The experimentally determined molar mass for what type ofsubstances is always lower than the true value when water is used as solvent. Explain. Give one example of such a substance and one example of a substance which does not show a large variation from the true value.
Answers
Experimentally determined molar mass of a substances that dissociates into two or more fragments on addition of water is always lower than the original molar mass of the substance.
# This anomaly occurs since the molar mass is experimentally determined with the help of methods that involve measurement of the elevation in boiling point or the depression in freezing point.
# We do not take the Vont Hoff factor(the i value) into consideration during our calculations, which results in the experimentally determined molar mass to be lower than the actual one.
#Molar mass(experimental)/Molar mass(actual)=1/i; where i is the vont hoff factor.
#NaCl is an example of such a substance since it dissociates into Na+ and Cl- ions on addition of water.
#An example of a substance that does not show large variation from the true value can be one that does not either dissociates or associates on addition of water to it.Glucose is an example of such a substance.
The experimentally determined molar mass for substances with lower colligative value is always lower than the true value when water is used as solvent.
Explanation:
- When the solute is dissolved in solvent, then there is dispersion of ions in the solvent. Thus, the molar mass is lower than the true value.
- Due to dissociation, the number of particle increases.
- The colligative property depends on the number of particles in the solvent. If the colligative property is higher than true value.
- The ratio between colligative property and calculated value is called Vant Hoff factor.
Example of substance:
- The ionic compounds such as KCl and NaCl always have molar mass lower than true value.
- The organic compound such as glucose does not show much variation from the true value.