Class 11 Hydrogen - Structure and Properties 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: 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: 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: 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: 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: A sample of water under study was found to boil at \$102^{\circ}C\$ at normal pressure. Is the water pure? Will this water freeze at \$0^{\circ}C\$? Comment.

The boiling point of pure water is \$100^{\circ}C\$ at \$1 atm\$. The freezing point of pure water is \$O^{\circ}C\$. The sample of water under study boils at \$102^{\circ}C\$ at normal pressure. Hence, the sample is not pure water. Since the sample of water is not pure, it will not freeze at \$O^{\circ}C\$. It will freeze at a temperature below \$O^{\circ}C\$.

Q: The reagent commonly used to determine hardness of water titrimetrically is:

The hardness of water is estimated by simple titration of ethylene diamine tetraacetate (EDTA) solution. EDTA forms stable complexes with the metal ions present in the hard water since stability constants of calcium and magnesium complexes of EDTA are different, even the selective estimation of these ions is possible.

Q: The molecular formula of a commercial resin used for exchanging ions in water softening is \$C_8H_7SO_3Na\$ \$(\$molecular weight \$206)\$. What would be the maximum uptake of \$Ca^{2+}\$ ions by the resin when expressed in mole per gram resin?

The molecular weight of resin is \$206\$ g/mol.\$1\$ g of resin corresponds to \$\dfrac {1}{206}\$ moles. \$1\$ mole of calcium ions will displace \$2\$ moles of sodium ions from resin.\$1\$ mole of resin contains \$1\$ mole of sodium ions and can uptake upto \$0.5\$ moles of calcium ions.Hence, \$1\$ g of resin will uptake \$\dfrac {1}{206} \times 0.5 =\dfrac{1}{412}\$ moles of calcium ions.

Q: How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?

Washing soda is prepared by sodium carbonate by adding 10 molecules of water of crystallization to each formula unit and is also known as sodium carbonate decahydrate or washing soda.\$\underset{Sodium \\ Carbonate}{Na_2CO_3} + 10H_2O \longrightarrow \underset{washing \ soda}{ Na_2CO_3. 10H_2O}\$Washing soda is a sodium salt that is basic nature.Washing soda can be used to remove both temporary and permanent hardness in water. It is soluble in water and adds a large amount of carbonate ions in the water. These react with dissolved calcium and magnesium ions to produce insoluble precipitates. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.

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Structure and Properties of Water

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Water - Structure and Physical Properties

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Water - Chemical Properties

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Hard and Soft Water-Advanced

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Water and Its Structure

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Chemical Properties of Water

Chemical Properties of Water.

8 mins

Hard and Soft Water

5 mins

Removal of Temporary Hardness of Water

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Structure of Water and Ice

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Properties of Water

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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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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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A sample of water under study was found to boil at $10 2^{\circ} C$ at normal pressure. Is the water pure? Will this water freeze at $0^{\circ} C$ ? Comment.

Medium

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The reagent commonly used to determine hardness of water titrimetrically is:

Hard

View solution

The molecular formula of a commercial resin used for exchanging ions in water softening is $C_{8} H_{7} S O_{3} N a$ $($ molecular weight $206)$ . What would be the maximum uptake of $C a^{2 +}$ ions by the resin when expressed in mole per gram resin?

Hard

View solution

How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?

Medium

View solution

Revise with Concepts

Water - Structure and Physical Properties

Water - Chemical Properties

Hard and Soft Water-Advanced

Quick Summary With Stories

Structure of Water and Ice

Properties of Water

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 ?

Permanent hardness can be removed by simple boiling it.

Pressure cooker reduces cooking time because:

Write chemical reactions to show the amphoteric nature of water.

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]

A sample of water under study was found to boil at 102∘C at normal pressure. Is the water pure? Will this water freeze at 0∘C? Comment.

The reagent commonly used to determine hardness of water titrimetrically is:

The molecular formula of a commercial resin used for exchanging ions in water softening is C8​H7​SO3​Na (molecular weight 206). What would be the maximum uptake of Ca2+ ions by the resin when expressed in mole per gram resin?

How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?

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

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]

A sample of water under study was found to boil at $10 2^{\circ} C$ at normal pressure. Is the water pure? Will this water freeze at $0^{\circ} C$ ? Comment.

The molecular formula of a commercial resin used for exchanging ions in water softening is $C_{8} H_{7} S O_{3} N a$ $($ molecular weight $206)$ . What would be the maximum uptake of $C a^{2 +}$ ions by the resin when expressed in mole per gram resin?