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Mole Concept and Avogadro's Number

A mixture of ...

Question

A mixture of NaOH and $N a_{2} C O_{3}$ required 25mL of 0.1 M HCI using phenolphthalein as the indicator .However the same amount of the mixture required 30 mL of 0.1 M HCI When Methyl orange was used as the indicator. The molar ratio of NaOH and $N a_{2} C O_{3}$ in the mixture was:

A

2 : 1

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B

1 : 2

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C

4 : 1

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D

1 : 4

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Solution

The correct option is A 4 : 1
Phenolphthalein gives the end point corresponding to the reactions:
$N a O H + H C l \to N a C l + H_{2} O$
$N a_{2} C O_{3} + H C l \to N a H C O_{3} + N a C l$
$∴$ m-moles of $N a O H$ + m-moles of $N a_{2} C O_{3}$
= moles of $H C l = 2.5$
Methyl orange gives the end point corresponding to the reactions:
$N a O H + H C l \to N a C l + H_{2} O$
$N a_{2} C O_{3} + 2 H C l \to 2 N a C l + C O_{2} + H_{2} O$
$∴$ m-moles of $N a O H$ + m-moles of $N a_{2} C O_{3} \times 2$
= m-moles of $H C l = 30 \times 0.1 = 3$ .
milli moles of $N a_{2} C O_{3} = 0.5$
m-mole of $N a O H = 2.5 - 0.5 = 2$
Ratio of mole of $N a O H$ and $N a_{2} C O_{3}$
$= \frac{2}{0.5} = 4 : 1$

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Similar questions

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