Why is molality (m) preferred over molarity (M) when expressing concentrations at varying temperatures?

1. Why is molality (m) preferred over molarity (M) when expressing concentrations at varying temperatures? 1. विभिन्न तापमानों पर सांद्रता व्यक्त करते समय मोलैलिटी (m) को मोलैरिटी (M) की अपेक्षा क्यों प्राथमिकता दी जाती है? +

Detailed Answer

Molality (m) is preferred over molarity (M) when concentration needs to be compared at different temperatures because molality depends on the mass of the solvent, whereas molarity depends on the volume of the solution.

Molality = Number of moles of solute / Mass of solvent in kg

m = n / W

Molarity = Number of moles of solute / Volume of solution in litre

M = n / V

Why is Molality Temperature Independent?

Temperature can change the volume of a substance because of thermal expansion or contraction. Therefore, the volume of a solution may change when temperature changes.

Since molarity is based on the volume of solution, its value may change with temperature.

However, the mass of solvent does not appreciably change with temperature. Therefore, molality remains essentially constant with temperature.

Hence, molality is preferred when concentration has to be expressed or compared at varying temperatures.

Key Difference

Molality: Based on mass of solvent → essentially independent of temperature.

Molarity: Based on volume of solution → changes with temperature.

Conclusion

Molality is preferred over molarity for temperature-dependent studies because mass remains essentially constant with temperature, whereas the volume of a solution changes with temperature.

विस्तृत उत्तर

मोलैलिटी (m) को मोलैरिटी (M) की अपेक्षा विभिन्न तापमानों पर सांद्रता व्यक्त करने के लिए अधिक उपयुक्त माना जाता है, क्योंकि मोलैलिटी विलायक के द्रव्यमान पर आधारित होती है, जबकि मोलैरिटी विलयन के आयतन पर आधारित होती है।

मोलैलिटी = विलेय के मोल / विलायक का द्रव्यमान (kg)

m = n / W

मोलैरिटी = विलेय के मोल / विलयन का आयतन (L)

M = n / V

मोलैलिटी तापमान पर निर्भर क्यों नहीं होती?

तापमान बदलने पर पदार्थों के आयतन में प्रसार या संकुचन हो सकता है। इसलिए विलयन का आयतन तापमान के साथ बदल सकता है।

चूँकि मोलैरिटी विलयन के आयतन पर निर्भर करती है, इसलिए तापमान बदलने पर इसकी मात्रा बदल सकती है।

इसके विपरीत, विलायक का द्रव्यमान तापमान बदलने पर सामान्यतः नहीं बदलता। इसलिए मोलैलिटी तापमान से लगभग स्वतंत्र रहती है।

इसी कारण अलग-अलग तापमानों पर सांद्रता की तुलना करने के लिए मोलैलिटी अधिक विश्वसनीय है।

मुख्य अंतर

मोलैलिटी: विलायक के द्रव्यमान पर आधारित → तापमान से लगभग स्वतंत्र।

मोलैरिटी: विलयन के आयतन पर आधारित → तापमान के साथ बदल सकती है।

निष्कर्ष

विभिन्न तापमानों पर सांद्रता की तुलना के लिए मोलैलिटी को प्राथमिकता दी जाती है क्योंकि द्रव्यमान तापमान से लगभग अपरिवर्तित रहता है, जबकि विलयन का आयतन तापमान के साथ बदल सकता है।

