F4C3 Chemical Formulae and Equations

CHEMISTRY FORM 4 CHEMICAL FORMULAE AND EQUATIONS

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CHAPTER 3: CHEMICAL FORMULAE AND EQUATIONS

RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS

Relative atomic mass of an element , Ar

= The average mass of an atom of the element1/12 x the mass of an atom of carbon-12

Example:Ar of C=12Ar of O=16Ar of Mg=24

1. The Relative atomic mass of an element is …………………………………………………..

……………………………………………………………… when compare with 1/12 of the

mass of an atom of carbon – 12.

2. Carbon-12 is chosen because it is a ………………………. and can be easily handled.

3. Find the relative atomic masses of these elements.

Element Relative Atomic Mass Element Relative Atomic Mass

Calcium, Ca Argon, ArSodium, Na Silver, Ag

Iron, Fe Caesium, CsCopper, Cu Lead, PbCarbon, C Chlorine, Cl

Hydrogen, H Flourine, FPotassium, K Aluminium, Al

Lithium, Li Zinc, ZnBromine, Br Helium, He



Relative molecular mass of a substance, Mr

= The Average mass of a molecule of the substance1/12 x the mass of an atom of carbon-12

Calculating Relative molecular mass,Mr

Mr= The sum of Ar of all atoms present in one molecule

Example:

Mr of Water, H2O = 2(1) + 16 = 18

Mr of Carbon dioxide, CO2 = 12 + 2(16) = 44

For ionic substance , Relative formula mass , Fr

= The sum of Ar of all atoms present in the formula

Example:Fr of Magnesium oxide, MgO = 24 + 16 = 40Fr of Sodium chloride, NaCl = 23 + 35.5 = 58.5

1. The relative molecular mass of a molecule is ………………………………………………

………………………………………………………. when compared with 1/12 of the mass

of one atom of ……………………………………………

2. Calculate the relative molecular masses of the substances in the table below.

Substance Molecular formula Relative molecular mass, Mr

Hydrogen gas H2 2(1) = 2Propane C3H8

Ethanol C2H5OHBromine gas Br2

Methane CH4

Glucose C6H12O6

Ammonia NH3

[Relative atomic mass : H,1; C,12; O,16; Br,80 ; N,14 ]

2 HydrogenatomsMolecular

formula

Relative atomic massfor OxygenRelative atomic mass

for Hydrogen

All Ar, Mr andFr have no unit



3. Calculate the relative formula masses of the following ionic compounds in the table.

Substance Compound formula Relative formula mass, Fr

Potassium oxide K2O 2(39) + 16 = 94

Aluminium sulphate Al2(SO4)3 2(27)+3[32+4(16)]=342

Zinc nitrate Zn(NO3)2

Aluminium nitrate Al(NO3)3

Calcium carbonate CaCO3

Calcium hydroxide Ca(OH)2

Hydrated copper(II)sulphate

CuSO4.5H2O 64 + 32 + 4(16) + 5[2(1) + 16]=250

Hydrated sodiumcarbonate

Na2CO3.10H2O

Sodium hydrogensulphate

NaHSO4

Aluminium chloride AlCl3

Copper(II) sulphate CuSO4

Zinc carbonate ZnCO3

Potassiumcarbonate

K2CO3

[Relative atomic mass: O,16; C,12; H,1; K,39 ; Cu,64 ; Zn, 65; Cl, 35.5 ; Al, 27 S,32 ;Ca, 40; Na,23; N, 14]



THE MOLE AND THE NUMBER OF PARTICLES

1. To describe the amount of atoms, ions or molecules , mole is used.

2. A mole is an amount of substance that contains as many particles as the ………………..

…………………………………………………………….. in exactly 12g of carbon-12.

