MPSC State Service 2021 · CSAT · Question 31 of 73

Passage: Read the following passage and answer the questions. The mol is a SI unit of the amount of a chemical species/substance. It is always associated with the formula and contains 6.022 × 1023 Avogadro՚s number of particles of the species represented by that formula. The molar mass of any chemical substance is the mass in gram of one mol of that substance, this can be obtained by adding the atomic weights of the component elements present in the formula of the substance (eg. molar mass of HCl is obtained by adding atomic weight of H i.e. 1 and Cl i.e. 35.5 36.5 g). The molar volume, i.e. 22.4 dm3 is the volume of one mol of any gas (assumed to behave ideally) at 273 K and 1 atm = 101.325 kPa pressure (STP). This volume can be converted to the volume under any condition P1V 1 P2V 2 of temperature and pressure by using general gas equation T1 = T2 . Chemists express the concentration of chemical species on the basis of volume (temperature dependent) i.e. molarity (M), normality (N), formality (F) etc. as well as mass (temperature independent) i.e. molality (m), mole fraction (χ) etc. The molar concentration, M, of a solution of any chemical species is the number of mol of that species present per dm3 of the solution. Normality, N, is a more convenient unit of concentration as the number of equivalents of an analyte (unknown concentration) present = the number of equivalents of the standard reagent (known concentration) added. As a consequence, the stoichiometry of the reaction need not be derived every time a titrimetric analysis is performed. Normality is the number of equivalents of the solute present per dm3 of the solution. The number of equivalents is the ratio of weight in gram of the solute to the equivalent weight of that solute. The equivalent weight of the solute then needs to be known which is defined as “the weight which contains, combines with or displaces one part by weight of hydrogen”. Commercially available reagents i.e. acids and bases are highly concentrated. To use them in laboratory for routine experiments, one needs to dilute the solutions with definite quantity of water. The total number of equivalents or mol in both, concentrated and dilute solution, must be the same is the basis of this dilution. The mass based concentration units are molality, m, i.e. the number of mol of solute dissolved in a kg of solvent and mole fraction, χ, i.e. the fraction of the total number of mol of that component present in the given solution. Atomic weights: (H = 1, He = 4, Ne = 20, N = 14, O = 16, S = 32, CI = 35.5, Ar = 40, Ag = 108) 7g of gaseous nitrogen N2 occupies ________ dm3 at 546 K and 2 atm.

Reading Comprehension

1. A5.6Correct
2. B11.2
3. C22.4
4. D313.6