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SECTION 1 (7%): ATOMS, MOLECULES AND STOICHIOMETRY

SECTION 1 (7%): ATOMS, MOLECULES AND STOICHIOMETRY. Uses of isotopes (e.g. radioactive dating & leak detection) Mass spectrometer (e.g. determination of relative atomic or molecular mass from relative intensities of peaks in mass spectrum)

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SECTION 1 (7%): ATOMS, MOLECULES AND STOICHIOMETRY

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  1. SECTION 1 (7%): ATOMS, MOLECULES AND STOICHIOMETRY

  2. Uses of isotopes (e.g. radioactivedating & leak detection) • Mass spectrometer (e.g. determination of relative atomic or molecular mass from relative intensities of peaks in mass spectrum) • Electrolysis (e.g. calculation involving volume of gas evolved / mass of solid deposited / thickness of electroplating layer, oxidation states of metals)

  3. Ideal gas equation (e.g. calculation of partial pressure & densities for a mixture of gases at 2 different temperature) • Calculation involving volumetric analysis / back titration (e.g. determination of % by mass of active chlorine in bleaching solution, % of mass of N in ammonium fertilizer) • Balancing nuclear reactions

  4. 1.1 THE ATOMIC STRUCTURE • Relative atomic mass: • Atomic number: equal to the number of p_____ in the nucleus • Mass number: number of protons plus no. of neu_____s • Isotopes: e.g. 12C, 13C, 14C; 35Cl, 37Cl

  5. 1.2 RADIOACTIVITY • Radioactivity: the disinte________ of certain unstable nuclei with emission of ra______ • 3 types of radiation from radioactive substance 1.  ray ( particle) 2.  ray (particle) 3.  ray

  6. Nuclear equations: all the following equations which involve changes in the number of neutrons and / or protons are known as nuclear equations. (balancing in both mass and atomic number on both sides) e.g. 9Be + 4He 12C + 1n 9Be + 1H 10B +  baX+dcY feZ b + d = f a + c = e

  7. decay: • mass no.-4, atomic no. –2 •  decay: • mass no. no change, atomic no. +1

  8. Half life: the time for the radiation to be halved, or for the radio-nuclei to be reduced to half. • Radioactive decay: ln Io / It = kt & k = ln 2 / t1/2 (t1/2 : half life)

  9. Uses of radioactivity • Leak detection • radiotherapy:  ray in cancer treatment • nuclear power • tracers: for studying metabolism in living organism • carbon-14 dating

  10. 1.3 RELATIVE ISOTORIC, ATOMIC AND MOLECULAR MASSES • Isotope: atoms with the s______ number of protons but dif_____ numbers of neutrons. • Relative atomic mass: e.g. relative atomic mass of Cl atom = 0.75 * 35 + 0.25 * 37 = 35.5 • Relative molecular mass: e.g. the relative molecular mass of water H2O = (2 * at. mass H) +(at. mass O)

  11. Mass spectrometer: used for the determination of relative masses of particles. It responds to the mass-charge ratio. NOT mass!

  12. Mass spectrum: (i) y-axis  peak height (or relative intensity) is directly proportional to the % abundance of the ionized species (e.g. 14N+ or CO2+), (ii) x-axis  m/e ratio

  13. 1.4 THE MOLE CONCEPT, P-V-T RELATIONSHIPS OF GASES • A mole: the amount of a substance (6.02 * 1023). • Avogadro’s law: equal volumes of gases at same conditions  equal no. of moles. • Molar volume of gas at s.t.p.: 22.4 dm3 in volume. • Ideal gas equation: PV = nRT

  14. Relationship between density & molar mass: the ideal gas equation can be rewritten for the calculation of the gas density (, in g m-3 [or kg m-3]). For a gas of mass m (in g [or kg]) & a molar mass of M (in g mol-1 [or kg mol-1]), a volume of V (in m3) at a pressure P (Nm-2, Pa): PV =nRT &  =m/V  PV = (m/M)RT  P = (/M)RT

  15. Partial Pressure: in a gas mixture, the partial pressure of each component gas is the pressure that the gas would exert if it were the only gas occupying the total volume at that temperature. • Dalton’s Law of Partial Pressure: for three gases 1, 2 and 3 in mixture P total = P1+ P2 + P3 rewritten as P total = (n1+n2+n3)(RT/V) = n total (RT/V)

  16. Mole fraction: the mole fraction (I) of a component in a mixture is defined as the no. of moles of that component over the total no. of moles of gases, i.e. 1 = n1/(n1+n2+n3) = n1/ntotal • Partial pressure of component gas i is given by Pi = i * P total where i = ni/ntotal

  17. 1.5 THE FARADAY AND THE MOLE • Charge [Q] = current [I] * time [t] Unit: C unit: A unit: s • No. of moles of e- = no. of faradays = charge / 96500 C

  18. 1.7 CHEMICAL EQUATIONS AND STOCHIOMETRY • Primary standard: a substance that can be used to prepare a standard solution without standardization (i.e. by dissolving a known mass of that substance in a solvent to give a known volume of the solution, the molarity of that solution can be calculated), e.g. (COOH)2.2H2O [acid], aminosulphonic acid (sulphamic acid) NH2SO3H [acid], anhydrous Na2CO3(s) [base], K2Cr2O7(s) [OA], KIO3(s) [OA], Na2C2O4(s) [RA], Fe(NH4)2(SO4)2.6H2O ammonium iron(II) sulphate(VI)-6-water [RA]

  19. Primary stand must be very pure / with constant composition does not absorb moisture or lose water easily, stable in air, soluble. • Preparation of primary standard: • Standardization involving redox agents: • Preparation of standard I2(aq): • Titration of iodine (in flask) against thiosulphate (in burette) • Standardization of other oxidizing agents: • Back titration

  20. Common Errors in Section 1

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