3. A mole is an amount of substance which contains a constant number of particles

atoms, ions, molecules which is 6.02 x 1023

4. The number 6.02 x 1023 is called …………………………………… (NA)

5. In other words:

1 mol of atomic substance contains ……………………………. atoms

1 mol of molecular substance contains ……………………………. molecules

1 mol of ionic substance contains ………………………………….. formula units

6. Relationship between number of moles and number of particles (atom/ion/molecules):

x Avogadro Constant

∻ A vogadro Constant

number of moles number of particles



Number of moles Number of particles

0.5 mol of carbon atoms …………………………………… atoms ofcarbon

0.2 moles of hydrogen gas ( H2) (i) …………………………..moleculesof hydrogen gas

(ii) …………………………….Atoms ofhydrogen

2 mol of carbon dioxide molecules ………………x 10 23 molecules of carbon dioxidegas contains :

………………. atoms of C and

…………………. atoms of O

0.007 mol of calcium ions ……………………… calcium ions

…………………………. mol of water 6.02 x 10 25 molecules of water

0.4 mol of ozone gas ( O3) ………………….x 10 23 molecules of ozone,

contains :

……………………… atoms of O

7. Complete these sentences .

a) 1 mol of calcium contains ………………………………………….. atoms

b) 2 mol of iron contains ……………………………………………….. atoms

c) 2 mol of magnesium oxide, (MgO) contains ………………………………………….. ions

d) 2 mol of sodium carbonate, (Na2CO3) contains ……………………………………….

e) 3 mol of carbon dioxide, (CO2) contains …………………………………….. molecules

f) 0.5 mol Copper (II) nitrate, Cu(NO3)2 contains ………………………………….. Cu2+ ions

and …………………………………………………. NO3- ions


Cikg

NUMBER OF MOLES AND MASS OF SUBSTANCES

1. The molar mass of a substance= The molar mass of _________________ mole of the substance.

= The mass of (NA) number of particles

= The mass of ____________________ particles

x Molar mass

2. C

Ther

Exam

Exam

3. C

a) 2M

Massin g

NumberOf

u2u.com @2009 Page 6

∻ Molar mass

alculating the Mass from a number of Moles

Number of moles = . mass of the substance .Mass of 1 mole of the substance

efore :

Mass of substance = Number of moles x Mass of 1 mole

ple 1 : What is the mass of 2 moles of carbon ?

Mass = 2 x 12= 24g

ple 2 : What is the mass of 2 moles of H2O ?

Mass = 2 x [ 2(1) + 16 ]

= 36g

alculate the masses of these substances

moles of aluminium atomsass =

b) 10 moles of iodine atomsMass =

moles



c) 3 moles of lithium atomsMass =

d) 0.5 moles of oxygen gas (O2)Mass =

e) 0.1 moles of sodiumMass =

f) 2 moles of chlorine molecules (Cl2)Mass =

g) 1 mole of carbon dioxide ( CO2)Mass =

h) 3 moles of nitric acid, ( HNO3 )Mass =

i) 2 moles of calcium carbonate (CaCO3 )Mass =

j) 0.25 moles of calcium chloride (CaCl2 )Mass =

k) 0.25 moles of sodium hydroxide (NaOH)Mass =

l) 0.25 moles of sodium carbonate (Na2CO3)Mass =

m) 0.5 moles of potassium manganate (VII)(KMnO4)Mass =

n) 0.25 moles of hydrated magnesiumsulphate (MgSO4.7H2O)Mass =



4. Calculate the Number of Moles from a given Mass

Example : How many moles are there in 88g of CO2

Number of moles = 88 = 2 moles44

a) 2g of helium atomsNumber of moles =

b) 6g of carbon atomsNumber of moles =

c) 16g of helium atomsNumber of moles =

d) 4g of sulphur atomsNumber of moles =

e) 4g of oxygen molecules (O2)Number of moles =

f) 213g of chlorine molecules (Cl2)Number of moles =

g) 0.56g of nitrogen molecules (N2)Number of moles =

h) 254g of iodine molecules (I2)Number of moles =

i) 88g of carbon dioxide (CO2)Number of moles =

j) 3.1g of sulphur dioxide (SO2)Number of moles =

k) 560g of potassium hydroxide (KOH)Number of moles =

l) 392g of sulphuric acid (H2SO4)Number of moles =

m) 170g of ammonia (NH3)Number of moles =

n) 120g of magnesium oxide (MgO)Number of moles =

o) 4g of sodium hydroxide (NaOH) p) 73g of hydrogen choride (HCl)



