Question

How much Ca(OH)2, must be added in 5 litre of
water to change its pH by 3 units -
(A) 1.37 gm
(B) 0.0185 gm
(C) 0.037 gm
(D) 1.85 gm​

Answer 1

Answer:

The correct answer is option B.

Explanation:

The pH of the water = 7

The pH of water after adding calcium hydroxide =  7 + 3 = 10

The pH is calculated by using formula:

pH + pOH = 14

pOH = 14 - 10 = 4

$4=\log [OH^-]$

$[OH^-]=0.0001 M$

$Ca(OH)_2(aq)\rightarrow Ca^{2+}(aq)+2OH^-(aq)$

1 mole of calcium hydroxide gives 2 moles  of hydroxide ions.

$[OH^-]=2\times [Ca(OH)_2]$

Then $10^{-4} M$ of hydroxide will be obtained from:

$[Ca(OH)_2]=\frac{[OH^-]}{2}=\frac{0.0001 M}{2}=0.00005 M$

$Moles=Molarity\times Volume$

$Moles=0.00005 M\times 5 L=0.00025 mol$

Mass of 0.00025 moles of calcium hydroxide:

0.00025 mol × 74 g/mol = 0.0185 g