Analytical Chemistry The Chemical Analysis Treatment of analytical data Chemical Calculations

The Chemical Analysis

Chemical analysis is divided into two main methods (classic and instrumental methods).

Classic Methods of Analysis:

Classic methods provide accurate and precise results but require relatively large quantities of sample and long time.

They involve gravimetry (based on weight) and

volumetry or titrimetry (based on volume)

 

Instrumental Methods of Analysis:

Instrumental methods based on using electric instruments.

They are generally more sensitive, selective and rapid.

They involve electrochemical methods, spectrochemical methods, graphimetric methods, …..etc.

Treatment of analytical data

Units of Measure

Compare these instructions:

“Transfer 1.5 of your sample to a 100 volumetric flask, (incomplete since the units of measurement are not stated).

“Transfer 1.5 g of your sample to a 100-mL volumetric flask. (complete)

Measurements usually consist of a unit and a number.

The system international units, (SI units).

 

Measurement	Unit	Symbol
Mass	Kilogram	Kg
Volume	Liter	L
amount of substance	Mole	Mol

UNIT CONVERSIONS

Unit conversions based on proportional rules, for example convert 5 ml to liter ?

                      1 Liter   =  1000 ml

                       X liter   =   5      ml

X  = (1 l  x 5 ml ) / 1000  ml  = 0.005 liter  or 5x10^-3 liter   

Convert 34.5 mg to g

        Science   1 g     =  1000 mg

                        x g  =  34.5 mg 

 

For simplicity the very large or very small measurements, for example, a mole contains 602,000,000,000,000,000,000,000 particles, and some analytical techniques can detect as little as 0.000000000000001 g of a compound.

We use scientific notation; thus, a mole contains 6.02 ´ 10²³ particles, and the stated mass is   1 ´ 10^–15 g.

Sometimes it is preferable to use prefixes (like kilo, milli, micro,……. ).

Chemical Calculations

  Atomic Weight, Molecular Weight and Equivalent Weight  

Atomic Weight  (Aw)

An atom's mass can be found by summing the number of protons and neutrons.  It is measured in atomic mass units (amu).

For example, the atomic weight of Na is 23 amu,

Molecular Weight (M.wt)

Simply add the individual mass of each atom in the compound.

 For example, the molecular mass of glucose (C₆H₁₂O₆) is (6x12+12x1+6x16) = 180 amu,

 

The Moles

The mole is equal to the chemical formula mass, except that the unit is grams per mole instead of amu.

One mole of any element is the atomic mass of that element expressed as a weight in grams.                                                                                            

One mole of any compound is the molecular mass of that compound expressed as a weight in grams.

Molar Weight (Mol.wt)

The molar weight (or molar mass) is exactly the same as the atomic weight of an element, or the molecular weight of a compound.                                                                                                        

For example, the atomic weight of oxygen is 16.00 amu, so its molar mass is 16.00 g/mol and the atomic weight of Fe = 55.8 amu, so its molar mass is  55.8 g/mol,

For species with more than one element, (such as compounds), we simply add the individual atomic mass of each element in the compound (found on the periodic table) to obtain the molar mass of the compound.

In general, Molar mass is the mass (in grams) of one mole of a substance.

For example, the formula mass of glucose (C₆H₁₂O₆)  is (6x12+12x1+6x16) = 180 amu, so its molar mass is 180 g/mol.

Another example, sulfur trioxide gas is made up of sulfur and oxygen, whose atomic weights are 32.06 and 16.00 respectively.

                           NaCl = 58.5 g/mol,           Na₂CO₃ = 106 g/mol,                

                          H₂SO₄ = 98 g/mol,             NaOH = 40 g …..etc

Aluminium carbonate, for example, contains aluminium, carbon, and oxygen. To find the molar mass, we have to be careful to find the total number of atoms of each element. Three carbonate ions each containing three oxygen atoms gives a total of nine oxygens. The atomic weights of aluminium and carbon are 26.98 and 12.01 respectively.

