CLS 231 questions



 

 

1. Both classical and instrumental methods can be used to determine the concentration of a specific element or species in a compound. Which of the following pairs is the WRONG combination of the type of analysis with its characteristic? 

Gravimetric analysis - mass

Potentiometric analysis - voltage

Volumetric analysis - volume

Spectrophotometric analysis – density

Actually, spectrophotometric exploits the light properties of chemical compounds. In a spectophotometric analysis, one measures how much light is absorbed by a certain solution 

 

 

 

2. The unit molarity (mole per liter) is used to express the concentration of a certain species of chemical element in a concentrated solution. Meanwhile, other more appropriate units are more commonly used to describe the concentration of more diluted solutions. One of these units, which is equivalent to milligram per liter, is? 

Part per million (ppm)

Part per billion (ppb)

Part per thousand (ppt)

Part per trillion (pptr)

For example, a solution with a concentration of 1.5 ppm sodium is interpreted as there is only 1.5 milligram of sodium in 1 liter of solution.
The concentration of salt (sodium chloride) in typical sea water is around 32-35 ppt. 1 ppt (part per thousand) means 1 milligram per 1 liter of solution) 

 

 

 

 

4. In chemical equilibrium, a principle states that if a stress (for example, a change in concentration, pressure, temperature or volume of the vessel) is applied to a system in equilibrium, the equilibrium will shift in such a way to lessen the effect of the stress. This principle is called?

The Hess Principle

The Aufbau Principle

The Le Chatelier’s Pinciple

The Robert Principle

 

 

 

 

3. Many statistical tests are performed for the data obtained in analytical chemistry. One of the tests decides whether a given datum (either the value is too big or too small compared to the other data) can be rejected or not. This test is?

F test

Q test

T test

A test

The F test is used to test whether the standard deviations for 2 sets of data are significantly different from each other. On the other hand, the T test, also known as the Student T test, is used to compare the obtained value in experiments with the real (actual) value

 

4. In chemical equilibrium, a principle states that if a stress (for example, a change in concentration, pressure, temperature or volume of the vessel) is applied to a system in equilibrium, the equilibrium will shift in such a way to lessen the effect of the stress. This principle is called?

The Hess Principle

The Aufbau Principle

The Le Chatelier’s Pinciple

The Robert Principle

 

 

5. Amphoteric salts, also known as amphiprotic salts, are often used in the preparation of buffer solutions. An amphoteric salt can act as an acid or a base, depending on the values of its dissociation constants. Which of the followings is (are) an amphiprotic molecules(s)?

 

 

 

 

 

 

6. In acid-base titration in analytical chemistry, the Kjeldahl method involves dissolution of a substance in sulfuric acid, treatment with sodium hydroxide and also back titration using hydrochloric acid. It was developed by Johan Kjeldahl and is used to determine the percentage of a specific element in chemical compounds. Which element is it?

Oxygen, O

Hydrogen, H

Carbon, C

Nitrogen, N

The nitrogen in a substance will react with sulfuric acid, H2SO4 to produce ammonia, NH3. The ammonia produced will then be neutralized by sodium hydroxide, NaOH. The excess NaOH will be neutralized again with hydrochloric acid, HCl.
Using the backward calculation, we will know the percentage of nitrogen in the given substance.

 

 

 

 

 

 

 

 

 

 

7. In complexometric titration, the most common used chelating agent is EDTA (ethylenediamine tetraacetic acid). EDTA’s molecules will combine with metals to form chelate. EDTA is classified as a?

Bidentate ligand

Hexadentate ligand

Tetradentate ligand

Monodentate ligand

A monodentate ligand only has one lone pair of electron that can be donated to the metal ion. Bidentate has two and tetradentate has four. For example, water (containing 2 lone pairs of electrons) is a bidentate ligand, while ammonia (containing a lone pair of electrons) is a monodentate ligand. An EDTA molecule has 6 lone pairs of electrons (4 from the H+ nand 2 from the diamine groups). Hence, it is a hexadentate liugand.
For complexometric analysis, the pH of the solution is normally buffered at pH 11 to give a maximum yield of the Y^4- species (from the H4Y EDTA) that we desire. Y^4- will combine with metal ion, M^2+ to form chelate, MY^2

8. The gravimetric analysis involves preparation of solution, digestion, filtration, washing, drying, weighing and igniting. The term analyte refers to the elements that we are interested in the analysis. What is the term that is used to refer to the other elements or species that we are not interested in?

Matrix

Complex

Peptide

Debris

For example, we wish to calculate the percentage of arsenic in a food sample for a food poisoning case. So, the analyte here is arsenic, while the rest of the compounds in the food sample are matrices.

 

 

 

 

9. All of the following methods are used for the analysis of precipitate, EXCEPT?

o    The Haber's process

o    Fajan's method

o    Volhard's method

o    Mohr's method

 

is used for the manufacturing of ammonia, NH3. The Mohr's method requires a blank analysis and it is usually carried out in a slightly basic solution.
On the contrary, the Volhard's method is carried out in an acidic medium, and it requires back titrations. Lastly, the Fajan's method uses indicators such as dichlorofluorescein. The colloidal particles that are formed will react with the indicator to produce a complex of different colour, indicating the end point has been reached for the chemical reaction.

 

10. In redox titration, what chemical substance is added to the iodometry and iodimetry analysis?

 Argentum

Glucose

Starch

Phenolphthalein

For iodimetry, iodine is added to a solution and the end point is signaled by the change of colour from a colourless solution to a blue colour.
In contrast, a change from blue colour to a colourless solution signals the end point for the iodometry analysis, where excess iodide is added to the solution