Chapter 3c EF of MgO & Molecular Formula

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Determining empirical formula of Determining empirical formula of

magnesium oxidemagnesium oxide

Procedure :Procedure :

A crucible & its lid are weighed.A crucible & its lid are weighed.

Mg ribbon is cleaned to remove the oxide layer.Mg ribbon is cleaned to remove the oxide layer.

The ribbon is coiled & is placed in the crucible. Weigh.The ribbon is coiled & is placed in the crucible. Weigh. The crucible is heated strongly.The crucible is heated strongly.

When Mg starts to burn, crucible is covered with lidWhen Mg starts to burn, crucible is covered with lid

Lid is raised a little at intervals.Lid is raised a little at intervals.

When burning is complete, lid is removed & crucible isWhen burning is complete, lid is removed & crucible isheated strongly.heated strongly.

Crucible is cooled to room temperature.Crucible is cooled to room temperature.

TheThe process of heating, cooling & weighing are process of heating, cooling & weighing are

repeatedrepeated until a constant mass is obtained.until a constant mass is obtained.



The lid is removed at intervals to allowThe lid is removed at intervals to allow oxygenoxygen

to enter the crucible & react with Mg.to enter the crucible & react with Mg. The crucible is quickly covered with lid toThe crucible is quickly covered with lid to

prevent the white fumes of magnesium oxide prevent the white fumes of magnesium oxidefrom escaping.from escaping. Otherwise, it would affect theOtherwise, it would affect the

accuracy of the mass obtained.accuracy of the mass obtained. Heating, cooling & weighing are repeated toHeating, cooling & weighing are repeated to

ensure that theensure that the magnesium reacts completelymagnesium reacts completelywith oxygenwith oxygen to form magnesium oxide.to form magnesium oxide.

This method can be used to determineThis method can be used to determineempirical formula of oxides of empirical formula of oxides of high reactivityhigh reactivitymetals such as calcium oxide, aluminiummetals such as calcium oxide, aluminium

oxide & zinc oxide.oxide & zinc oxide.



Determining molecular formulaeDetermining molecular formulae

The molecular formula of a compound canThe molecular formula of a compound can

be determined if the following data is be determined if the following data is

known :known :

(a) the(a) the empirical formulaempirical formula

(b) Its(b) Its relative molecular mass / molar relative molecular mass / molar

mass.mass.Molecular formula =Molecular formula = (empirical formula)(empirical formula)nn

n is a positive integer n is a positive integer



Example 1Example 1

A carbon compound has an empiricalA carbon compound has an empirical

formula of CHformula of CH22 & a relative molecular & a relative molecular

mass of 70. Find themass of 70. Find the molecular formulamolecular formulaof the compound.of the compound.

[ Relative atomic mass: H, 1; C, 12 ][ Relative atomic mass: H, 1; C, 12 ]



Example 2Example 2

8.5 g of hydrogen peroxide contains8.5 g of hydrogen peroxide contains

0.5 g of hydrogen. If the molar mass of 0.5 g of hydrogen. If the molar mass of

hydrogen peroxide is 34 g molhydrogen peroxide is 34 g mol--11

, find, finditsits molecular formula.molecular formula.

[ Relative atomic mass: H, 1; O, 16 ][ Relative atomic mass: H, 1; O, 16 ]



Example 3Example 3

What is theWhat is the mass of copper mass of copper needed toneeded to

combine completely with 0.5 mole of combine completely with 0.5 mole of

chlorine atoms to form copper (II)chlorine atoms to form copper (II)chloride, CuClchloride, CuCl22 ??

[ Relative atomic mass: Cu, 64 ][ Relative atomic mass: Cu, 64 ]



Example 4Example 4

2.07 g of element Z reacts with bromine2.07 g of element Z reacts with bromine

to form 3.67 g of a compound with theto form 3.67 g of a compound with the

empirical formula of ZBr empirical formula of ZBr 22. Find the. Find therelative atomic mass of element Z.relative atomic mass of element Z.

[ Relative atomic mass: Br, 80 ][ Relative atomic mass: Br, 80 ]



Percentage of an element by massPercentage of an element by mass

in a compound =in a compound =

Mass of the element inMass of the element in

1 mole of the compound1 mole of the compound x 100 %x 100 %Mass of 1 mole of theMass of 1 mole of the

compoundcompound



ExampleExample

Calculate theCalculate the percentage percentage of potassiumof potassium

by mass in potassium carbonate, K by mass in potassium carbonate, K 22COCO33..

