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>> $Removal of Temporary Hardness of Water$

Removal of Temporary Hardness of Water

Q: Calculate the temporary and permanent hardness of water sample having the following the following constituents per litre:\$ Ca(HCO_{3})_{2} = 162\, mg, MgCl_{2} = 95 =\, mg, \$\$ NaCl = 585\, mg, Mg(HCO_{3})_{2} = 73\, mg, \$\$ CaSO_{4} = 136\, mg \$

mole of \$ Ca(HCO_{3})_{2} = \dfrac{162\times 10^{-3}mg}{162\,g/mole} = 1\times 10^{-3}\,moles \$Mole of \$ Ca(SO_{4}) = \dfrac{136\times 10^{-3}g}{136\,g/mole} = 1\times 10^{3}\,mole \$Total mole of \$ Ca = 2\times 10^{-3}\,mole \$mass of \$ CaCO_{3} = 2\times 10^{-3}\times 100 = 0.2\,g \$\$ \therefore \$ ppm (permanent hardness) \$ = \dfrac{6.2}{1000}\times 10^{6} = 200\,ppm \$Mole of \$ MgCl_{2} = \dfrac{95\times 10^{-3}}{95} = 1\times 10^{-3}\,mole \$ Mole Mg \$ (HCg)_{2} = \dfrac{73\times 10^{-3}}{146} = 5\times 10^{-4}\,mole \$mole Mg \$ = 1.5\times 10^{-4}\,mole \$Mole g \$ CaCO_{3} \$ (In terms of mg) \$ = 1.5\times 10^{-3} \$mass \$ = 1.5\times 10^{-3} = 0.150\,g \$ppm (temporary hardness) \$ = \dfrac{0.150}{100}\times 106 = 150\,ppm \$Hence, the correct option is \$\text{C}\$

Q: What causes the temporary and permanent hardness of water ?

The presence of calcium and magnesium bicarbonates \$Ca(HCO_3)_2\$ and \$Mg(HCO_3)_2\$ causes temporary hardness in water. The presence of soluble salts of calcium and magnesium, i.e., sulphates and chlorides of calcium and magnesium cause permanent hardness in water.

Q: Write chemical reactions to show the amphoteric nature of water.

Amphoteric nature of water is represented by following chemical reactions.(1) Water as a base:\$\displaystyle H_2O(l)+H_2S(g) \rightleftharpoons H_3O^+(aq) + HS^-(aq) \$(2) Water as an acid:\$\displaystyle H_2O(l) + NH_3(aq) \rightleftharpoons OH^-+NH_4^+ \$(3) Self ionization of water in which water simultaneous acts as acid and base.\$\displaystyle 2H_2O \rightarrow H_3O^+ + OH^- \$

Q: Pressure cooker reduces cooking time because:

Because under pressure, the heat builds faster and maintains temperature better, so cooking time is reduced. ... The increased pressure inside the cooker increases the boiling point of water above \$100 ^0C\$ so more cooking is done before the water actually starts to boil. So ultimately food is cooked faster.

Q: Permanent hardness can be removed by simple boiling it.

Permanent hardness is caused by the presence of sulfates of \$Ca/Mg\$. Which further increases the hardness on boiling because boiling removes only water in the form of vapour while none of its mineral content boil away-they remain in the vessel.

Q: Which of the following methods is used for the removal of temporary hardness of water?

In Clark's process, slaked lime,\$ Ca(OH)_2\$ is added to temporary hard water. Insoluble calcium carbonate precipitates out and no longer produce hardness.Clark's process involves the addition of a controlled quantity of slaked lime (calcium hydroxide)\$ Ca(HCO_3)_2(aq) + Ca(OH)_2(aq)\rightarrow 2CaCO_3(s) + 2H_2O(l)\$ \$Mg(HCO_3)_2(aq) + Ca(OH)_2(aq)\rightarrow CaCO_3(s) + MgCO_3(s) + 2H2O(l)\$Slaked lime is itself a source of calcium ions (and hence hardness) so care must be taken to avoid adding an excess.

Q: The formula of calgon is:

The formula of calgon is \$Na_2[Na_4(PO_3)_6]\$. Calgon is a trade name of a complex salt, sodium hexametaphosphate \$(NaPO_3)_6\$. It is used for softening hard water. Calgon ionizes to give a complex anion.

