Chemistry practice Test 05.docx

Chemistry practice test 5 Question 1 The boiling point of benzene is 80OC. Estimate its molar heat of vaporization. Assume that it obeys Trouton's rule. 25.64 kJ mol-1 31.064 kJ mol-1 29.96 kJ mol-1 39.54 kJ mol-1 40.67 kJ mol-1 Correct Answer : b) 31.064 kJ mol-1 Explanation: From the Trouton's law ?H/Tb = 88 J mol-1 K-1 The given data is Tb = 80OC = 80+273 = 353 K Therefore, ?H = (88J mol-1 K-1) (353 K) = 31064 J mol-1 = 31.064 kJ mol-1 Question 2 The force of attraction between gaseous particles is Strong Weak Very strong Very weak Negligible Correct Answer : e) Negligible Explanation: Due to very high kinetic energy, gaseous particles easily escape from the attractions of other particles and tend to maintain very long distances from each other. So, the attraction between gaseous particles is negligible. Question 3 The coordination number of hcp structure is 6 4 8 12 2 Correct Answer : d) 12 Explanation: Each sphere in hcp structure is in contact with 12 neighbouring spheres. Six spheres are on its own layer, three on the above layer and remaining three on the below layer. So, the coordination number of hcp structure is 12. For BCC structure it is 8. For ZnO it is 4. Question 4 Diamond is an example for Ionic crystals Metallic crystals Covalent crystals Molecular crystals None of these Correct Answer : c) Covalent crystals Explanation: In diamond, atoms are linked together by continuous system of covalent bonds. In ionic crystals, atoms are held together by ionic bonds. In metallic crystals, atoms are held together by metallic bonds. In molecular crystals, molecules are held together by weak Van der Waals forces. Question 5 Select the incorrect phrase(s) about the significance of salt bridge I. Connects the solutions of two half-cell reactions. II. It keeps the solution of two half-cells electrically charged. III. Prevents liquid-liquid junction potential. I & II I & III II & III II only III only Correct Answer : d) II only. Explanation: It keeps the solution of two half-cells electrically neutral. In anodic half-cell, positive ions will accumulate around the anode due to deposition of negative ions by oxidation. To neutralise these positive ions, sufficient number of negative ions are provided by salt bridge. Similarly for cathodic half-cell, it is vice-versa. Question 6 Hess's law is related to ________ of the system. Free energy change Entropy change Enthalpy change Internal energy None of these Correct Answer : c) Enthalpy change Explanation: Hess's law states that the enthalpy or heat energy change accompanying a chemical reaction is independent of the pathway between the initial and final states. ?H for a single reaction can be calculated from the difference between the enthalpy (heat) of formation of the product and reactant. ?H0reaction = ? ?Hf0 (products) – ? ?Hf0 (reactant) Question 7 7) Which one of the following molecules is held together by dative bond? AlBr3 NaCl Al2 Cl6 C2 H6 H2 O2 Correct Answer : c)Al2 Cl6 Explanation: In Al2 Cl6 lone pairs of electron from chlorine are donated to electron deficient aluminium in such a way that it is held together by dative bond, where aluminium acts as an electron acceptor and chlorine acts as an electron donor. AlBr3 is a neutral ionic molecule. NaCl- ionic bond, C2 H6 - covalent bond,H2 O2– Hydrogen bond. Question 8 Arrange the following atoms and ions in the increasing order of atomic size Mg, Mg2+, Al, Al3+ Al3+> Al > Mg2+> Mg Mg2+ Mg > Al3+> Al Mg > Mg2+ > Al > Al3+ Mg > Al > Mg2+> Al3+ Al3+> Mg2+ > Al > Mg Correct Answer : d) Mg > Al > Mg2+> Al3+ Explanation: Atomic radii decrease across the period. Cations are smaller than their parent atoms. Among these isoelectronic ions, the one with larger positive nuclear charge will have the smaller radius. So, here the largest atom is Mg and the smallest one is Al3+. Question 9 Which of the following groups has –I effect? -CH3 -C2 H5 -C(CH3)3 –C6 H5 Both a and c Correct Answer : d) –C6 H5 Explanation: The polarisation of the bond is due to electron withdrawing or electron donating effect of adjacent atoms or groups. Such a type of electron displacement along a carbon chain is called Inductive effect. Eg: C –>--- C –>--- C –>--- C6 H5 The electron withdrawing nature of groups or atoms is called negative inductive effect. C6H5 is the only group, which has –I effect. Since --C6 H5 is electron-withdrawing group, it pulls up the electrons towards itself. Thereby it creates a partial positive charge in adjacent carbon atoms and partial negative charge in phenyl group (C6 H5). Question 10 An example for interpolation error is Incorrect identification of indicator's color change in titration Guessing the correct value between two calibrated marks on the metre scale Zero setting of the needle in analog display Calibration of measured instrument None of these Correct Answer : b) Guessing the correct value between two calibrated marks on the metre scale Explanation: Guessing the correct value between two calibrated marks on the metre scale is an example for interpolation error. It is one of the two types of human or personal errors. Incorrect identification of indicator's color change in titration – Operative error. Zero setting of the needle in analog display – static error. Calibration of measured instrument - Instrument error. Question 11 The one which is most commonly used as a detection of developed colorless chromatogram spots in T.L.C plate is Iodine Phosphorus Water Copper salts Ammonia Correct Answer : a) Iodine Explanation: The spots of colorless compounds can be detected by placing the T.L.C plate in a closed jar containing few crystals of iodine. Spots of compounds, which absorb iodine, will indicate as a brown color. Question 12 The Henderson-Hasselbalch equation for acid is pH = pKa- log ([A-] / [HA]) pH = pKa + log ([HA] / [A-]) pH = pKb + log ([B] / [HB+]) pH = pKa+ log ([A-] / [HA]) pH = pKa+ 2log ([A-] / [HA]) Correct Answer : d) pH = pKa+ log ([A-] / [HA]) Explanation: The Henderson-Hasselbalch equation explains whether a compound will exist in its acidic form or in its basic form at a particular pH. [A-] = Molar concentration of a conjugate base. [HA] = Molar concentration of an undissociated weak acid (M). Question 13 An azeotropic mixture is also called Increased boiling mixture Decreased boiling mixture Constant boiling mixture Either a or b None of these Correct Answer : c) Constant boiling mixture Explanation: A pure chemical compound boils at a constant temperature and distils over completely at the same temperature without any change in composition. It is also known as constant boiling mixture. Question 14 A system is said to be in equilibrium at all the times. Such a process is called Irreversible process Equilibrium process Static process Quasi-static process Reversible process Correct Answer : d) Quasi-static process Explanation: A quasi-static process is the one in which the system may be considered to be in equilibrium at all times. Question 15 Which one of the following is an intensive property? Volume Density Mass Energy None of these. Correct Answer : b) Density Explanation: Density is the only intensive property of the above-mentioned properties. All other properties are extensive properties. Intensive property of a system is that which is independent of the amount of the substance present in the system. Extensive property of a system depends upon the amount of substance or substances present in the system.