TEST 1 CHM 256 REVISION QUESTION

By : Khairul Anwar Bin Abu Mansor

QUESTION 1

a)      Analytical chemistry concerned with the chemical characterization of matter and provide the information on “what” and “how much”?. Distinguish between qualtitative analysis and quantitative analysis with reference to one (1) selected sample of your choice.

b)      Give three (3) examples of science disciplines where analytical chemistry is being applied

1.       In medicine

2.       In industry

3.       Environmental quality

4.       The nutritional value of food.

c)       List the seven (7) steps commonly employed in an analytical chemistry

1.       Define the problem

2.       Select a method

3.       Obtain a Representative sample

4.       Prepare the sample for analysis

5.       Perform any necessary chemical separations

6.       Perform the measurement

7.       Calculate the results and report.

QUESTION 2

a)       i)    We can’t analyse the whole sample, so what should we do?

Preparing the sample of for the analysis. By measure the amount of the sample to be analyzes. It’s used to calculate the percent composition from the amount of analyte found. The analytical sample size must be measured to the degree of precision and accuracy required for the analysis.

ii)    Describe the sampling procedure for a heterogonous soil sample

The sample dried by applied some heat on it. Then we, dissolve inorganic solids. After that we destruction of organic materials for inorganic analysis for burning or acid oxidation.

iii)  What are the three (3) factors that may cause contamination or alteration of the sample?

-          container

-          atmosphere

-          light

b)      i)     What is a reagent-grade chemicals?

Chemicals conform to the minimum standards set forth by the Reagent Chemical Committee of the American Chemical Society (ACS) and are used wherever possible in analytical work. Some suppliers label their products with the maximum limits of impurity by the ACS specification; other print actual concentrations for the various impurities.

ii)    Name any three (3) types of grade for chemical reagents

1.       Technical or commercial C.P (Chemically pure)

2.       U.S.P

3.       A.C.S. reagent

4.       Primary standard

iii)   Give five (5) rules of handling chemical reagents.

1.       Select the best grade of chemical available for analytical work. Whenever possible, pick the smallest bottle that will supply the desired quantity.

2.       Replace the top of every container immediately after removal of the reagent; do not rely on someone else to do this.

3.       Hold the stoppers of reagent bottles between your fingers; never set a stopper on a desk top.

4.       Unless specifically directed otherwise, never turn any excess reagent to a bottle. The money saved by returning excesses is seldom worth the risk of contaminating the entire bottle.

5.       Never insert spatulas, spoons, or knives into a bottle that contains a solid chemical. Instead, shake the capped bottle vigorously or tap it gently against a wooden table to break up an encrustation; then pour out the desired quantity.

6.       Keep the reagent shelf and the laboratory balance clean and neat. Clean up any spills immediately, even though someone else is waiting to use the same chemical or reagents

7.       Observe local regulations concerning the disposal of surplus ragents and solutions.

c)       i)     What is a standard solution?

        Solution in which the concentration of a solute is known with high reliability

ii)    Why is sodium hydroxide is not a primary standard?

        Its not stable in water concentration

iii)   Calculate the volume required to prepare 100mL of HCl solution. Given that pHCl = 1.19g/ml, %(w/w)= 37^oC

QUESTION 3

a)      i)     Name the three (3) glassware for accurate measurement

        Burette, Pipette and measuring cylinder

ii)    If you need to dilute a stock standard solution accurately, which type of pipette would you use?

        The volumetric pipette

b)      i)     What is the temperature of the oven recommended to dry sample materials?

        105^oC to 110^oC for 1 or 2 hours

ii)    What is the normal temperature of the muffle furnace for dry ashing?

        400^OC to 700^OC

iii)   Give the commonly acid used for dissolving inorganic solid sample.

        Hydrochloric acid

c)       The following set of chloride analyses on separate aliquots of pooled serum were reported: 103, 106, 108, 109 and 114 meq/L. One value appears suspect.

i)                    Determine if it can be an outlier or otherwise, at the 95% confidence level.

ii)                   Calculate the confidence limit at 95% confidence level.

iii)                 If a true value is 107 meq/L, what is the relative error in parts per thousand?

Titration

By :               Khairul Anwar Bin Abu Mansor

                        Faculty Of Applied Science, Universiti Teknologi MARA Malaysia

Definition

A method of volumetric analysis in which a volume of one reagent (the titrant) is added to a known volume of another reagent slowly form a burette until an end point is reached. The volume added before the end is reached is noted. If one of is reached. If one of the solution has a known concentration, that of other can be calculated.

Process

Titration  os the process of the analyte react with a measured volume reagent of known concentration.

