MCQEasyJEE 2024Internal Energy & Enthalpy

JEE Chemistry 2024 Question with Solution

Standard enthalpy of vaporization for CCl4\text{CCl}_4 is 30.5kJ/mol30.5 \, \text{kJ/mol}. Heat required for vaporization of 284g284 \, \text{g} of CCl4\text{CCl}_4 at constant temperature is:

  • A

    56kJ56 \, \text{kJ}

  • B

    45kJ45 \, \text{kJ}

  • C

    65kJ65 \, \text{kJ}

  • D

    60kJ60 \, \text{kJ}

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: Standard enthalpy of vaporization of CCl4\text{CCl}_4 is ΔHvap=30.5kJ mol1\Delta H_{\text{vap}} = 30.5 \, \text{kJ mol}^{-1} and mass of CCl4\text{CCl}_4 is 284g284 \, \text{g}.

Find: Heat required for vaporization of 284g284 \, \text{g} of CCl4\text{CCl}_4.

First, calculate the molar mass of CCl4\text{CCl}_4:

Molar mass=1×12+4×35.5=154g mol1\text{Molar mass} = 1 \times 12 + 4 \times 35.5 = 154 \, \text{g mol}^{-1}

Now calculate the number of moles:

moles of CCl4=284154=1.844mol\text{moles of } \text{CCl}_4 = \frac{284}{154} = 1.844 \, \text{mol}

Use the relation:

Heat required=moles×ΔHvap\text{Heat required} = \text{moles} \times \Delta H_{\text{vap}}

So,

Heat required=1.844×30.5=56.222kJ\text{Heat required} = 1.844 \times 30.5 = 56.222 \, \text{kJ}

On rounding, the heat required is 56kJ56 \, \text{kJ}. Therefore, the correct option is A.

Direct Proportionality Method

Given: 11 mole of CCl4\text{CCl}_4 requires 30.5kJ30.5 \, \text{kJ} for vaporization.

Find: Heat required for 284g284 \, \text{g} of CCl4\text{CCl}_4.

Since molar mass of CCl4\text{CCl}_4 is 154g/mol154 \, \text{g/mol}, the required heat is directly proportional to mass:

Heat=30.5×284154\text{Heat} = 30.5 \times \frac{284}{154} =56.24kJ= 56.24 \, \text{kJ}

Therefore, the required heat is approximately 56kJ56 \, \text{kJ}, so the correct option is A.

Common mistakes

  • Using the given mass directly in the enthalpy formula is incorrect because 30.5kJ/mol30.5 \, \text{kJ/mol} is given per mole, not per gram. First convert 284g284 \, \text{g} into moles, then multiply by ΔHvap\Delta H_{\text{vap}}.

  • Calculating the molar mass of CCl4\text{CCl}_4 incorrectly leads to the wrong answer. Do not forget that there are 44 chlorine atoms, so the molar mass is 12+4×35.5=154g/mol12 + 4 \times 35.5 = 154 \, \text{g/mol}.

  • Confusing vaporization enthalpy with some other thermodynamic quantity is wrong because the question specifically asks for heat required for vaporization at constant temperature. Use standard enthalpy of vaporization, not combustion or formation enthalpy.

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