Chemical 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: 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: 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 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: 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: State the disadvantages of using hard water.

Disadvantages of using hard water are-1. Hard water is unfit for washing as it is difficult to form lather with soap.2. Scum may form in a reaction with soap, wasting the soap.3. Furring of tea kettles will take place due to the formation of carbonates of calcium and magnesium.4. Hard blocks hot water pipes. This is due to the formation of layers of carbonates of calcium and magnesium.

Q: One litre of a samle of hard water contains 1 mg of \$CaCl_2\$ and 1 mg of \$MgCl_2\$.Then the total hardness in terms of parts of \$CaCO_3\$ per \$10^8\$ parts of water by mass is:

\$1\$ mole of \$CaCl_2\equiv1\$ mole of \$CaCO_3\equiv1\$ mole of \$MgCl_2\$\$\therefore\$ \$100mg\$ \$CaCO_3\$ is produced by \$95mg\$ \$MgCl_2\$\$\therefore\$ \$1mg\$ of \$MgCl_2\$ gives \$\cfrac {100}{95}mg\$ \$CaCO_3=\$ \$1.05mg\$ \$CaCO_3\$Similarly, \$1mg\$ of \$CaCl_2\$ gives \$\cfrac {100}{111}mg\$ \$CaCO_3=\$ \$0.90mg\$ \$CaCO_3\$\$\therefore\$ Total \$CaCO_3\$ per litre of water= \$1.05+0.90=1.95\$ \$mg\$Weight of \$1000ml\$ of water= \$10^3g\$=\$10^6mg\$\$\therefore\$ Total hardness in terms of parts of \$CaCO_3\$ per \$10^6\$ parts of water by weight = \$\cfrac {1.95}{10^6}\times 10^6\$=\$1.95\$ \$ppm\$Answer: (A) \$1.954\$ \$ppm\$