Midterm Exam 2024-1 Answers PDF

1. (6 pts) Consider the following pure substances at the temperature of 25 oC and ambient pressure listed in the Table below. Classify each of them as a homogeneous mixture, a heterogeneous mixture, an ionic compound, a molecular substance, a metal or a non-metal (check all boxes that ap...

1. (6 pts) Consider the following pure substances at the temperature of 25 oC and ambient pressure listed in the Table below. Classify each of them as a homogeneous mixture, a heterogeneous mixture, an ionic compound, a molecular substance, a metal or a non-metal (check all boxes that apply). Hg(l) O3(g) Sea foam P4(s) NH4F(s) Seawater Homogeneous mixture X Heterogeneous mixture X Ionic compound X Molecular substance X X Metal X Non-metal X X 2. (6 pts) a) Indicate the number of electrons, protons and neutrons in the cation 37Cl+: # of protons: ___17____ # of electrons: ___16___ # of neutrons: __20_ b) Write the formula of a species that contains the following subatomic particles. Use the format AXn where n is the charge if different from 0: # of protons: 79 # of electrons: 80 # of neutrons: 118 Answer: __197Au-____ 3. (6 pts) How many molecules are there in 2.30 g of an element with the molecular mass of 123.9 amu? Show your work: Convert grams to moles; note that molar mass is 123.9 g/mol: 2.30g of the substance contain 2.30g / 123.9 g/mol = 0.0186 mol of molecules; Hence, the sample contains 6.022×1023 molecules/mol × 0.0186 mol = 1.12×1022 mol molecules Answer: __1.12×1022___ molecules 4. (6 pts) Consider a mixture of oxygen isotopes of the following composition: 74.1% 18O (isotopic mass 18.00 amu), 1.2% 17O (isotopic mass 17.00 amu) and 24.7 % 16O (isotopic mass 16.00 amu). Calculate the element’s atomic weight in this sample in amu. Show your work: Atomic weight of O is 0.741×18.00 + 0.012×17.00 + 0.247×16.00 = = 13.3 + 0.20 + 3.95 = 17.5 (amu) Answer: ____17.5___ amu 5. (6 pts) One atom of an element, on average, weighs 1.678×10-22 g. Identify the element. Show your work: The molar mass of the element is 6.022×1023 = 101.1 g This corresponds to the element Ruthenium, Ru Answer: ___Ru_____ 6. (6 pts) Consider the following combinations of atoms of two elements below. Provide formulas of the expected ionic compounds. If no ionic compound can result, write N/A. a) O2(g) and Mg(s): _______MgO________________ b) F2(g) and H2(g): ________N/A__________________ c) Cl2(g) and Sc(s): ________ScCl3_______________ 7. (10 pts) Provide a systematic name or a chemical formula for the following pure substances: a) silver carbonate ____Ag2CO3________ b) _____ammonium nitrate_______________________________ NH4NO3 c) ______octaoxygen_________________________________ O8 d) ______phosphorus pentachloride______________________________ PCl5 e) dichlorine monoxide _____Cl2O_________ 8. (6 pts) An excited atom relaxes to its ground state with the emission of a photon of light with the wavelength of 589.6 nm. Calculate the energy in J/mol of the emitted photons. Show your work: Energy of a photon is E = hv and for 1 mol of photons it is 6.022×1023×hv Frequency of an electromagnetic wave is = c/ Hence, the energy of the emitted photons is E = 6.022×1023×hc/ = E = 6.022×1023 mol-1×(6.626×10-34 J×s)×(2.998×108 m×s-1)/589.6×10-9 m = 2.029×105 J/mol Energy: __2.029×105 __ J/mol 9. (8 pts) Indicate the following: a) the number of subshells in the 6th principal electron shell of the hydrogen atom. Answer: ___6_______ b) the maximum number of electrons a 5f-subshell can hold. Answer: ___14_______ c) the values of the orbital quantum numbers n and l for the 5d orbital. Answer: n=__5___, l=__2___ 10. (8 pts) Consider cross-sections of the atomic orbitals below. White and dark colors represent areas with different sign of the wavefunction. For each of the orbitals provide a designation, e.g., 5pz: a) Answer: _2py__ ; x y b) Answer: __3s_. 11. (8 pts) Provide the full orbital diagram for the Se+ ion in its ground-state: 12. (6 pts) Indicate the number of the core, valence and unpaired electrons in the following atoms: Atom; electron configuration # Core e-’s # Valence e-’s # Unpaired e-’s 5 1 24Cr; [Ar] 3d 4s 18 6 6 14 10 2 2 82Pb; [Xe] 4f 5d 6s 6p 78 4 2 13. (8 pts) Arrange the following species according to their increasing a) atomic radius: In, Ar, Si: (smallest) ___Ar < Si < In__ (largest) b) ionic radius: K+, Cl-, Mg2+ (smallest) ___Mg2+ < K+ < Cl ___ (largest) c) ionic lattice energy: KBr, NaF, RbI: (most negative) ____NaF < KBr < RbI____ (least negative) d) 1st ionization energy: Be, Li, Na: (lowest) _____Na < Li < Be_____ (highest) 14. (10 pts) Determine the empirical formula of a naturally occurring boron mineral that contains (by mass) 19.50% Ca, 15.78% B, 2.45% H and 62.27% O. Show your work: 1) Consider exactly 100 g of the compound. It contains 19.50 g Ca which correspond to 19.50 g / 40.08 g/mol Ca = 0.4865 mol Ca 15.78 g B which correspond to 15.78 g / 10.81 g/mol B = 1.460 mol B 2.45 g H which correspond to 2.45 g / 1.008 g/mol H = 2.43 mol H 62.27 g O correspond to 62.27 g / 16.00 g/mol O = 3.892 mol O 2) therefore, the elements’ atomic ratio is Ca : B : H : O = 1 : 3 : 5 : 8 Empirical formula: _CaB3H5O8_