In molality mass of solvent is taken and mass is independent of temperature. In the case of molarity, the volume is taken and volume depends upon temperature change.
03. 100g of 98% H2SO4 has a volume of 54.34 cm3 of H2SO4. Explain.
The density of H2SO4 is 1.84 gcm-3
. So,
v = m / d
d = 100g / 1.84 gcm3
d = 54.34 cm3
04. Relative lowering of Vapour pressure is independent of temperature. Explain.
The relative lowering of the vapour pressure of a solution is equal to the mole fraction of solute or ∆p/p = X2.
χ2 = n2 / n1 + n2
The number of moles of a substance does not depend upon temperature. Hence relative lowering of Vapour pressure is independent of temperature.
05. Colligative properties are obeyed when the solute is non-electrolyte. Explain.
Colligative properties depend upon the number of particles. One mole has 6.02 x 1023 no. of particles. If the solute is volatile the no. of particles increases. For example, if NaCl is dissolved it dissociates to form Na+ and Clients, in this way no. of particles become doubles. So colligative properties don’t obey.
06. Colligative properties are obeyed when the solution is dilute. Explain.
In the case of the concentrated solution, Raoult’s law is not obeyed. When the concentration of solute in a solvent is high, the force of attraction between solute and solvent particles dominates. Due to this Vapour pressure of solvent lowers. All colligative properties are related to each other and show non-ideal behavior at high concentrations.
07. The total volume of the solution by mixing 100cm3 of water with 100 cm3 of alcohol may not be equal to 200cm3 . Justify it.
Water and alcohol mix to form non-ideal solution. The force of attraction between water and alcohol molecules in pure state is greater than the force of attraction between water and alcohol.
08. One molal solution of urea in water is dilute as compared to one molar solution of urea, but the number of particles of the solute is same. Justify it.
In molal solution mass of solvent is 1 kg but in molar solution the total volume of solution is 1 liter. In molal solution water is more than in molar solution. So 1 molal solution is dilute than 1 molar solution but same have one mole of solute.
09. Non-ideal solutions don’t obey the Raoult’s law. Explain.
Non-ideal solutions have more or less intermolecular attractive forces than present in pure components.
Lowering of Vapour pressure is more or less than the expected lowering according to Raoult’s law. Nonideal solutions do not obey ∆p = poX2.
10. Boiling point of solvent increases due to the presence of solutes. Explain.
A liquid boils when its Vapour pressure becomes equal to external pressure. When some solute is added to solvent its Vapour pressure decreases. More heat is required equalize its Vapour pressure to external pressure, so its boiling point increases.
11. Freezing points are depressed due to the presence of solute. Explain.
Freezing point of a substance depends upon its Vapour pressure. It is the temperature at which the Vapour pressure of solid and Vapour phases co-exists and there Vapour pressure become equal. When some solute is added in the solvent the Vapour pressure of solvent decreases so its freezing point also decreases.
12. The boiling point of one molal urea solution is 100.52oC but the boiling point of two molal urea solutions is less than 101.04°C. Explain.
Elevation in boiling point is a colligative property. It obeys only when solution is dilute. In concentrated solutions the solute don’t dissociate completely. Less lowering of Vapour pressure takes place so elevation of boiling point is less than the expected value.
13. Why Beckmann thermometer is used to note the depression in freezing point and elevation of boiling point.
Why Beckmann thermometer is very sensitive. It can read upto 0.01 K. Depression in freezing point and elevation of boiling points are very small temperature changes. So it is used to read these small changes.
14. In summer the antifreezing solutions protect the liquid of the radiator from boiling over. Explain. Or
In winter the antifreezing solutions protect the liquid of the radiator from freezing. Explain.
Addition of antifreeze, for example ethylene glycol in water of the radiator decreases its Vapour pressure.
Decrease in Vapour pressure causes the elevation of boiling point and depression in freezing point. So in summer it protects the water from boiling over and in winter it protects the water from freezing over.
15. NaCl and KNO3 are used to lower the melting point of ice. Explain.
NaCl and KNO3 are used as antifreeze. This mixture depresses the freezing point of ice. This method is used to melt the ice from roads in winter.
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