CBSE Class 12 Chemistry Solved Question Paper 2018

CBSE Class 12 Chemistry Exam 2019 is scheduled to be conducted on 12 March, 2019 (Tuesday). While preparing for the board exams, every student is keen to know the questions asked in the previous years and they also search for the appropriate solutions to all those questions.

In this article, we are providing the CBSE Class 12 Chemistry Solved Question Paper 2018. This paper will not only help to know the important questions but also to write proper explanation to them. We have provided a detailed solution to each question that involves the appropriate key points as mentioned in the CBSE marking scheme.

All the solutions have been collated in the form of PDF which students may download to check the right criteria to write perfect exam and score high marks.

Some of the questions and their solutions from the CBSE Class 12 Chemistry Solved Question Paper 2018,are given below:

Q. Give reasons for the following:

(a) Measurement of osmotic pressure method is preferred for the determination of molar masses of macromolecules such as proteins and polymers.

(b) Aquatic animals are more comfortable in cold water than in warm water.

(c) Elevation of boiling point of 1M KCl solution is nearly double than that of 1 M sugar solution.

Ans.

(a) Reasons to use osmotic pressure method are as follows:

• As compared to other colligative properties, magnitude of osmotic pressure is large even for very dilute solutions
• Osmotic pressure can be measured at room temperature whereas other colligative properties need either high temp or low temp conditions. 
• Macromolecules are generally not stable at higher temperatures and polymers have poor solubility.
• Values of other colligative properties are very small that a small error can account to large difference.
• In this method, molarity of the solution is used instead of molality.

(b) Because oxygen is more soluble in cold water or at low temperature.

(c) 1 mol of KCl ionises into 1 mol of K⁺ and 1 mol of Cl^–. So, the total number of ions get doubled. Since, elevation in boiling point is a colligative property (depends on no. of molecules),so it gets doubled in case of KCl and remains same in case of glucose which is a non-electrolyte.

Q. What happens when

(a) a freshly prepared precipitate of Fe(OH)₃ is shaken with a small amount of FeCl₃ solution?

(b) persistent dialysis of a colloidal solution is carried out?

(c) an emulsion is centrifuged?

Ans.

(a) Peptisation occurs which leads to the formation of a colloidal solution of Fe(OH)₃.

(b) Coagulation occurs.

(c) Demulsification of immiscible mixture occurs which leads to the separation of the constituent liquids.

CBSE Class 12 Mathematics Solved Question Paper 2018

Q. Give reasons :

(a) E^o value for Mn³⁺ /Mn²⁺ couple is much more positive than that for Fe³⁺/Fe²⁺.

(b) Iron has higher enthalpy of atomization than that of copper.

(c) Sc³⁺ is colourless in aqueous solution whereas Ti³⁺ is coloured.

Ans.

(a) E^o value for Mn³⁺ /Mn²⁺ couple is much more positive due to the fact that Mn⁺² has higher stability then Mn⁺³ due to half filled d⁵ configuration.

(b) Iron has higher number of unpaired electrons than that in copper due to which extent of covalent bonding is more hence enthalpy of atomization is more.

(c) It is due to the absence of unpaired d-electron in Sc³⁺ whereas in Ti³⁺ there is one unpaired electron which shows d-d transition imparting colour to its solution.

Q. (a) Why is bithional added to soap?

(b) What is tincture of iodine? Write its one use.

(c) Among the following, which one acts as a food preservative?

       Aspartame, Aspirin, Sodium Benzoate, Paracetamol

Ans.

(a) Bithional is added to soap to impart antiseptic properties.

(b) Tincture of iodine is 2-7 % elemental iodine along with potassium iodide or sodium iodide in mixture of ethanol and water.

Use of tincture of iodine: It is used as an antiseptic and is applied on minor cuts, scraps and burns to prevent infection.

(c) Sodium Benzoate work as food preservative

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