Advertisement

30 Related MCQs 30 संबंधित बहुविकल्पीय प्रश्न

1. Which concentration term is independent of temperature?

A. Molarity

B. Molality

C. Normality

D. Volume percentage

Answer: B. Molality

2. Molality is defined as moles of solute per:

A. Litre of solution

B. Litre of solvent

C. Kilogram of solvent

D. Kilogram of solution

Answer: C. Kilogram of solvent

3. Molarity is expressed as:

A. mol kg⁻¹

B. mol L⁻¹

C. kg mol⁻¹

D. L mol⁻¹

Answer: B. mol L⁻¹

4. Molality is expressed as:

A. mol L⁻¹

B. mol kg⁻¹

C. kg mol⁻¹

D. L mol⁻¹

Answer: B. mol kg⁻¹

5. Which quantity changes with temperature?

A. Mass of solvent

B. Number of moles

C. Volume of solution

D. Molecular mass

Answer: C. Volume of solution

6. Why can molarity change with temperature?

A. Moles change

B. Molecular mass changes

C. Solution volume changes

D. Solute disappears

Answer: C. Solution volume changes

7. Molality depends on the mass of:

A. Solute

B. Solvent

C. Solution

D. Container

Answer: B. Solvent

8. Molarity depends on the volume of:

A. Solute

B. Solvent

C. Solution

D. Gas only

Answer: C. Solution

9. Which concentration unit is most suitable for colligative property calculations?

A. Molality

B. Molarity only

C. Volume percentage

D. Mole fraction only

Answer: A. Molality

10. The SI-related unit commonly used for molality is:

A. mol L⁻¹

B. mol kg⁻¹

C. kg L⁻¹

D. L kg⁻¹

Answer: B. mol kg⁻¹

11. If temperature increases, the volume of most solutions generally:

A. Decreases

B. Increases

C. Becomes zero

D. Remains exactly unchanged

Answer: B. Increases

12. Which concentration is based on kg of solvent?

A. Molarity

B. Molality

C. Formality

D. Volume fraction

Answer: B. Molality

13. Which concentration is based on litres of solution?

A. Molality

B. Molarity

C. Mole fraction

D. Mass fraction

Answer: B. Molarity

14. If 1 mole of solute is dissolved in 1 kg solvent, molality is:

A. 0.1 m

B. 1 m

C. 10 m

D. 100 m

Answer: B. 1 m

15. Which quantity remains essentially constant when temperature changes?

A. Solution volume

B. Solvent mass

C. Density

D. Molarity

Answer: B. Solvent mass

16. The symbol for molality is:

A. M

B. m

C. N

D. X

Answer: B. m

17. The symbol for molarity is:

A. m

B. M

C. X

D. W

Answer: B. M

18. Which one is a temperature-independent concentration unit?

A. Molarity

B. Molality

C. Volume percentage

D. Normality based on volume

Answer: B. Molality

19. For accurate comparison of solutions at different temperatures, one should prefer:

A. Molarity

B. Molality

C. Volume fraction

D. Density only

Answer: B. Molality

20. Molality is calculated using:

A. Moles / volume

B. Moles / mass of solvent

C. Mass / volume

D. Volume / moles

Answer: B. Moles / mass of solvent

21. Which factor causes the volume of solution to change?

A. Temperature

B. Atomic number only

C. Proton number

D. Neutron number

Answer: A. Temperature

22. Molality is particularly useful in:

A. Colligative properties

B. Atomic structure only

C. Nuclear reactions only

D. Periodic classification only

Answer: A. Colligative properties

23. If 2 moles solute are dissolved in 4 kg solvent, molality is:

A. 0.25 m

B. 0.5 m

C. 2 m

D. 8 m

Answer: B. 0.5 m

24. If the volume of a solution increases while moles remain constant, its molarity:

A. Increases

B. Decreases

C. Remains same

D. Becomes infinite

Answer: B. Decreases

25. Which statement is correct?

A. Molality depends on solution volume

B. Molarity depends on solvent mass

C. Molality depends on solvent mass

D. Both depend only on temperature

Answer: C. Molality depends on solvent mass

26. Which concentration unit is represented by mol kg⁻¹?

A. Molarity

B. Molality

C. Normality

D. Mole fraction

Answer: B. Molality

27. Which concentration unit is represented by mol L⁻¹?

A. Molality

B. Molarity

C. Mole fraction

D. Mass fraction

Answer: B. Molarity

28. The denominator in molality contains:

A. Volume of solution

B. Mass of solvent

C. Mass of solute

D. Volume of solute

Answer: B. Mass of solvent

29. The denominator in molarity contains:

A. Mass of solvent

B. Mass of solute

C. Volume of solution

D. Volume of solvent only

Answer: C. Volume of solution

30. Why is molality preferred over molarity at varying temperatures?

A. It depends on solution volume

B. It depends on solvent mass

C. It depends on density only

D. It depends on temperature directly

Answer: B. It depends on solvent mass

Advertisement