[ Relative atomic mass: C, 12; O, 16;[ Relative atomic mass: C, 12; O, 16;K, 39 ]K, 39 ]



Chemical formulae of Chemical formulae of

ionic compoundsionic compounds Ionic compounds consists of Ionic compounds consists of

(a)(a) cationscations (positively(positively--charged ions)charged ions)

(b)(b) anionsanions (negatively(negatively--charged ions)charged ions)

Ionic compounds areIonic compounds are electrically neutralelectrically neutral

(total positive charges are equal to the(total positive charges are equal to the

total negative charges).total negative charges).



Steps in constructing chemical formula of Steps in constructing chemical formula of

anan ionic compound :ionic compound :1, From the name,1, From the name, write the formulaewrite the formulae

of the cation & anion.of the cation & anion.

2, Determine the number of cations &2, Determine the number of cations &

anions byanions by balancing the positive & balancing the positive &

negative charges.negative charges.



Iron (II) chloride ?Iron (II) chloride ?

Iron (II) ion, FeIron (II) ion, Fe2+2+ Chloride ion, ClChloride ion, Clíí

+ 2+ 2 í 1í 1

x 2x 2

Total :Total : + 2+ 2 í 2í 2

Hence,Hence, FeClFeCl22



Iron (III) chloride ?Iron (III) chloride ?

Zinc sulphate ?Zinc sulphate ?

Copper (I) nitrate ?Copper (I) nitrate ?

Aluminium carbonate ?Aluminium carbonate ?

Ammonium bromide ?Ammonium bromide ?

Potassium hydroxide ?Potassium hydroxide ?

Lead (II) iodide ?Lead (II) iodide ?

Potassium manganate (VII) ?Potassium manganate (VII) ? Potassium dichromate (VI) ?Potassium dichromate (VI) ?

Magnesium hydride ?Magnesium hydride ?



NomenclatureNomenclature

Chemical compounds are namedChemical compounds are named

systematically according to the guidelinessystematically according to the guidelines

given by thegiven by the International Union of PureInternational Union of Pure& A pplied Chemistry (IUPAC).& A pplied Chemistry (IUPAC).



Ionic compounds :Ionic compounds :

Name of cation come first, followed by Name of cation come first, followed byanion.anion.

Sodium ion + chloride ion = sodium chlorideSodium ion + chloride ion = sodium chloride

Transition metalsTransition metals can form more than onecan form more than onetype of ions. Roman numerals ( I, II,type of ions. Roman numerals ( I, II,

III«. ) are used.III«. ) are used.

CuCu++ ---- copper (I) ioncopper (I) ion

CuCu2+2+ ---- copper (II) ioncopper (II) ion



PbOPbO22

PbOPbO

CuOCuO

CuCu22OO

MnOMnO22

MnOMnO



Molecular compounds :Molecular compounds :

The more electronegative element isThe more electronegative element is

written last & is added with µwritten last & is added with µ ± ± ideide¶.¶.

COCO ± ± carbon monoxcarbon monoxideideCClCCl44 ± ± Carbon tetrachlor Carbon tetrachlor ideide

SOSO22 ± ± sulphur dioxsulphur dioxideide



Greek prefixes are used to show theGreek prefixes are used to show the

number of atoms of each element in anumber of atoms of each element in amolecule.molecule.

PrefixPrefix MeaningMeaning PrefixPrefix MeaningMeaning

MonoMono-- 11 HexaHexa-- 66

DiDi-- 22 HeptaHepta-- 77

TriTri-- 33 OctaOcta-- 88TetraTetra-- 44 Nona Nona-- 99

PentaPenta-- 55 DekaDeka-- 1010



CO : carbonCO : carbon monomonoxidexide

SOSO22 : sulphur : sulphur didioxideoxide

SOSO33 : sulphur : sulphur tritrioxideoxide

CClCCl44

: carbon: carbon tetratetrachloridechloride

PClPCl55 : phosphorus: phosphorus penta pentachloridechloride

N N22OO44 :: didinitrogennitrogen tetratetraoxideoxide

ClCl22OO77 :: didichlorinechlorine heptheptoxideoxide



Aperson¶s happiness stems not

Aperson¶s happiness stems notfrom how much he ownsfrom how much he owns

but how little he complains. but how little he complains.~ Still Thoughts~ Still Thoughts

Chapter 3c EF of MgO & Molecular Formula

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