2025 DHS H1 Chem Prelim Paper 1_Worked Solutions

2025 DHS Preliminary Examination H1 Chemistry 8873 Paper 1 Suggested Solutions © DHS Chemistry Unit Page 1 of 4 Answer Key 1 2 3 4 5 6 7 8 9 10 D D B B D A C D C B 11 12 13 14 15 16 17 18 19 20 D B A A A B C A C C 21 22 23 24 25 26 27 28 29 30 B D A B A D C D B B 1 D Since the gas is diatomic, the number of molecules of this gas in the 5 g sample = (3.76 x 1022)  2 = 1.88 x 1022 Therefore, the number of moles of this gas = (1.88 x 1022)  (6.02 x 1023) = 0.03123 Thus, Mr of the gas = 5  (0.03123) = 160 2 D  A Since both particles have a positive angle of deflection, they are deflected in the same direction towards the same plate. Similar to 4He2+, X must be positively charged.  B The beam of positively charged particles X travels in a curved path towards the negatively charged plate.  C Charge to mass ratio of 12C+ = + 1 12 Charge to mass ratio of 4He2+ = + 2 4 = + 1 2 Since angle of deflection is proportional to charge to mass ratio, the angle of deflection of 12C+ is expected to be 1 6(1) = +0.17  D None of the above options are correct. 3 B  A Element with el ectronic c onfiguration of 1s22s22p63s23p1 is Al. Al can lose three electrons to form Al3+.  B Element with el ectronic c onfiguration of 1s22s22p63s23p3 is P. P can gain three electrons to form P3–.  C Element with el ectronic c onfiguration of 1s22s22p63s23p63d14s2 is Sc. Sc can lose three electrons to form Sc3+.  D Element with el ectronic c onfiguration of 1s22s22p63s23p63d74s2 is Co. Co tends to lose electrons to form simple ions such as Co2+ or Co3+. 4 B 1. Determine the number of neutrons in Sr (find proton number of Sr using Data Booklet.) ⇒ 84 – 38 = 46 2. Find number of electrons present in Sr 2+ (using proton number of Sr given in Data Booklet.) ⇒ 38 – 2 = 36 ⇒ W has 36 electrons 3. Find number of protons present in W W: 36 protons 4. Calculate the nucleon number of W W: 36 + 46 = 82 5 D  A Electronic c onfiguration of Cr atom is 1s22s22p63s23p63d54s1. There are six unpaired electrons.  B Electronic c onfiguration of C u+ ion is 1s22s22p63s23p63d10. There are no unpaired electrons.  C There are no unpaired electrons.  D There is one unpaired electron on the N atom in the NO2 molecule. 6 A (1 and 2 only)  1 C=O and C=S are polar covalent bonds due to the difference in electronegativity between the atoms.  2 The dipole moments of the polar bonds are equal and opposite in O=C=O and S=C=S molecules so they cancel out and the molecules are non -polar. There exists only instantaneous dipole –induced dipole (id –id) interactions between these non –polar molecules. As O is more electronegative than S, C=O bond is more polar than C=S bond and there is a net dipole moment in O=C=S. Hence, COS molecule is polar and it has both id –id and permanent dipole –permanent dipole interactions between molecules