Number of moles = Number of moles =

q) 15.8g of potassium manganate (VII)KMnO4

Number of moles =

r) 8g of ammonium nitrate (NH4NO3)Number of moles =

s) 0.78g of aluminium hydroxide Al(OH)3

Number of moles =t) 0.92g of ethanol (C2H5OH)Number of moles =

5. Complete the following table.

Element/compoundChemicalformulae Molar mass Calculate

Copper Cu RAM= 64 (a)Mass of 1 mol = ……………g

(b) Mass of 2 mol = …………. g

(c)Mass of ½ mol = ………….g

(d)Mass of 3.01x1023 Cu atoms

=

Sodium hydroxide NaOH RFM= 40 (a) Mass of 3 mol of sodiumhydroxide =

(b) Number of moles of sodiumhydroxide in 20 g =

Zinc nitrate Zn(NO3)2 RFM = a) Number of moles in 37.8 g ofzinc nitrate :



NUMBER OF MOLES AND VOLUME OF GAS

1. The molar volume of a gas is defined as the ………………………………………………….

…………………………………………………………….

2. One mole of any gas always has the …………………………………………… under the same

temperature and pressure.

3. The molar volume of any gas is

24 dm3 at ……………………………………………… or

22.4 dm3 at …………………………………………….

Example :

1 mol of oxygen gas, 1 mol of ammonia gas, 1 mol helium gas and 1 mol sulphur dioxide gasoccupies the same volume of 24 dm3 at room condition

x 22.4/24 dm3

∻22.4/24 dm3

Number of moles of gas Volume of gas



4. Calculate the volume of gas in the following numbers of moles at STP

Example : Find the volume of 1 mole of CO2 gas

Volume = number of moles x 22.4 dm3

= 1 x 22.4 dm3

= 22.4 dm3

a) 3 moles of oxygenVolume =

b) 2 moles of CH4

Volume =

c) 0.3 moles of ArgonVolume =

d) 0.2 moles of SO3

Volume =

e) 0.1 moles of N2

Volume =f) 1.5 mol of N2

Volume =

5. Complete the diagram below . (Refer to Page 33,34 & 38-Chemistry textbook)

Volume of gas (dm3)

Number of molesMass in gram No of particles



Solve these numerical problems

1. What is the volume of 0.3 mole of sulphur dioxide gas at STP?[Molar volume: 22.4 dm3 mol-1 at STP]

(Ans: 6.72 dm3)

2. Find the number of moles of oxygen gas contained in a sample of 120 cm3 of the gasat room conditions.[Molar volume: 24 dm3 mol-1 at room conditions]

(ans: 0.005 mol)

3. Calculate the number of water molecules in 90 g of water, H2O.[Relative atomic mass: H, 1; O, 16. Avogadro constant, NA: 6.02 x 1023 mol-1]

(Ans; 3.01x 1024

molecules)

4. What is the volume of 24 g methane ,CH4 at STP?[Relative atomic mass: H, 1; C, 12. Molar volume: 22.4 dm3 mol-1 at STP]

(Ans: 33.6 dm3)



5. How many aluminium ions are there in 20.4 g of aluminium oxide, Al2O3?[Relative atomic mass: O, 16; Al, 27. Avogadro constant, NA: 6.02 x 1023 mol-

(2 x 0.2 x 6.02 x1023

)

6. Calculate the number of hydrogen molecules contained in 6 dm3 of hydrogen gas atroom conditions.[Molar volume: 24 dm3 mol-1 at room conditions Avogadro constant, NA: 6.02 x 1023

mol-1]

