REVISION NOTE ON PRACTICAL

REVISION NOTE ON PRACTICAL

Definition of some terms

  1. Concentration: This is the amount of substance dissolved in a known volume of solution. It can be expressed as:
  2. Mass concentration (gdm-3)
  • Amount of substance dissolved in Idm3 of solution
  1. Molar concentration (moldm-3)

  1. Percentage concentration
  • Amount of substance dissolved in 100cm3 of solution e.g 10% glucose solution is 10g glucose in 100cm3
  1. Primary Standard: This is a soluble substance that can be obtained in a high degree of purity and stable when exposed to the atmosphere. e.g Na2CO3, C6H5COOH, Nacl, H2C2O4
  2.  Tools for titration:
    a. Birette, pipette, conical flask, beaker, funnel, retort stand and clamp, white file, indicator bottle
    4. Some measures taken during titration to ensure accuracy:
    a. Rinse Pipette with alkali before using in drawing alkali
    b. Rinse birette with acid before filling for use
    c. Ensure birette jet is filled
    d. Ensure birette tap is not leaking
    e. Avoid parallax errors when taking birette readings
    f. Obtain concordant titre valves for calculation
    g. Use 2-3 drops only of indication
    5. Common indicators used during titration
    a. Mettyl Orange b. Mettyl red c. Phenolphthalein d. Screened mettyl Orange e. Bromothymol blue
  3. Example of

Strong acid        H2SO4, Hcl, HNO3

Strong base       NaOH, KOH

Weak acid         CH3COOH, H2CO3, H2C2O4, 2H2O

Weak alkali       NH3(aq) Na2CO3

For a titration involving

  • Strong acid/base –     Any indicator can be used
  • Strong acid/weak base –         mettyl orange, mettyl red, screened mettyl Orange
  • Weak acid/strong base –         Phenolphthalein
  • Weak acid/weak base      –         No suitable indicator

SOME NOTES ON QUANTITATIVE ANALYSIS

  1. Gases are given off when substances are heated or when a mineral acid is added to a substance.

Types of gases –

  1. acidic gas e.g CO2, SO2, H2S, NO2, Hd
  2. Alkaline – NH3
  3. Neutral – O2, H2, H2O
  4. Acidic/bleaches – Cl2, Bv2, I2

Also gases can be classified based in Colour:

  1. Colourless gases – CO2, SO2, Hcl, H2S, O2, H2O
  2. Brown or reddish brown – NO2, Bv2
  3. Greenish yellow – Cl2
  4. Violet/purple – I2

Also odour can be used to classify gases:

  1. Odourless – H2, O2, H2O, CO2
  2. Irritating/choking/pungent – SO2, NO2, HCl, Cl2, NH3, Cl2
  3. Rotten egg smell – H2S

During analysis of salts, an eggervescence indicates evolution of a gas. The colour and odour must be noted. If a gas bites the nose, describe as choking, pungent or irritating as the case may be. Where you cannot see the gas but there is evidence of gas coming off, describe as colourless. Where there is no evidence of gas evolving write no eggervescence. Gases emanate from the anion. Always indicate the anion from which the gas comes from e.g CO2(g) from CO32-

  1. SOLUBILITY OF SOME COMMON SUBSTANCE IN WATER
  2. All salts of Na K and NH4 are soluble in water
  3. All trioxonitrate (v) salts are soluble
  4. All tetraOxosulphate (VI) salts are soluble except Pb, Ba, Ca is slightly soluble in water
  5. All chloride salts are soluble except Pb, Hg and Ag
  6. All trioxocarbonate (IV) salts are insoluble in water except those of Na, K and NH4
  7. All trioxoxulphate (IV) salts are insoluble except Na, K and NH4
  8. All sulphide salts are insoluble in water except Na, K, NH4, Ca, Mg and Ba

 

  1. Reagents commonly used to identify cations
  2. NaOH(aq) – Preliminary test
  3. NH4OH(aq) – Confirmatory test
  4. H2S
  5. H2SO4

The commonly used ones in our laboratory are NaOH(aq) and NH4OH.

The commonly identifiable cations in our school laboratory are – Zn2+, Ca2+, Fe2+, NH4+, CU2+, Al3+, Pb2+

Common anions identified in the laboratory include

CO32, SO32, SO42, S2, 52, NO3, Cl, HCO3

Common reasents used in the laboratory to identify anions include Bacl2 or BaNo3, PbNO3, Pb (CH3COO)2, AgNO3, NH4OH, H2SO4, KMNO4, FeSO4, HCl, HNO3

Exercises

  1. C is an inorganic salt of sodium. The tests recorded in the table were performed as indicated copy and complete the table as appropriate.
S/N Test Observation Inference
a. (+ water, stirred thoroughly)
b. C+Cinc.H2SO4 + heat Colourless gas with pungent smell evolved. Gas turned blue litmus paper red and gave dence white fumes with concentrated ammonia
c. C+Cinc.H2SO4 + MnO2 + heat
d. Aqueous Solution of C+ litmus

 

  1. State one drying agent for chlorine

What precaution should be taken in the laboratory to prevent excessive inhalation of chlorine gas during its preparation

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