CBSE Class 12 Chemistry Solutions Worksheet Set A

Question. An ‘A’molal solution of a compound in benzene has mole fraction of solute = 0.2. The value of A is
(a) 1.4
(b) 14.0
(c) 3.2
(d) 2.0
Answer. C

Question. What will be the normality of resulting solution in which 5 mL of MHCl, 20 mL of M/2 H₂SO₄ and 30 mL of M/3 HNO₃ are mixed together and volume made to 1 L ?
(a) M/40
(b) M/10
(c) M/20
(d) M/5
Answer. A

Question. Which of the following aqueous solutions has the highest concentration of sodium ion?
(a) Solution having 15.4 mg Na⁺ /mL
(b) Solution having 2.07 g NaCl/100 mL
(c) 0.208 M Na SO ₂ 4
(d) All of these have equal concentration of Na+
Answer. A

Question. When bottles A and B have 2Mand 2maqueous solutions of sulphuric acid respectively, then
(a) A is more concentrated than B
(b) B is more concentrated than A
(c) The concentration of both A and B are equal
(d) It is not possible to compare the concentrations
Answer. A

Question. The vapour pressure of a liquid in a closed container depends upon
(a) surface area of the container
(b) amount of liquid
(c) temperature
(d) None of these
Answer. C

Question. H₂S gas is used in qualitative analysis of inorganic cations. Its solubility in water at STP is 0.195 mol kg ^{- 1}. Thus, Henry’s law constant (in atm molal ^{- 1}) for H₂S is
(a) 3.826 × 10³
(b) 5.128
(c) 1.80 ×10²
(d) 2.628 × 10^{- 4}
Answer. B

Question. A solid dissolves in water when ...... is ...... than hydration energy.
(a) lattice energy, more
(b) ionisation energy, less
(c) lattice energy, less
(d) ionisation energy, more
Answer. C

Question. At higher altitudes the boiling point of water is low because
(a) temperature is low
(b) atmospheric pressure is low
(c) atmospheric pressure is high
(d) temperature is high
Answer. B

Question. When two liquids A and B form an ideal solution, then
(a) the Gibbs free energy is zero
(b) the entropy of mixing is zero
(c) the Gibbs free energy and entropy of mixing both are zero
(d) the enthalpy of mixing is zero
Answer. D

Question. The statement ‘‘The partial vapour pressure of each volatile component in the solution is directly proportional to its mole function’’ is
(a) Henry’s law
(b) Raoult’s law
(c) Dalton’s law
(d) None of these
Answer. B

Question. What will the value of DVmix if a solution of CHCl₃ is mixed with a solution of acetone?
(a) DVmix = + ve
(b) DVmix = - ve
(c) DV = 0
(d) Cannot be predicted
Answer. B

Question. Identify the substance which exerts lowest vapour pressure.
(a) Kerosene
(b) Water
(c) Mercury
(d) Ethanol
Answer. C

Question. What will be the mole fraction of the solute when the vapour pressure of a dilute aqueous solution of glucose is 750 mm Hg at 373 K?
(a) 1/7.6
(b) 1/76
(c) 1/46
(d) 1/38
Answer. B

Question. Osmosis is involved in processes given below:
I. Interchange of nutrients and waste products between tissue cells and their surroundings.
II. Reverse osmosis
III. Excretion of urine
IV. Evaporation
Select the correct processes.
(a) I and III
(b) I, II and III
(c) I, II and IV
(d) All of these
Answer. B

Question. The increasing order of osmotic pressure of equimolar solutions of BaCl₂ , NaCl and glucose will be
(a) NaCl > BaCl₂ > glucose
(b) Glucose > BaCl₂ > NaCl
(c) BaCl₂ > NaCl > Glucose
(d) NaCl > Glucose > BaCl₂
Answer. C

Very Short Answer

Question. Define solvent?
Answer. The component that is present in the largest quantity is called solvent.

Question. How can we describe the composition of a solution?
Answer. Composition of a solution can be described by expressing its concentration.

Question. Why mole fraction unit is useful?
Answer. Mole fraction is useful when two reactive components are mixed together, as the ratio of the two components is known if the mole fraction of each is known.

Question. Define solution?
Answer. Solutions are homogeneous mixtures of two or more than two components.

Question. What is solute?
Answer. One or more components present in the solution other than the solvent are called solutes.

Short Answer

Question. Calculate the molarity of a solution containing 6g of NaOH in 450mL solution?
Answer. Molar mass of NaOH 39.97 mL, so we can take 40g mol
Moles of NaOH = 6g / 40g mol^-1 = 0.15 mol
Volume of the solution in litres = 450mL / 1000 mL L^-1
Molarity = 0.15 mol * 1000mL L^-1 / 450 mL
= 150 / 450 = 0.333 M
= 0.333 mol L^-1
= 0.333 mol dm^-3.

Question. Define solubility?
Answer. Solubility is a property referring to the ability for a given substance, the solute, to dissolve in a solvent. It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution.

Question. What do you understand by Henry’s law?
Answer. Henry’s law states that at a constant temperature the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid or solution.

Question. What do you mean by the Le Chatelier’s principle?
Answer. Le Chatelier's principle is an observation about chemical equilibria of reactions. It states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.