(Ans: 1.505x1023

molecules)

7. Find the volume of nitrogen in cm3 at STP that consists of 2.408 x 1023 nitrogenmolecules.[Molar volume: 22.4 dm3 mol-1 at STP. Avogadro constant, NA: 6.02 x 1023 mol-1]

(Ans: 8.96 dm3

)



CHEMICAL FORMULAE

1) A Chemical formula - A representation of a chemical substance using letters for

……………………………………… and subscripts to show the numbers of each type of

…………………….. that are present in the substance.

2) Complete this table

Chemical subtance Chemical

formulae

Notes

Water …………….. 2 atoms of H combine with 1 atom of O

……….. NH3 ……. atoms of H combine with 1 atom of N

Propane

C3H8 …….. atoms of C combine with …….

atoms of H

Magnesium oxide

…………….. …………………………………………….

……………….. H2SO4 ……………………………………………

H2

Subscript shows 2hidrogen atoms ina molecule

The letter Hshows…………….…………….



3). There are two types of chemical formulae. Complete the following:

** Empirical Formula The simplest ………… ……….. ratio of atoms of each ……….

in the compound.

** Molecular Formula The actual …………… of atoms of each …………… that are

present in a molecule of the compound

Remember:

Example: (i) Compound – Ethene (ii) Compound – Glucose

Molecular formula - 42HC Molecular formula - 6126 OHC

Empirical formula - ................... Empirical formula - ....................

1 Find the empirical formula of a compound

Example of calculation:

a) When 11.95 g of metal X oxide is reduced by hydrogen, 10.35 g of metal X is

produced. Find the empirical formula of metal X oxide [ RAM; X,207; O,16 ]

Element X O

Mass of element(g) 10.35 11.95-10.35

Number of moles of atoms 10.35÷207 (11.95-10.35)÷16

Ratio of moles

Simplest ratio of moles

Empirical formula : ……………

Molecular formula = (Empirical formula)n



b) A certain compound contains the following composition:

Na 15.23%, Br 52.98% , O 31.79%, [ RAM : O, 16; Na, 23; Br,80]

(Assume that 100g of substance is used)

Element Na Br O

Mass of element(g) 15.23 52.98 31.79

Number of moles atoms 15.23 ÷23 52.98÷80 31.79÷16

Ratio of moles

Simplest ratio of moles

Empirical formula:: ……………………………………………….

c) Complete the table below.

Compound Molecular Formula Empirical formula Value of n

Water H2O

Carbon Dioxide CO2 CO2

Sulphuric Acid H2SO4

Ethene C2H4 CH2

Benzene C6H6

Glucose C6H12O6

d) 2.52g of a hydrocarbon contains 2.16 g of carbon. The relative molecular mass of the

hydrocarbon is 84. [RAM H,1; C,12]

i. Find the empirical formula of the hydrocarbon

ii. Find the molecular formula of the carbon.



Chemical Formula for ionic compounds:

Complete the table below :

Cation Formula Anion Formula

Hydrogen ion H Flouride ion F

Lithium ion Chloride ion

Sodium ion Bromide ion

Potassium ion Iodide ion

Magnesium ion Hydroxide ion

Calcium ion 2Ca Nitrate ion

Barium ion 2Ba Manganate(VII) ion

Copper(II) ion Ethanoate ion COOCH 3

Iron(II) ion 2O

Iron (III) ion Sulphate ion

Lead (II) ion Sulphide ion 2S

Zinc ion Carbonate ion

Chromium (III) ion Dichromate (VI) ion 2

72OCr

Aluminium ion 3Al 3

4PO

Ammonium ion Chromate (VI) ion



a) Chemical formula of an ionic compound comprising of the ions Xm+ and Yn- is constructed

by exchanging the charges of each element. The formula obtained will XnYm

Example : Sodium oxide Copper (II) nitrate

Na+ O2- Cu2+ NO3-

+1 -2 +2 -1

2 1 1 2

= Na2O = ....................

