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JIPMER 2007
Exam
JIPMER
Year
2007
Subject
Chemistry
Chapter
Classification of Elements and Periodicity in Properties

JIPMER 2007 · Chemistry · Classification of Elements and Periodicity in Properties

What is the correct relationship between the pHs of isomolar...

What is the correct relationship between the pHs of isomolar solutions of sodium oxide (pH1)\left(\mathrm{pH}_1\right), sodium sulphide \left(\mathrm{pH}_2\right),sodiumselenide(pH3), sodium selenide \left(\mathrm{pH}_3\right) and sodium telluride \left(\mathrm{pH}_4\right) ?

Options

  1. ApH1>pH2≈pH3>pH4\mathrm{pH}_1\gt\mathrm{pH}_2 \approx \mathrm{pH}_3\gt\mathrm{pH}_4
  2. BpH1<pH2<pH3<pH4\mathrm{pH}_1 \lt \mathrm{pH}_2 \lt \mathrm{pH}_3 \lt \mathrm{pH}_4
  3. CpH1<pH2<pH3≈pH4\mathrm{pH}_1 \lt \mathrm{pH}_2 \lt \mathrm{pH}_3 \approx \mathrm{pH}_4
  4. DpH1>pH2>pH3>pH4\mathrm{pH}_1\gt\mathrm{pH}_2\gt\mathrm{pH}_3\gt\mathrm{pH}_4

Correct answer

D. pH1>pH2>pH3>pH4\mathrm{pH}_1\gt\mathrm{pH}_2\gt\mathrm{pH}_3\gt\mathrm{pH}_4

Step-by-step solution

The correct order of pH of isomolar solution in sodium oxide (pH1)\left(\mathrm{pH}_1\right), sodium sulphide \left(\mathrm{pH}_2\right),sodiumselenide(pH3)andsodiumtelluride(pH4), sodium selenide \left(\mathrm{pH}_3\right) and sodium telluride \left(\mathrm{pH}_4\right) is pH1>pH2>pH3>pH4\mathrm{pH}_1\gt\mathrm{pH}_2\gt\mathrm{pH}_3\gt\mathrm{pH}_4 because in aqueous solution, they are hydrolysed as follows. Na2O+2H2O⟶2NaOH Base +H2ONa2 S+2H2O⟶2NaOH+ Strong base H2 S Weak acid Na2Se+2H2O⟶2NaOH+ Strong base H2Se Weak acid Na2Te+2H2O⟶2NaOH+ Strong base H2Te Weak acid \mathrm{Na}_2 \mathrm{O}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text{ Base }}{2 \mathrm{NaOH}}+{\mathrm{H}_2 \mathrm{O}}{ } \mathrm{Na}_2 \mathrm{~S}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text{ Strong base }}{2 \mathrm{NaOH}+} \underset{\text{ Weak acid }}{\mathrm{H}_2 \mathrm{~S}} \mathrm{Na}_2 \mathrm{Se}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text{ Strong base }}{2 \mathrm{NaOH}+} \begin{gathered}\mathrm{H}_2 \mathrm{Se} \\ \text{ Weak acid }\end{gathered} \mathrm{Na}_2 \mathrm{Te}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text{ Strong base }}{2 \mathrm{NaOH}+} \begin{gathered}\mathrm{H}_2 \mathrm{Te} \\ \text{ Weak acid }\end{gathered} Order of acidic strength H2Te>H2Se>H2 S>H2O\mathrm{H}_2 \mathrm{Te}\gt\mathrm{H}_2 \mathrm{Se}\gt\mathrm{H}_2 \mathrm{~S}\gt\mathrm{H}_2 \mathrm{O} Order of neutralisation of NaOH H2Te>H2Se>H2 S>H2O\mathrm{H}_2 \mathrm{Te}\gt\mathrm{H}_2 \mathrm{Se}\gt\mathrm{H}_2 \mathrm{~S}\gt\mathrm{H}_2 \mathrm{O} Hence, their aqueous solutions have the following order of basic character due to neutralisation of NaOH with H2O,H2 S.H2SeandH2TeNa2O>Na2 S>Na2Se>Na2Te(∴pH\mathrm{H}_2 \mathrm{O}, \mathrm{H}_2 \mathrm{~S} . \mathrm{H}_2 \mathrm{Se} and \mathrm{H}_2 \mathrm{Te} \mathrm{Na}_2 \mathrm{O}\gt\mathrm{Na}_2 \mathrm{~S}\gt\mathrm{Na}_2 \mathrm{Se}\gt\mathrm{Na}_2 \mathrm{Te} (\therefore \mathrm{pH} of basic solution is higher than acidic or least basic solution.)
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