Question

A sample of water is alkaline to both phenolphthalein as well as methyl

orange. 100 mL of this water sample required 10 mL of N/50 H2SO4 for

phenolphthalein end point and 10 mL of the acid to methyl orange end

point on separate titration. Calculate the types and extent of alkalinity

present in water.​

Answer 1

Answer:

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Answer 2

Given:

A sample of water is alkaline to both phenolphthaleins as well as methyl orange.

100 mL of water sample required 10 mL of  H2SO4 for phenolphthalein endpoint.

10 mL of the acid to methyl orange endpoint.

To find:

The alkalinity of water =?

Explanation:

$\displaystyle H_2So_4$ dibasic in nature.

Normality of sulphuric acid  = $\displaystyle \frac{N}{50} = \frac{10}{50}$

                            Normality = 0.2N

Molarity of sulphuric acid = $\displaystyle \frac{0.02}{2}$

                Molarity             = 0.01

Mols of sulphuric acid = 10mL × 0.01M

                                     =   0.1mmol

Amount of base = 2×0.1 mmol

                           = 0.2mmol

Conc of bas  e    = $\displaystyle \frac{ (0.2\times 10^{-3}\times 1000 )}{100}$

                           =$2 \times 10^{-3} M$

Now

      POH = -log[OH-]

               = -log (2×$10^{-3}$)

               = 2.7

The sum of  pH and pOH  is 14.

pH +pOH = 14

pH = 14 - pOH

     = 14 -2.7

pH  = 11.3

So alkalinity of water is 11.3