, it gets precipitated in the form of insoluble salts of calcium and magnesium. Hardness of water is a measure of the total concentration of the calcium and magnesium ions expressed as calcium carbonate. There are two types of hardness

1.Temporary hardness 2. Permanent hardness

Temporary Hardness is due to the presence of bicarbonates of calcium and magnesium. It can be easily removed by boiling.
Permanent Hardness is due to the presence of chlorides and sulphates of calcium and magnesium. This type of hardness cannot be removed by boiling.

A water sample is buffered to pH 10.1 and taken in to a conical flask. If an indicator dye like EBT, when added to a solution containing Calcium and Magnesium ions, the color of the solution turns to wine red. EDTA, the titrant, complexes with Magnesium and Calcium ions, removing them from association with the indicator. When all the Mg and Ca are complexed with EDTA, the indicator will turn blue. This is the end point of the titration.

1. The cation that cannot cause hardness is _____.

a) Ca
b) Mg
c) Fe
d) H

2. Addition of complexometrically neutral Mg to buffer is to

a) get a clear precise end point
b) neutralize sodium present
c) neutralize calcium present
d) maintain pH of the buffer

3. The hard water does not lather with soap because Soap is precipitated

a) by Ca and Mg ions
b) only by Mg ions
c) only by Ca ions
d) by Na ions

4. The buffers can be stored

a) for a month
b) no time limit
c) for an year
d) as long as the solution is clear without any turbidity

5. The need for buffer in the titration is ___.

a) for maintaining volume
b) for maintaining the pH
c) for maintaining the temperature
d) to nullify the error

6. The hard water consumes more soap for cleaning purposes.

a) True
b) False

7. E.D.T.A. means

a) Ethylene diamine tetra acetic acid
b) Erichrome diamine tetra acetic acid
c) Ethylene dye toluene acid
d) Erichrome dye toluene acid

8. The hard water

a) is corrosive
b) forms scales
c) is tasteless
d) is costly

9. Buffer solution is the solution which resists change in its

a) pH
b) colour
c) turbidity
d) conductivity

10. The Erichrome Black T is used as a catalyst.

a) True
b) False

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