Question. What are the applications in industry and some biological phenomena found by the Henry’s law?
Answer. 
1. To increase the solubility id CO₂ in soft drinks and soda water the bottle is sealed under the high pressure.
2. At high altitudes the partial pressure of oxygen is less than that at the ground level. This leads to the low concentration of oxygen in the blood and the tissues of people living at the high altitudes or climbers.
3. Scuba divers must cope with high concentrations of dissolved gases while breathing air at the high pressure underwater. Increased pressure increase the solubility of the atmosphere gases in the blood.

Long Answer

Question. State the difference between saturated solution and supersaturated solution?
Answer. 
1. A saturated solution is a solution is a chemical solution containing the maximum concentration of a solute dissolved in the solvent whereas a supersaturated solution is a solution which contains more dissolved solute than could ordinarily dissolve into the solvent.
2. A saturated solution can be prepared by evaporating a solvent from a solution until the solute begins to crystallize or precipitate whereas a supersaturated solution can be prepared by adding the solute in excess at a high temperature and suddenly cooling down at a rate that is faster than the precipitation of the solute.
3. A saturated solution has the maximum amount of solutes that can be dissolved in the solvent at room temperature whereas a supersaturated solution has more than the maximum amount of solutes that can be dissolved in the solvent at room temperature.
4. A saturated solution does not show precipitation at the bottom of the container whereas a supersaturated solution shows precipitation at the bottom of the container if the physical conditions are changed.

Question. State the difference between solution and solute?
Answer. 
1. A solvent is a substance in which different compounds can be dissolved in order to form a solution whereas a solute is a substance that can be dissolved in a solution.
2. Solubility of a solute in a solvent depends on the solvent properties such as polarity whereas solubility of a solute in a solvent depends on solute properties such as surface area of the solute.
3. Solvents are in liquid phase almost all the time, but there can be solid or gaseous solvents as well whereas solutes are found in solid, liquid or gaseous phase.
4. The boiling point of a solvent is usually lower than that of a solute whereas the boiling point of a solute is usually higher than that of a solvent.

Question. Explain the types of solutions?
Answer.  The types of solution are:
1. Solid solution: Solid solution, mixture of two crystalline solids that coexist as a new crystalline solid, or crystal lattice. Much of the steel used in construction, for example, is actually a solid solution of iron and carbon. There are two classes of solid solutions, substitutional solid solutions and interstitial solid solutions.A solid solution is formed by mixing a foreign element B (called an impurity or solute) with a perfect crystalline element A (called the host or solvent) such that the atoms of B share the various crystal sites of element A.
2. Liquid solution: If the solvent is a liquid, then almost all gases, liquids, and solids can be dissolved, for example: Gas in liquid, oxygen in water, Carbon dioxide in water, because the solution is accompanied by a chemical reaction (formation of ions). The visible bubbles in carbonated water are not the dissolved gas, but only an effervescence of carbon dioxide that has come out of solution; the dissolved gas itself is not visible since it is dissolved on a molecular level.
3. Gaseous solution: If the solvent is a gas, only gases are dissolved under a given set of conditions. An example of a gaseous solution is air (oxygen and other gases dissolved in nitrogen). A solution in which a gaseous solute is dissolved in a gaseous solvent. Other solutes include argon, carbon dioxide, neon, helium and methane. Some trace solutes include ozone, carbon monoxide and ammonia.

Question. What are the several ways to describe the concentration of the solution quantitatively?
Answer. The several ways to describe the concentration of the solution quantitatively are:
1. Mass percentage: Mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100% i.e. mass of the component in the solution/ total mass of the solution * 100.
2. Volume percentage: volume percentage is a measure of the concentration of a substance in a solution. It is expressed as the ratio of the volume of the solute to the total volume of the solution multiplied by 100 i.e., volume % of a component = volume of the component/ total volume of solution *100.
3. Mass by volume percentage: Another version of a percentage concentration is mass by volume percent, which measures the mass or weight of solute in grams (e.g., in grams) vs. the volume of solution (e.g., in mL). It is commonly used in medicine and pharmacy.
4. Parts per million: when a solute is present in the trace quantities it is convenient to express concentration in parts per million. Parts per million = number of parts of the component/ total number of parts of all components of the solution * 106.
5. Mole fraction: Mole fraction is a unit of concentration, defined to be equal to the number of moles of a component divided by the total number of moles of a solution, because it is a ratio, mole fraction is a unit less expression, mole fraction of a component = number of moles of the component / total number o moles of all the components.
6. Molarity: Molarity is used to express the concentration of a solution. Also known as molar concentration, molarity is the number of moles of solute per liter of solution molarity = moles of solute/ volume of solution in litre.
7. Molality: Molality is a property of a solution and is defined as the number of moles of solute per kilogram of solvent. The SI unit for molality is mol/kg. Molality
= moles of solute/ mass of solvent in kg.

Question. State the difference between molarity and molality?
Answer. 
1. Molarity is moles of solute dissolved in 1 litre of solution whereas molality is moles of solute dissolved in 1kg of moles.
2. Molarity formula is moles / litre whereas molality formula is mole / kg.
3. Molarity is denoted by M whereas molality is denoted by m.
4. Molarity may be imprecise or inaccurate whereas molality is very precise and accurate.
5. Molarity is affected by changes in the pressure whereas molality is not affected by changes in the pressure.
6. Molarity is affected by changes in temperature whereas molality is not affected by changes in temperature.

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