Chm256 Tutorial 4-Answer


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CHM256 Tutorial 4 with answer
  Tutorial 4: ANSWER SCHEME 1. Define sampling and explain the purpose of sampling: SAMPLING  is a process to get a representative and homogeneous sample to produce meaningful information. PURPOSE OF SAMPLING  is to obtain a representative samples of the whole sample that can be taken to the laboratory for chemical analysis and the results obtain will be accurate. 2. Describe a sampling technique for solid sample.   To collect a solid sample, coning and quartering method  is used. a. Divide a pile of material into quarter and take a sample from each quarter of the pile and crush these samples to a smaller conical pile. b. Flatten the conical pile and cut into equal quarters. c. Two opposite quarters are chosen at random. d. Then, crush the quarter further. e. The whole steps are repeated until a laboratory samples obtain. 3. Briefly describe the ‘wet digestion’ techniques  and give two examples of reagent used in this technique. Wet Digestion Techniques   A method use to decompose or eliminate an organic material by treatment with strong liquid oxidizing agents or mixture of acids. The acids oxidize organic matter to CO 2 , H 2 O and other volatile products, which are driven off, leaving behind inorganic constituents without losing element to be determined.    PROCESS :   The technique will performed in Kjedahl flask, and undergo in a fume hood.   The sample will treat with acid digestion to achieve a complete dissolution. (mixture of H 2 SO 4  (small amount) and HNO 3 , (large amount)   )    HNO 3   will destroys the bulk of organic matter, but not destroy the last traces.   During acid digestion process, the solution is boiled off and white SO 3  fumes evolve, until H 2 SO 4 remains and dense and destroy the remaining organic matter.   The digestion is continued until producing clear solution.   Reagent used: Mixture of H 2 SO 4  and HNO 3  or mixture of HCl  and HNO 3    4. Give two examples of primary standard and list the properties of primary standard.    Na 2 CO 3   solution (Sodium carbonate)    KHC 8 H 4 O 4   solution (Potassium hydrogen phthalate) Properties of primary standard solution o  must exhibit HIGH PURITY, typically 100 ± 0.05 %. o  must exhibit HIGH STABILITy, must be stable with respect to both spontaneous decomposition and reaction with moisture. o  should not contain water of hydration o  High equivalent weight (more stable) to standardize the solution o  must not be costly. o  easily available. o  must be readily soluble 5. If a water sample is to be analyzed for trace levels of metals, briefly explain why a glass container for sampling and storage is inappropriate .   Metal will undergo retention on a glass container   Metal will react with the vessel or walls of the glass container, this will cause contamination of the sample and loss of analytes that want to be analyzed 6. Explain briefly the preparation of 250 mL 10% (w/v) of Ca (NO 3 ) 2  in the laboratory. 10% = weight of solute (g) / volume solution (mL) x 100 10 g/mL x 250 mL = 25 g Ca(NO 3 ) 2  100 How to prepare : Weigh accurately 25 g Ca(NO 3 ) 2 , dissolve in small amount of distilled water, transfer to a 250 mL volumetric flask and dilute with distilled water to the mark.  7. List advantages and disadvantages of wet digestion and dry ashing. WET ASHING DRY ASHING ADVANTAGES DISADVANTAGES ADVANTAGES DISADVANTAGES Superior in term of rapidity Introduction of impurities from the reagent necessary for the reaction Simpilicity, safe -Losses due to retention to the ashing container or walls of the vessel -Losses due to volatilazation of elements Freedom from loss by retention Free from contaminations since few or no reagents are added 2-4 hour are needed for dry ashing Low level of temperature maintained  Adsorbed metals on the vessel may in turn contaminate future samples 8. An analyst wants to know the level of pesticide residue on the leaves of tobacco plants grown in 100 m 2  plot of land. Describe the correct sampling procedure that should be carried out.   Obtain the representative sample of the leaves of tobacco plants grown in 100 m 2  plot of land   Take a gross sample of the leaves of tobacco plant by taking 1/50 to 1/100 of the total leaves of tobacco plant in a 100 m 2  plot of land   Reduce the gross sample until obtain laboratory samples by using coning and quartering method 9. A river water samples was collected a week before it was analyzed for a trace level of metals. Discuss the problem encountered during storage of the sample and ways to overcome the problem. Problems:   Loss of water from hygroscopic material   Loss of volatile analytes from water samples   Precipitation of metals from water sample   Decomposition of sample   Sample can be contaminated by foreign matter   Side chain reaction and reaction of sample between wall of container occurred may change the properties of sample  To overcome:   The sample kept in bottles with stoppers   The bottles or container should be clean and free from impurities   Avoid using glass container 10. Primary standard sodium carbonate solutions containing 0.25 g of Na 2 CO 3  required 30.50 mL of hydrochloric acid solution for standardization of HCl. Identify the titrant and primary standard and then calculate the molarity of the HCl solution. (molar mass Na 2 CO 3  = 106 g/mol) Titrant : HCl, Primary standard : Na 2 CO 3 Balance equation : Na 2 CO 3 + 2HCl   2NaCl + CO 2  + H 2 O Mol Na 2 CO 3 = 0.25g /106 g/mol = 2.3585 x 10 -3  mol 1 mol Na 2 CO 3 ≈ 2 mol HCl  2.3585 x 10 -3  mol Na 2 CO 3 x 2 mol HCl = 4.717 x 10 -3  mol HCl 1 mol Na 2 CO 3 Molarity HCl = Mol/L = 4.717 x 10 -3  mol HCl/ 30.5 x 10 -3  L = 0.155 M
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