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Preparation of Alcohols from Grignard Reagents

Titration of ...

Question

Titration of $N a_{3} P O_{4}$ with $H C l$ using phenolphthalein end point produces $N a_{2} H P O_{4}$ while subsequent titration using methyl orange end point produces $N a H_{2} P O_{4}$ . Find the mg of $N a_{2} H P O_{4}$ present in a additional $35$ mL of the same acid to reach the methyl orange end point.

A

492

mg

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B

170

mg

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C

662

mg

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D

Insufficient data

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Solution

The correct option is B $170$ mg
For phenolphthalein end point,
$N a_{3} P O_{4} + H C l (25 m l, 0.12 N) \to N a_{2} H P O_{4} + N a C l$
Milliequivalents of $H C l = 25 \times 0.12 = 3 =$ Milliequivalents of $N a_{3} P O_{4}$ (in sample) $=$ Milliequivalents of $N a_{2} H P O_{4}$ (produced)
For methyl orange end point,
$N a_{3} P O_{4} + H C l (25 m l, 0.12 N) \to N a_{2} H P O_{4} + N a C l$
Milliequivalents of $H C l = 35 \times 0.12 = 4.2 =$ Milliequivalents of $N a_{2} H P O_{4}$ (in sample + produced from $N a_{3} P O_{4}$ )
Milliequivalents of $N a_{2} H P O_{4}$ in sample $= 4.2 - 3 = 1.2$
Milliequivalents of $N a_{2} H P O_{4}$ in sample $= 1.2 \times 142$ $= 170.4$ $\approx 170$
Hence, $N a_{2} H P O_{4}$ present is $170$ mg.

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0.6234

g sample that might contain

N a O H, N a_{2} C O_{3}, N a H C O_{3}

or a mixture of

N a O H + N a_{2} C O_{3}

N a_{2} C O_{3} + N a H C O_{3}

is titrated with

0.1062 M H C l

by two-indicator method. It is found that

40.38

mL of the acid is required to reach the phenolphthalein end-point. Methyl orange is then added to the solution and the titration continued using an additional

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500 m L

of a solution of

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was first titrated with

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using phenolphthalein indicator.

18.5 m L

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2.5 m L

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Q. A mixture of

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N a_{2} C O_{3}

solution is titrated with

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\frac{N}{10} H C l

using phenolpthalein. At this stage methyl orange was added and addition of acid was continued. The second end point was reached after further addition of

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0.4 g

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N a_{2} C O_{3}

and some inert impurities were first titrated with

\frac{N}{10} H C l

using phenolphthalein as an indicator.

17.5 m L

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Q. A solution contains a mixture of

N a_{2} C O_{3}

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20 m L

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