CHM256 tutorial 4 PDF

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CHMTutorial 41. Define sampling and explain the purpose of sampling. Sampling is an act of collecting sample to produce a meaningful information. The purpose of sampling is to obtain a representative of the whole sample that can be taken to the laboratory for chemical analysis and the result obtaine...

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CHM256 Tutorial 4 1. Define sampling and explain the purpose of sampling. Sampling is an act of collecting sample to produce a meaningful information. The purpose of sampling is to obtain a representative of the whole sample that can be taken to the laboratory for chemical analysis and the result obtained.

2. The sample must be stored appropriately to avoid contamination after sample was taken. However, there are some problems that encountered during sample storage. List three (3) problems encounter during storage of samples. i.

Losses of analyte by the volatilization

ii. Incomplete dissolution of the analyte iii. Introduction of analyte as a solvent contaminant 3. Explain sampling technique for solid sample. Sampling technique for solid sample solids come in variety of form, each of which is sampled differently. Examples of solid sample are large particulates solid such as ores, metals and alloy, tissue sample, and sample particulates. The easiest way to do sampling technique for solid sample is grab sampling where sample are taken at random and assume to be representative. For reliable results, it is best to take 1/50 to 1/100 of the total bulk. The larger the particle size, the larger the gross sample should be. Easiest way but less reliable because only satisfactory when the sample is homogenous.

4. Discuss the process of the appropriate method of decomposition and dissolution of inorganic materials which will not dissolve in acids. Name the method and list one advantage of using the method. The appropriate method of decomposition and dissolution of inorganic materials which will not dissolve in acids: I. II.

Sample is put in muffle furnace (550- 600°C) Water and volatile compounds are vaporized and organic compounds are burned in the presence of the oxygen in air to CO2, H20, and N2.

III.

Most minerals are converted to oxides, sulfates, phosphates, chlorides, or silicates.

Name of method: Dry Ashing Advantage: Free from contamination since few or no reagents are added. 5. Contamination has been detected in 20 % of baby food samples compared to other foods. Propose a suitable method of dissolution of food sample that can minimize loss of analytes in sample due to retention and volatilization and explain the method. Using wet digestion method. This method usually performed in a Kjeldahl flask. Mixture of HNO3 and H2SO4 is often used. HNO3 is boiled off during the digestion process until only H2SO4 remains and dense, white SO3 fumes are evolved and begin to reflux in the flask. At this point, the solution gets very hot, H2SO4 acts on the remaining organic material. If the organic matter persists, more HNO3 may be added. Digestion is continued until the solution clears.

6. List advantages and disadvantages of method wet digestion and dry ashing. Advantages of wet digestion: superior in term of rapidity Disadvantages of wet digestion: labor-intensive

Advantages of dry ashing: required limited operator attention Disadvantages of dry ashing: adsorbed metals on the vessel may in turn contaminate future sample 7. List two (2) apparatus that must be used to prepared standard solution. i.

Volumetric flask

ii. Analytical balance 8. Give four (4) examples of primary standard and list the important requirement for primary standard reagent. i.

Potassium hydrogen phthalate

ii. Potassium hydrogen iodate iii. Sodium carbonate iv. Sodium tetraborate decahydrate Important requirement for primary standard reagent: i.

Should essentially be available in pure form

ii. Stable towards light and heat iii. React in stoichiometric proportion

9. 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 that will cause contamination of the sample and loss of analytes that want to be analyzed

10. 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: i.

Loss of water from hygroscopic material

ii. Decomposition of sample iii. Sample can be contaminated by foreign matter Ways to overcome the problems: i.

The sample kept in bottles with stoppers

ii.

The bottles or container should be clean and free from impurities

iii.

Avoid using glass container

11. Explain briefly the preparation of 250 mL 10% (w/v) of Ca (NO3)2 in the laboratory.

Weight of solute = (10 g/mL x 250mL) ÷ 100 = 25 g Ca(NO3)2 Preparation: Weigh 25 g Ca(NO3)2 accurately, dissolve in small amount of distilled water, transfer into a 250mL volumetric flask and dilute with distilled water to the mark.

12. Primary standard sodium carbonate solutions containing 0.25 g of Na2CO3 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 Na2CO3 = 106 g/mol)

Titrant: HCl Primary standard: Na2CO3 Balance equation: Na2CO3 + 2HCl → 2NaCl + CO2 + H2O

No. of moles of Na2CO3 = 0.25 g ÷ 106 g/mol = 2.3585 x 10-3 mol

1 mol of Na2CO3 = 2 mol of HCl 2.3585 x 10-3 mol of Na2CO3 = 4.717 x 10-3 mol of HCl. ⸫ The mol of HCl is 4.717 x 10-3 mol.

Molarity of HCl = no. of moles ÷ volume = 4.717 x 10-3 mol ÷ (30.50/1000) L = 0.1547 M

⸫ The molarity of HCl is 0.1547 M....