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Current Question (ID: 10675)

Question:
$\text{The correct relationship between the pH of isomolar solutions of sodium oxide}(\text{pH}_1)\text{, sodium sulphide}(\text{pH}_2)\text{, sodium selenide}(\text{pH}_3)\text{ and sodium telluride}(\text{pH}_4)\text{ is -}$
Options:
  • 1. $\text{pH}_1 > \text{pH}_2 \approx \text{pH}_3 > \text{pH}_4$
  • 2. $\text{pH}_1 < \text{pH}_2 < \text{pH}_3 < \text{pH}_4$
  • 3. $\text{pH}_1 < \text{pH}_2 < \text{pH}_3 \approx \text{pH}_4$
  • 4. $\text{pH}_1 > \text{pH}_2 > \text{pH}_3 > \text{pH}_4$ (Correct)
Solution:
$\text{Hint: Down the group acidity order is } \text{H}_2\text{O} < \text{H}_2\text{S} < \text{H}_2\text{Se} < \text{H}_2\text{Te}$ $\text{Step 1:}$ $\text{The correct order of pH of isomolar solution of sodium oxide}(\text{pH}_1)\text{, sodium sulphide}(\text{pH}_2)\text{, sodium selenide}(\text{pH}_3)\text{ and sodium telluride}(\text{pH}_4)\text{ is pH}_1 > \text{pH}_2 > \text{pH}_3 > \text{pH}_4\text{ because in aqueous solution, they are hydrolysed as follows.}$ $\text{Na}_2\text{O} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\text{O}$ $\text{base}$ $\text{Na}_2\text{S} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\text{S}$ $\text{strong base}\quad\text{weak acid}$ $\text{Na}_2\text{Se} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\text{Se}$ $\text{strong base}\quad\text{weak acid}$ $\text{Na}_2\text{Te} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\text{Te}$ $\text{strong base}\quad\text{weak acid}$ $\text{order of acidic strength } \text{H}_2\text{Te} > \text{H}_2\text{Se} > \text{H}_2\text{S} > \text{H}_2\text{O}$ $\text{Step 2:}$ $\text{Hence their aqueous solutions have the following order of basic character due to the neutralisation of NaOH with } \text{H}_2\text{O}\text{, } \text{H}_2\text{S}\text{, } \text{H}_2\text{Se} > \text{H}_2\text{Te}\text{.}$ $\text{Na}_2\text{O} > \text{Na}_2\text{S} > \text{Na}_2\text{Se} > \text{Na}_2\text{Te}$ $(\because\text{pH of basic solution is higher than acidic or least basic solution})$

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{
  "question": "The mass of carbon present in 0.5 mole of $\\mathrm{K}_4[\\mathrm{Fe(CN)}_6]$ is:",
  "options": [
    {
      "id": 1,
      "text": "1.8 g"
    },
    {
      "id": 2,
      "text": "18 g"
    },
    {
      "id": 3,
      "text": "3.6 g"
    },
    {
      "id": 4,
      "text": "36 g"
    }
  ],
  "solution": "\\begin{align}\n&\\text{Hint: Mole concept}\\\\\n&1 \\text{ mole of } \\mathrm{K}_4[\\mathrm{Fe(CN)}_6] = 6 \\text{ moles of carbon atom}\\\\\n&0.5 \\text{ mole of } \\mathrm{K}_4[\\mathrm{Fe(CN)}_6] = 6 \\times 0.5 \\text{ mol} = 3 \\text{ mol}\\\\\n&1 \\text{ mol of carbon} = 12 \\text{ g}\\\\\n&3 \\text{ mol carbon} = 12 \\times 3 = 36 \\text{ g}\\\\\n&\\text{Hence, 36 g mass of carbon present in 0.5 mole of } \\mathrm{K}_4[\\mathrm{Fe(CN)}_6].\n\\end{align}",
  "correct_answer": 4
}