chemical formulae
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CHEMICAL FORMULAE
The chemical formula of an element shows the symbol representing that element and the number of atoms making up that element.
H2 O2 N2
CH4CH3CH2OH
CuCO3
CuSO4.5H2O
1. Moleculer formula
2. Empirical formula
3. Structural formula
Chemical formulae for molecular elements and molecular/covalent compounds
Chemical formulae for covalent and ionic componds
Chemical formulae for molecular elements and covalent compounds that show the relative arrangement of atoms in a molecule.
The chemical formula of a chemical compound shows the symbols of all elements and the number of atoms of each element in one molecule of the compound.
The molecular formula of a compound is the chemical formula which shows the actual number of atoms of each combining element in one molecule of a compound
1. Molecular formula
Oxygen Gas
O2
One molecule of oxygen contains two oxygen atoms that combined together
Ammonia
NH3
In one molecule of ammonia three hydrogen atoms combined with one nitrogen atom
Trichloromethane CHCl3
One molecule of trichloromethane contains one carbon atom, one hydrogen atom and three chlorine atoms
2. EMPIRICAL FORMULA
Ethane
Molecular formula
C2H6
Emphirical formula
CH3
Glucose
Molecular formula
C6H12O6
Emphirical formula
CH2O
Water H2O
Emphirical formula
H2O
Oxalic Acid
Molecular formula
H2C2O4
Emphirical formula HCO2
The empirical formula of a compound is the chemical formula which shows the smallest ratio of atom of each element that made up a compound.
Molecular formula
Na+
ions Cl- ions
Sodium chloride
Chemical formula for an ionic compound is stated in term of its empirical formula. Ionic compounds do not exist in the form of simple molecules. At room conditions all ionic compounds are crystalline solid that consists of positively and negatively charged ions that are held together by a network of electrostatic attractive forces of attraction (ionic bond). “The whole crystalline solid is a molecule”
Compound Empirical formula
Calcium hydroxide
Ca(OH)2
Aluminium sulphate
Al2(SO4)3
Lithium carbonate
Li2CO3
Zinc sulphate ZnSO4
Ammonium sulphate
(NH4)2SO4
3. Structural formula
EthanolMolecular formula
C2H6O
Structural formula
The chemical formulae for molecular elements and covalent compounds that show the relative arrangement of atoms in a molecule. The formula also shows the actual number of atoms of each element present in one molecule.
Cyclohexane
Molecular formula
C6H12
3-methyl-2-pentane
Structural formula
Molecular formula
C6H12
Structural formula
Calculating the Empirical Formula
In general, the empirical formula of a compound can be determine from the masses of the elements that combine together.
Determination of the empirical formula of magnesium oxide:
Mass of the crucible + the lid = 21.02 g
Mass of the crucible + the lid + Mg ribbon = 21.50 g
Mass of the crucible + the lid + magnesium oxide = 21.82 g
Mass of the crucible + the lid = 21.02 g
Mass of the crucible + the lid + Mg ribbon = 21.50 g
Mass of the crucible + the lid + magnesium oxide = 21.82 g
Mass of magnesium burnt = (21.50 – 21.02)g = 0.48 g
Mass of oxygen used = ( 21.82 – 21.50) = 0.32 g
elements Mg O
Mass 0.48 g 0.32 g
Number of moles of atom 0.48/24 = 0.02 mol 0.32/16 = 0.02
The simplest ratio 0.02/0.02 = 1 0.02/0.02 = 1
The empirical formula = MgO
Example 1: 3.12 g metal X reacts with oxygen to form 4.56 g X oxide. Determine the empirical formula of X oxide.(Relatif atomic mass: O = 16, X = 52)
Element X O
Mass (g) 3.12 (4.56-3.12)
= 1.44 g
Number of moles of atom 3.12/52
= 0.06 mol
1.44/16
= 0.09 mol
Simplest ratio of the number of moles
0.06/0.06 = 1 0.09/0.06 = 1.5
2 3
Empirical formula = X2O3
Elements Fe O
Mass, g 1.12 0.48Number of moles 1.12 = 0.02
560.48 = 0.03
16
Divide by the smallest number
0.02 = 1 0.02
0.03 = 1.5 0.02
Ratio of atom 2 3
Empirical Formula : Fe2O3
• A sample of ferum oxide consists of 1.12 g ferum and 0.48g oxygen.Find the empirical formula of this compound.
• [Relative atomic mass : Fe, 56 ; O,16 ]
Example 2:A compound of zinc bromide contains 28.89% zinc. What is the empirical formula of the compound?(R.A.M : Zn, 65 ; Br,80 )
Elements Zn Br
Mass per 100g compound
28.89 100 – 28.89 = 71.11
Number of moles of atoms
28.89 = 0.44
65
71.11 = 0.89
80
Divide by the smallest number
0.44 = 1
0.44
0.89 = 2
0.44
Ratio of atom 1 2
Empirical formula : ZnBr2
• Example 3
The empirical formula of a compound is given as CH2 . If the RMM of the compound is 56, what is the molecular formula? [ RAM C,12 ; H , 1 ].
( Empirical formula ) n = RMM
( CH2 ) n = 56
( 1x 12 + 2 x 1 ) n = 56
14 n = 56
n = 4
Molecular formula (CH2 )4 = C4H8
• Example 2
A compound CxHyOz contains 40% carbon and 53.3% oxygen. If the relative molecular mass of the compound is 180, finds it molecular formula
[ RAM : H , 1 : C , 12 ; O , 16 ]
Element C H O
Mass in 100g 40 40-53.3 = 6.7 53.3
Number of moles 40 =3.33
12
6.7 = 6.7
1
53.3 = 3.33
16
Simplest ratio 3.33 = 1
3.33
6.7 = 2
3.33
3.33 = 1
3.33
Empirical Formula : CH2O
• (Empirical formula ) n = RMM
• (CH2O)n = 180
• (12 + 2 + 16 ) n = 180
• 30n = 180
• n = 180/30 = 6
Molecular formula = (CH2O)6 = C6 H12 O6
CATION ANION
NAME FORMULA NAME FORMULA
Aluminium Al3+ Bromide Br-
Ammonium NH4+ Carbonate CO3
2-
Barium Ba2+ Chromate(VI) CrO42-
Calsium Ca2+ Dichromate(VI) Cr2O72-
Cromium(III) Cr3+ Fluoride F-
Cobalt(II) Co2+ Hidride H-
Copper(II) Cu2+ Hidrogen carbonate
HCO3-
Copper(I) Cu+ Hidrogen sulphate
HSO4-
Iron(II) Fe2+ Hidroxide OH-
Iron(III) Fe3+ Iodide I-
Lihtium Li+ Nitrate NO3-
Manganate(IV) Mn4+ Nitrite NO2-
Manganate (II) Mn2+ Nitride N3-
Magnesium Mg2+ Oxide O2-
Pottasium K+ Manganate(VII)
MnO4-
Tin(II) Sn2+ Peroksida O22-
Stanum(IV) Sn4+ Fosfat PO4 3-
Plumbum(II) Pb2+ Sulfat SO42-
Plumbum(IV) Pb2+ Sulfit SO32-
Vanadium(V) V5+ Sulfida S2-
Zink Zn2+ Tiosianat SCN-
Natrium Na+ Tiosulfat S2O32-
Titanium(IV) Ti4+ Oksalat C2O42-
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