The Equivalent Weight  (Eq. Wt)

 

                                                Eq.Wt =     

Where F is equivalent number  and it is defined as following :-

·        Number of H⁺  in acids 

For example      HCl  →  H⁺  + Cl^-                  F = H⁺  = 1

                         H₂SO₄  → 2H⁺  + SO₄^2-           F = H⁺  = 2

                     CH₃COOH  →  H⁺  + CH₃COO^-    F = H⁺  = 1

·        Number of OH^-  in bases 

                           Ba(OH)₂  →  Ba²⁺ + 2OH^-       F = 2

·        The equivalent (or charge) of acidic or basic part in a salt.

                          Na₂CO₃ →  2Na ⁺ +  CO₃^2-     F = 2

·        Number of electrons that gain or loss in an oxidation–reduction reaction, or the change in oxidation state (loss means increase and gain means decrease)

Some rules on oxidation number

Except in hydrides (e.g. NaH) hydrogen always is +1, H¹⁺

Except in peroxides (e.g. H₂O₂), oxygen always is -2, O^2-

Alkaline metals, group IA, (Li, Na, K, Rb, Cs) are in +1

Alkali earth metals ,IIA, (Mg, Ca, Sr, Ba, ..etc) are in +2

For neutral compounds, the total or sum equal zero, for example NaCl = 0, mean that Na⁺ + Cl^- = +1 + -1 = 0

H₂C₂O₄   2H + 2C +4O = 2(+1) +2(+3) + 4(-2) = +2+6-8= 0

For polyatomic ions, the sum equals the charge for example MnO₄^-  = +7-8 = -1    and in C₂O₄^2- 2C +4O = +2(+3) + 4(-2) = +6-8= -2

Question

Calculate the oxidation number of the following:       

                                                   C in CO, CO₂, H₂C₂O₄ and CO₃^--?        N in NO, NH₃, and HNO₃?                              S in SO₂ SO₄^2-?        Mn in KmnO₄?        Cr in Cr₂O₇^2-?  

For example, in the following reaction between Fe³⁺ and oxalic acid, H₂C₂O₄,

2Fe³⁺(aq) + H₂C₂O₄ (aq) + 2H₂O(l) ↔ 2Fe²⁺(aq) + 2CO₂(g) +2H₃O⁺(aq)

Iron is reduced since its oxidation state changes from +3 to +2.

Oxalic acid, on the other hand, is oxidized since the oxidation state for carbon increases from +3 in H₂C₂O₄ to +4 in CO₂.

So, the Eq.Wt of Fe is Mol.wt of Fe/1 = 56/1 = 56

     And the Eq.Wt of H₂C₂O₄, is Mol.wt of H₂C₂O₄,/2 (90/2 = 45)

Question

Calculate the equivalent weight (Eq. wt.) of Na, Fe and Sn according to the following equations ?

          Na  + H₂O  →  NaOH  + H₂ 

          2FeCl₃  + SnCl₂   → 2FeCl₂  + SnCl₄  

Substances	Chemical formula	Equivalent number (F)
Hydrochloric acid	HCl	1
Nitric acid	HNO3	1
Acetic acid	CH3COOH	1
Sulfuric acid	H2SO4	2
Oxalic acid	H2C2O4	2
Phosphoric acid	H3PO4	3
Sodium hydroxide	NaOH	1
Potassium hydroxide	KOH	1
Calcium hydroxide	Ca(OH)2	2
Barium hydroxide	Ba(OH)2	2
Sodium bromide	NaBr	1
Potassium chloride	KCl	1
Ferric chloride	FeCl3	3
Magnesium sulfate	MgSO4	2
Sodium carbonate	Na2 CO3	2
Sodium chloride	NaCl	1
Magnesium chloride	MgCl2	2
Ammonium chloride	NH4Cl	1
Potassium bicarbonate	K(HCO3)	1
Silver nitrate	AgNO3	1
Ammonium chloride	NH4Cl	1
Copper (II) sulfite	CuSO3	2
Calcium nitride	Ca3N2	6

 

Exercise

Calculate the equivalent weight (Eq.wt) for each compound in the above table ?

READING MORY 

INTRODUCTION TO ANALYTICAL CHEMISTRY