b) Construct a chemical formula for each of the following ionic compounds:

(i) Magnesium chloride (ii) Potassium carbonate

(iii) Calcium sulphate (iv) Copper (II) oxide

(v) Silver nitrate (vi) Zinc nitrate



(vii) Aluminium oxide (viii) Iron(II) hydroxide

(ix) Lead(II) sulphide (x) Chromium(III) sulphate

CHEMICAL EQUATIONS

Example: C (s) + 2O (g) 2CO (g)

Reactant product

1) Qualitative aspect of chemical equation:

a) Arrow in the equation the way the reaction is occurring

b) Substances on the left-hand side ……………………..

c) Substances on the right-hand side ………………………

d) State of each substance ………: (s), ………………(l), gas ……….and aqueous

solution ……………….

2) Quantitative aspect of chemical equations

Coefficients in a balanced equation the exact proportions of reactants and products in

equation.

Example: 2 2H (g) + 2O (g) 2 OH 2 (l)

(Interpreting): 2 molecules (2 mol) of 2H react with 1 molecule (1 mol) of 2O to produced 2

molecules(2 mol) of water



Complete the following word equations and write in chemical equation

a) Sodium + chlorine …………………………..

………… + …………… NaCl

b) Carbon + ……….. Carbon dioxide

………. + ………… ……………………..

c) Sulphur + oxygen ……………………………

……….. + ……….. …………………………..

d) Zinc + oxygen ………………………………..

………… + O2 ………………………………..

3) Write a balanced equation for each of the following reactions and interpret the equations

quantitatively.

(a). Carbon monoxide gas + oxygen gas carbon dioxide gas

………………………………………………………………………………………………………

Interpreting:

……………………………………………………………………………………………………………

(b). Hydrogen gas + nitrogen gas ammonia gas

……………………………………………………………………………………………………….

Interpreting:

…………………………………………………………………………………………………………..

(c). Aluminium + Iron (III) oxide aluminium oxide + Iron

……………………………………………………………………………………………………….

Interpreting:

…………………………………………………………………………………………………………….



Numerical Problems Involving Chemical Equations

Hydrogen peroxide decomposes according to the following equation:

2 22OH (l) 2 OH 2 (l) + 2O (g)

1). Calculate the volume of oxygen gas, 2O measured at STP that can be obtained from the

decomposition of 34 g of hydrogen peroxide, 22OH .

[Relative atomic mass : H, 1 ; O, 16. Molar volume : 22.4 3dm 1mol at STP]

(Ans: 11.2 dm3)

2).Silver carbonate Ag2CO3 breaks down easily when heated to produce silver metal

2 Ag2CO3(l) 4 Ag (s) + 2 2CO (g) + 2O

Find the mass of silver carbonate that is required to produce 10 g of silver

[Relative atomic mass: C, 12 ; O, 16 ; Ag, 108]

(Ans : 12.77g)



3). 16 g of copper (II) oxide, CuO is reacted with excess methane, 4CH . Using the equation

below, find the mass of copper that is produced.

[Relative atomic mass : Cu, 64 ; O, 16]

4 CuO (s) + 4CH (g) 4 Cu (s) + 2CO (g) + 2 OH 2 (l)

(Ans : 12.8 g)

4). A student heats 20 g of calcium carbonate 3CaCO strongly. It decomposes according to the

equation below:

3CaCO (s) CaO (s) + 2CO (g).

(a). If the carbon dioxide produced is collected at room conditions, what is its volume?

(b). Calculate the mass of calcium oxide, CaO produced.

[Relative atomic mass: C, 12 ; O, 16; Ca, 40. Molar volume :

24 dm3 1mol at room conditions]

(Ans : (a). 4.8 dm3

(b) 11.2 g)

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F4C3 Chemical Formulae and Equations