Find CBSE Class 12 Chemistry Solved Question Paper 2015. These questions not only help the students to prepare for exam in a better manner, but also help them in understanding the depth with which a topic should be studied.
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Few Sample Questions from the Question Paper are given below:
Q. What is the formula of a compound in which the element Y forms hcp lattice and atoms of X occupies 2/3rd of tetrahedral voids.
Let the number of atoms Y form hcp: N
The number of tetrahedral voids would be: 2N
Since 2/3 of tetrahedral voids are occupied therefore number of atom of element X would be: 2N 2/3 = 4/3 N
Ratio of element X: Y = 4/3: 1 or
Simple whole number ratio X: Y = 4: 3
Formula of compound: X4Y3.
Q. Write a method by which lyophobic colloids can be coagulated.
Ans. The common methods used for coagulation of lyofobic colloids are:
(ii) Addition of electrolyte
Addition of electrolyte: As we know that colloids solution bear some charge. When an electrolyte is added to this colloidal solution, the colloidal particles take up ions carrying opposite charge from the electrolyte. As a result, their charge gets neutralised. The neural molecules moves freely, come closer and form bigger molecules hence coagulated or precipitated. For example, if BaCl2 solution is added to As2S3 sol (negatively charged colloid) the Ba 2+ ions get attracted by the negatively charged sol particles and their charge gets neutralised. This leads to coagulation.
Q. Out of white phosphorus and red phosphorus, which one is more reactive and why?
Ans. White Phosphorus is more reactive than red phosphorus. The reactivity difference is due to difference in bonding.
White phosphorus exist as individual molecule made up of 4 phosphorus atom bonded together to form a tetrahedron. Each P4 molecule is held together by weak molecular forces. This is least stable form and hence more reactive form of phosphorus.
On the other hand red phosphorus is polymeric in nature. It forms additional P-P bond with neighbouring atoms and it forms chain like structure. Therefore there exists a chemical bond between individual P4 molecule hence it is more stable and less reactive.
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