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SECTION 1 (7%): ATOMS, MOLECULES AND STOICHIOMETRYPowerPoint Presentation

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

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- 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)

- 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

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

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

- 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

- decay:
- mass no.-4, atomic no. –2
- decay:
- mass no. no change, atomic no. +1

- 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)

Uses of radioactivity

- Leak detection
- radiotherapy: ray in cancer treatment
- nuclear power
- tracers: for studying metabolism in living organism
- carbon-14 dating

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)

- Mass spectrometer: used for the determination of relative masses of particles. It responds to the mass-charge ratio.

NOT mass!

- 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

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

- 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

- 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)

- 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

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

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]

- 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

Common Errors in Section 1 does not absorb moisture or lose water easily, stable in air, soluble.

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