General Tips for accurate and precise titrating

1.       Make sure the stopcock is secured tightly enough to prevent leakage.

2.       Check the tips for air bubbles. Work air bubbles out by rapid opening and closing of the stopcock to squirt the titrant through the tip or taping the tips while the solution is following. No bubbles should be in the barrel of burette.

3.       Initial reading taken by allowing it to drain slowly to the zero mark. The burette read to nearest 0.2ml. The initial reading may be 0.00ml or greater. It’s best taken by placing the finger just in back of the meniscus or by using a meniscus illuminator. Making the reading at eye level.

4.       Put the sample solution in an Erlenmeyer flask.  The flask placed on a white background and the burette tips is positioned within the neck of the flask.

5.       The flask is swirled with the right hand while the stopcock is manipulated with left hand. This grip on the burette maintains a slight inward pressure on the stopcock to ensure that leakage will not occur. More efficiently stirred by means of a magnetic stirrer and stirring bar.

6.       There will be a permanent changes in the colour at the end point when the fraction of a drop is added.

7.       Performed the titration in triplicate

8.       After a titration is complete, unused titrant should never be returned to the original bottle but should be discarded.

The Requirement

1.       In titration the test substance (analyte) in a flask reacts with a reagent added from a burette as a solution of known concentration.

2.       Titrant is known as a standard solution. The volume of titrant required to just completely react with the analyte and the reagent, we can calculate the amount of analyte.

3.       The requirement of a titration are :

-          Reaction must be stoichiometric – There must be a well defined and known reaction between the analyte and the titrant. Example : Titration of acetic acid in vinegar with sodium hydroxide:

                                HC₂H₃O₂  +  NaOH  à NaC₂H₃O₂  +  H₂O

-          Reaction should be rapid.

-          No side reactions, and the reaction should be specific. If there are interfering substances, these must be removed.

-          There should be a marked change in some property of the solution when the reaction is complete.  This may be change in colour of the solution or in some electrical or other physical property of the solution. A colour change is usually brought about by addition of indicator.

-          The point at which an equivalent or stoichiometric amount of titrant is added is called the equivalence point. The point at which the reaction is observed to be complete is called the end point. The end point should coincide with the equivalence point or be at a reproducible interval from it.

-          The reaction should be quantitative. The equilibrium of the reaction should be far to the right so that a sufficiently sharp change will occur at the end point to obtain the desired accuracy.

The General classification of volumetric or tirimetric method

1.       Acid-Base. Many compounds, both inorganic or organic, are either acids or bases and can be titrated with a standard solution of a strong base or a strong acid.  The end points of these titrations are easy to detect.

2.       Precipitation.  In the case of precipitation, the titrant forms an insoluble product with the analyte. Example : Titration of chloride ion with silver nitrate solution to form silver chloride precipitate. Again, indicators can be used to detect the end point, or the potential of the solution can be monitored electrically

3.       Complexometric. In complexometric titrations, the titrant is a reagent that forms a water-soluble complex with the analyte, a metal ions. The titrant is often a chelating agent. Ethylenediaminetetraacetic acid (EDTA) is one of the most useful chelating agents used for titration. It will react with a large number of elements, and reactions can be controlled by adjustment of the pH.

4.       Reduction-Oxidation. Involve the titration of an oxidizing agent with a reducing agent, or vice versa. An oxidizing agent gains electrons and a reducing agent loses electrons in a reaction between them.

Standardization and titration calculations – they are the reverse of one another.

When a titrant material of high or known purity is not available, the concentration of the approximately prepared titrant solution must be accurately determined by standardization; that is by titrating an accurately weighed quantity ( a known number of milimoles) of a primary standard. From the volume of titrant used to titrate the primary standard.

Taking the analyte A in Equation of aA x tT à P

                                Mmol_standard        = 

                                Mmol_titrant            = M_titrant[mmol/mL] x ml_titrant

                                                                = mmol_standard x t/a [mmol_titrant / mmol_standard]

                Mmol_titrant [mmol/ml]     =

Titration Curve

1.       By contracting a titration curve, we can easily explain how the end points of these titrations can be detected; the end point signals the completion of the reaction. A titrations can be detected; the end point signals the completion of the reaction.

2.        A titration curve is a constructed by plotting the pH of the solution as a function of titrant added. The titrant is always a strong acid or a strong base. The analyte may be either a strong base or acid or a weak base or acid.

3.       In the case of a strong acid versus a strong base, both the titrant and the analyte are completely ionized. Example; the titration of HCL and NaOH.

Source

1.        Gary D. Christian (2003) Analytical Chemistry 6^th Edition p: 11, 37, 158, 160, 165, 266