Case Study Questions Class 11 Chemistry Chapter 1 Some Basic Concepts of Chemistry
The identity of a substance is defined not only by the types of atoms or ions it contains, but by the quantity of each type of atom or ion. The experimental approach required the introduction of a new unit for amount of substances, the mole, which remains indispensable in modern chemical science. The mole is an amount unit similar to familiar units like pair, dozen, gross, etc. It provides a specific measure of the number of atoms or molecules in a bulk sample of matter. A mole is defined as the amount of substance containing the same number of discrete entities (atoms, molecules, ions, etc.) as the number of atoms in a sample of pure 12C weighing exactly 12g. One Latin connotation for the word “mole” is “large mass” or “bulk,” which is consistent with its use as the name for this unit. The mole provides a link between an easily measured macroscopic property, bulk mass, and an extremely important fundamental property, number of atoms, molecules and so forth. The number of entities composing a mole has been experimentally determined to be 6.02214179 × 1023.
6.02214179 × 1023, a fundamental constant named Avogadro’s number (NA ) or the Avogadro constant in honor of Italian scientist Amedeo Avogadro. This constant is properly reported with an explicit unit of “per mole,” a conveniently rounded version being 6.022×1023/mol. Consistent with its definition as an amount unit, 1 mole of any element contains the same number of atoms as 1 mole of any other element. The masses of 1 mole of different elements, however, are different, since the masses of the individual atoms are drastically different. The molar mass of an element (or compound) is the mass in grams of 1 mole of that substance, a property expressed in units of grams per mole (g/mol).
The following questions are multiple choice questions. Choose the most appropriate answer :
(i) A sample of copper sulphate pentahydrate contains 8.64 g of oxygen. How many grams of Cu is present in the sample ?
(ii) A gas mixture contains 50% helium and 50% methane by volume. What is the percent by \ weight of methane in the mixture?
(iii) The mass of oxygen gas which occupies 5.6 litres at STP could be
(a) gram atomic mass of oxygen
(b) one fourth of the gram atomic mass of oxygen
(c) double the gram atomic mass of oxygen
(d) half of the gram atomic mass of oxygen
(iv) What is the mass of one molecule of yellow phosphorus? (Atomic mass of phosphorus = 30)
(a)1.993 x 10-22 mg
(b)1.993 x 10-19 mg
(c) 4.983 x 10-20 mg
(d) 4.983 x 10-23 mg
(v) The number of moles of oxygen in 1L of air containing 21% oxygen by volume, in standard conditions is
(a) 0.186 mol
(b) 0.21 mol
(c) 2.10 mol
(d) 0.0093 mol
(i)-(b), (ii)-(d), (iii)-(d), (iv)-(b), (v)-(d)