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Molality

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Presence of $C a S O_{4}$ causes permanent hardness in water. It can be removed by precipitating it as $C a C O_{3}$ by adding $N a_{2} C O_{3}$ . If the amount of $N a_{2} C O_{3}$ required to soften 100 L of hard water, which is 200 ppm $C a C O_{3}$ , is $x$ g, then value of $x$ is:

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Solution

$N a_{2} C O_{3} 106 g + C a S O_{4} \to C a C O_{3} 100 g ↓ + N a_{2} S O_{4} 10^{6} m L o f h a r d w a t e r h a s = 200 g C a C O_{3} 100 L = 10^{5} m L o f h a r d w a t e r h a s = 20 g C a C O_{3} I f h a r d n e s s i s 100 g, N a_{2} C O_{3} r e q u i r e d = 106 g I f h a r d n e s s i s 20 g, N a_{2} C O_{3} r e q u i r e d = 21.2 g$

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Q. Permanent hardness is due to

S O_{4}^{2 -}

C l^{-}

C a^{2 +}

M g^{2 +}

and is removed by the addition of

N a_{2} C O_{3}

.

C a S O_{4} + N a_{2} C O_{3} ⟶ C a C O_{3} + N a_{2} S O_{4}

C a C l_{2} + N a_{2} C O_{3} ⟶ C a C O_{3} + 2 N a C l

100

C a C O_{3}

N a_{2} C O_{3}

required to soften

10

L of hard water is :

Q. Permanent hardness is due to

C l^{⊝}

S O_{4}^{2 -}

M g^{2 +}

C a^{2 +}

and is removed by adding

N a_{2} C O_{3}

C a S O_{4} + N a_{2} C O_{3} \to C a C O_{3} + N a_{2} S O_{4}

C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l

Which of the following statements is/are correct?

N a_{2} C O_{3}

is widely used in softening of hard water. If 1 L of hard water required

0.0106 g

N a_{2} C O_{3}

, The hardness in ppm (parts per million i.e.,

10^{6}

C a C O_{3}

Following reaction is used to remove temporary hardness which is due to dissolved bicarbonates of

C a^{2 +}

M g^{2 +}

C a (H C O_{3})_{2} + C a (O H)_{2} \to 2 C a C O_{3} ↓ + 2 H_{2} O

Actually lime (

C a O

) is added which in

H_{2} O

C a (O H)_{2}

and precipitates

C a C O_{3}

C a O + H_{2} O \to C a (O H)_{2}

10

L of hard water requires

0.56

C a O

, temporary hardness of

C a C O_{3}

x \times 10^{2}

x

Q. Hardness of water is measured in terms of ppm (parts per million) of

C a C O_{3}

. It is the amount (in g) of

C a C O_{3}

10^{6} g H_{2} O

. In a sample of water,

10

0.56

C a O

to the remove temporary hardness of

H C O_{3}^{-}

.

C a (H C O_{3})_{2} + C a O ⟶ 2 C a C O_{3} + H_{2} O

Temporary hardness is :

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