Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 7

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Q: 8. An antifreeze solution is prepared from of ethylene glycol and of water. Calculate the molality of the solution. If the density of the solution is , then what shall be the molarity of the solution?

Answer:

Molar mass of ethylene glycol

Number of moles of ethylene glycol

= 3.59 mol

Therefore, molality of the solution

Total mass of the solution

Given

Density of the solution

∴ Volume of the solution

⇒Molarity of the solution =

Q: 9. A sample of drinking water was found to be severely contaminated with chloroform supposed to be a carcinogen. The level of contamination was 15 ppm (by mass):

(i) Express this in percent by mass

(ii) Determine the molality of chloroform in the water sample.

Answer:

(i) 15 ppm (by mass) means 15 parts per million of the solution.

Therefore, percent by mass

(ii) Molar mass of chloroform

Now, according to the question,

of chloroform is present in of the solution.

i.e., 15 g of chloroform is present in of water.

∴ Molality of the solution

Q: 10. What role does the molecular interaction play in a solution of alcohol and water?

In pure alcohol and water, the molecules are held tightly by a strong hydrogen bonding. The interaction between the molecules of alcohol and water is weaker than alcohol-alcohol and water-water interactions. As a result, when alcohol and water are mixed, the intermolecular interactions become weaker and the molecules can easily escape. This increases the vapour pressure of the solution, which in turn lowers the boiling point of the resulting solution.

Q: 11. Why do gases always tend to be less soluble in liquids as the temperature is raised?

Answer:

Solubility of gases in liquids decreases with an increase in temperature. This is because dissolution of gases in liquids is an exothermic process.

Therefore, when the temperature is increased, heat is supplied and the equilibrium shifts backwards, thereby decreasing the solubility of gases.

Q: 12. State Henry's law and mention some important applications?

Answer:

Henry's law states that partial pressure of a gas in the vapour phase is proportional to the mole fraction of the gas in the solution. If p is the partial pressure of the gas in the vapour phase and x is the mole fraction of the gas, then Henry's law can be expressed as:

Where,

is Henry law constant

P = partial pressure of the gas in the vapour phase

X = mole fraction of the gas

Some important applications of Henry’s law are mentioned below.

(i) Bottles are sealed under high pressure to increase the solubility of in soft drinks and soda water.

(ii) Henry's law states that the solubility of gases increases with an increase in pressure. Therefore, when a scuba diver dives deep into the sea, the increased sea pressure causes the nitrogen present in air to dissolve in his blood in great amounts. As a result, when he comes back to the surface, the solubility of nitrogen again decreases and the dissolved gas is released, leading to the formation of nitrogen bubbles in the blood. This results in the blockage of capillaries and leads to a medical condition known as 'bends' or 'decompression sickness'.

Hence, the oxygen tanks used by scuba divers are filled with air and diluted with helium to avoid bends.

(iii) The concentration of oxygen is low in the blood and tissues of people living at high altitudes such as climbers. This is because at high altitudes, partial pressure of oxygen is less than that at ground level. Low-blood oxygen causes climbers to become weak and disables them from thinking clearly. These are symptoms of anoxia.