Q: When temporary hard water containing \$ Mg(HCO_{3})_{2} \$ is boiled, the ppt. formed is of

\$ Mg(OH)_{2} \$ is less soluble than \$ MgCO_{3} \$. On boiling temporary hard water containing \$ Mg^{2+} \$ ions, the ppt. obtained is of \$ Mg(OH)_{2} \$ and not that of \$ MgCO_{3} \$.\$MgCO_3+H_2O\xrightarrow{\Delta} Mg(OH)_2\downarrow+CO_2\$

Q: The hardness of a water sample containing \$10^{-3}\$M \$MgSO_4\$ expressed as \$CaCO_3\$ equivalents (in ppm) is ________. [molar mass of \$MgSO_4\$ is \$120.37\$ g/mol]

\$10^{-3}\$ molar \$MgSO_4 = 10^{-3}\$ moles of \$MgSO_4\$ present in \$1\$ Litre.\$n_{CaCO_3} =n_{MgSO_4} \$\$PPM\$ (in term of \$CaCO_3) = \dfrac{10^{-3}\times 100}{1000} \times 10^6\$ \$= 100\ ppm\$

Q: What do you understand by the term 'auto-protolysis' of water? What is its significance?

The reaction between two water molecules to produce hydronium ion and hydroxide ion is known as autoprotolysis of water. This is self ionization of water.\$\displaystyle 2H_2O \rightarrow H_3O^++OH^- \$ This reaction indicates the amphoteric nature of water. It can act as an acid as well as a base. One water molecule donates electron whereas other water molecule accepts electron.

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Hard and Soft Water-Advanced

ExampleDefinitionsFormulaes

Important Questions

Calculate the temporary and permanent hardness of water sample having the following the following constituents per litre:
$C a (H C O_{3})_{2} = 162 m g, M g C l_{2} = 95 = m g,$
$N a C l = 585 m g, M g (H C O_{3})_{2} = 73 m g,$
$C a S O_{4} = 136 m g$

Medium

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Which of the following methods is used for the removal of temporary hardness of water?

Hard

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The formula of calgon is:

Medium

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When temporary hard water containing $M g (H C O_{3})_{2}$ is boiled, the ppt. formed is of

Hard

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The hardness of a water sample containing $1 0^{- 3}$ M $M g S O_{4}$ expressed as $C a C O_{3}$ equivalents (in ppm) is ________.

[molar mass of $M g S O_{4}$ is $120.37$ g/mol]

Hard

JEE Mains

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What do you understand by the term 'auto-protolysis' of water? What is its significance?

Medium

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Removal of Temporary Hardness of Water

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Hard and Soft Water-Advanced

Calculate the temporary and permanent hardness of water sample having the following the following constituents per litre: Ca(HCO3​)2​=162mg,MgCl2​=95=mg, NaCl=585mg,Mg(HCO3​)2​=73mg, CaSO4​=136mg

What causes the temporary and permanent hardness of water ?

Write chemical reactions to show the amphoteric nature of water.

Pressure cooker reduces cooking time because:

Permanent hardness can be removed by simple boiling it.

Which of the following methods is used for the removal of temporary hardness of water?

The formula of calgon is:

When temporary hard water containing Mg(HCO3​)2​ is boiled, the ppt. formed is of

The hardness of a water sample containing 10−3M MgSO4​ expressed as CaCO3​ equivalents (in ppm) is ________. [molar mass of MgSO4​ is 120.37 g/mol]

What do you understand by the term 'auto-protolysis' of water? What is its significance?

Calculate the temporary and permanent hardness of water sample having the following the following constituents per litre:
$C a (H C O_{3})_{2} = 162 m g, M g C l_{2} = 95 = m g,$
$N a C l = 585 m g, M g (H C O_{3})_{2} = 73 m g,$
$C a S O_{4} = 136 m g$

When temporary hard water containing $M g (H C O_{3})_{2}$ is boiled, the ppt. formed is of

The hardness of a water sample containing $1 0^{- 3}$ M $M g S O_{4}$ expressed as $C a C O_{3}$ equivalents (in ppm) is ________.

[molar mass of $M g S O_{4}$ is $120.37$ g/mol]