TOPICS

Qualitative and Quantitative Analysis

Empirical and Molecular Formula

Combustion Analysis

PRESENTATION BY:Fawad Mueen Arbi

QUALITATIVE ANALYSIS

In chemistry, the chemical analysis designed to identify the components of a substance or mixture is called Qualitative Analysis of that substance. For instance, Color is a qualitative property of any substance because it can’t be quantified or simply measured in numbers.

QUANTITATIVE ANALYSIS

In chemistry, quantitative analysis is the determination of the absolute or relative abundance (often expressed as a concentration) of one, several or all particular substance(s) present in a sample.

DIFFERENCE BETWEEN QUALITATIVE AND QUANTITATIVE ANALYSIS

 Qualitative Analysis tells 'what' is in a sample, while Quantitative analysis is used to tell 'how much' is in a sample. For instance, suppose an indicator solution changes color in the presence of a metal ion. It could be used as a qualitative test: does the indicator solution change color when a drop of sample is added? It could also be used as a quantitative test, by studying the color of the indicator solution with different concentrations of the metal ion.

Empirical Formula:

The Empirical Formula of a chemical compound is the simplest positive integer ratio of atoms present in a compound.  A simple example of this concept is that the empirical formula of hydrogen peroxide, or H2O2, would simply be HO.

EXAMPLES:

The chemical compound n-hexane has the structural formula CH3CH2CH2CH2CH2CH3

which shows that it has 6 carbonatoms arranged in a chain, and 14 hydrogen atoms. Hexane's molecular formula is C6H14, and its empirical formula isC3H7, showing a C:H ratio of 3:7.

METHOD AND EXAMPLE:

Suppose you are given a compound such as methyl acetate, a solvent commonly used in paints, inks, and adhesives. When methyl acetate was chemically analyzed, it was discovered to have 48.64% carbon 8.16% hydrogen (H), and 43.20%oxygen (O). For the purposes of determining empirical formulas, we assume that we have 100 g of the compound. If this is the case, the percentages will be equal to the mass of each element in grams.

STEP 1

Change each percentage to an expression of the mass of each element in grams. That is, 48.64% C becomes 48.64 g C, 8.16% H becomes 8.16 g H, and 43.20% O becomes 43.20 g O.

STEP 2

STEP 3

STEP 4

If necessary, multiply these numbers by integers in order to get whole numbers; if an operation is done to one of the numbers, it must be done to all of them.Thus, the empirical formula of methyl acetate is C3H6O2. This formula also happens to be methyl acetate's molecular formula.

Molecular formula:

Formula which gives the whole number ratio of atoms of different element present in a molecule of a compound

n=Molecular formula / Empirical formula Molecular formula =n(empirical formula)

FORMULA:

SUBSTANCES HAVING SAME EMPIRICAL AND MOLECULAR FORMULA

There are also some compounds which have same molecular and empirical formula e.g.NO2

CO2

H2O

COMBUSTION ANALYSIS:

WHAT IS COMBUSTION ANALYSIS ?•  is a method used in both organic

chemistry and analytical chemistry to determine the elemental composition (more precisely empirical formula) of a pure organic compound by combusting the sample under conditions where the resulting combustion products can be quantitatively analyzed. Once the number of moles of each combustion product has been determined the empirical formula or a partial empirical formula of the original compound can be calculated.

• Empirical and molecular formulas for compounds that contain only carbon and hydrogen (CaHb) or carbon, hydrogen, and oxygen (CaHbOc) can be determined with a process called combustion analysis

HISTORY

The method was invented by Joseph Louis Gay-Lussac. Justus von Liebig studied the method while working with Gay-Lussac between 1822 and 1824 and improved the method in the following years to a level that it could used as standard procedure for organic analysis.

METHOD: Obtaining Empirical and Molecular Formulas from Combustion Data Empirical and molecular formulas for compounds that contain only carbon and hydrogen (CaHb) or carbon, hydrogen, and oxygen (CaHbOc) can be determined with a process called combustion analysis. The steps for this procedure areWeigh a sample of the compound to be analyzed and place it in the apparatus shown in the image below.Burn the compound completely. The only products of the combustion of a compound that contains only carbon and hydrogen (CaHb) or carbon, hydrogen, and oxygen (CaHbOc) are carbon dioxide and water.The H2O and CO2 are drawn through two tubes. One tube contains a substance that absorbs water, and the other contains a substance that absorbs carbon dioxide. Weigh each of these tubes before and after the combustion. The increase in mass in the first tube is the mass of H2O that formed in the combustion, and the increase in mass for the second tube is the mass of CO2 formed.Assume that all the carbon in the compound has been converted to CO2 and trapped in the second tube. Calculate the mass of carbon in the compound from the mass of carbon in the measured mass of CO2 formed.Assume that all of the hydrogen in the compound has been converted to H2O and trapped in the first tube. Calculate the mass of hydrogen in the compound from the mass of hydrogen in the measured mass of water.If the compound contains oxygen as well as carbon and hydrogen, calculate the mass of the oxygen by subtracting the mass of carbon and hydrogen from the total mass of the original sample of compound.Use this data to determine the empirical and molecular formulas in the usual way. 

MODERN TECHNIQUES:

Nowadays, modern instruments are sufficiently automated to be able to do these analyses routinely. Samples required are also extremely small - 3 mg of sample is sufficient to give satisfactory CHN analysis.

MODERN TECHNIQUES

The water vapor, carbon dioxide and other products can be separated via gas chromatography and analyzed via thermal conductivity detector.

GAS CHROMATOGRAPHY:

Gas chromatography (GC), is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, or separating the different components of a mixture (the relative amounts of such components can also be determined). In some situations, GC may help in identifying a compound. In preparative chromatography, GC can be used to prepare pure compounds from a mixture.

THERMAL CONDUCTIVITY DETECTOR

The thermal conductivity detector (TCD), also known as a Katharometer, is a bulk property detector and a chemical specific detector commonly used in gas chromatography. This detector senses changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas. Since most compounds have a thermal conductivity much less than that of the common carrier gases of helium or hydrogen, when an analyte elutes from the column the effluent thermal conductivity is reduced, and a detectable signal is produced.

THANK YOU!ANY QUESTIONS?