3 Chemical Formulae and Equations

WAJA F4 Chemistry 2009 Chapter 3 : Chemical Formulae and Equations

CHAPTER 3 : CHEMICAL FORMULAE AND EQUATIONS

A RELATIVE ATOMIC MASS (RAM) AND RELATIVE MOLECULAR MASS (RMM)

Learning OutcomesYou should be able to: state the meaning of relative atomic mass based on carbon-12 scale, state the meaning of relative molecular mass based on carbon-12 scale, state why carbon-12 is used as a standard for determining relative atomic mass and

relative molecular mass, calculate the relative molecular mass of substances.

Activity 1 (refer text book pg 28 )

Relative atomic mass of an element , Ar

= The average mass of an atom of the element 1/12 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

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Relative molecular mass of a substance, Mr

= The Average mass of a molecule of the substance 1/12 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 ]

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2 Hydrogen atomsMolecular

formula

Relative atomic mass for OxygenRelative atomic mass

for Hydrogen

All Ar, Mr and Fr 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 sodium carbonate

Na2CO3.10H2O

Sodium hydrogen sulphate

NaHSO4

Aluminium chloride AlCl3

Copper(II) sulphate CuSO4

Zinc carbonate ZnCO3

Potassium carbonate

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]

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B THE MOLE AND THE NUMBER OF PARTICLES

Learning Outcomes You should be able to:

define a mole as the amount of matter that contains as many particles as the number of atoms in 12 g of 12C,

state the meaning of Avogadro constant, relate the number of particles in one mole of a substance with the Avogadro constant, solve numerical problems to convert the number of moles to the number of particles of a

given substance and vice versa.


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 10 23

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

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number of moles

number of particles


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

0.2 moles of hydrogen gas ( H2) (i) …………………………..molecules of hydrogen gas(ii) …………………………….Atoms

of hydrogen

2 mol of carbon dioxide molecules ………………x 10 23 molecules of carbon dioxide gas 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

C NUMBER OF MOLES AND MASS OF SUBSTANCES

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Learning Outcomes You should be able to:

state the meaning of molar mass, relate molar mass to the Avogadro constant, relate molar mass of a substance to its relative atomic mass or relative molecular mass, solve numerical problems to convert the number of moles of a given substance to its mass and

vice versa.


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

∻ Molar mass

2. Calculating the Mass from a number of Moles

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

Therefore :

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

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

Mass = 2 x 12= 24g

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

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

= 36g

3. Calculate the masses of these substances

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NumberOf

moles

Massin g


a) 2 moles of aluminium atomsMass =

b) 10 moles of iodine atomsMass =

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 magnesium sulphate (MgSO4.7H2O)Mass =

Activity 5

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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 moles 44

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)Number of moles =

p) 73g of hydrogen choride (HCl)Number of moles =

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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 =

Activity 6

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 sodium hydroxide =

(b) Number of moles of sodium hydroxide in 20 g =

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

D NUMBER OF MOLES AND VOLUME OF GAS

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Learning OutcomesYou should be able to:

state the meaning of molar volume of a gas, relate molar volume of a gas to the Avogadro constant, make generalization on the molar volume of a gas at a given temperature and pressure, calculate the volume of gases at STP or room conditions from the number of moles and

vice versa, solve numerical problems involving number of particles, number of moles, mass of substances

and volume of gases at STP or room conditions.

Activity 7 (refer text book pg 36, 37 )

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 gas occupies the same volume of 24 dm3 at room condition

x 22.4/24 dm3

∻22.4/24 dm3

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

Example : Find the volume of 1 mole of CO2 gas

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Number of moles of gas

Volume of 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)

Activity 8

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]

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Volume of gas (dm3)

Number of moles Mass in gram No of particles


(Ans: 6.72 dm3)

2. Find the number of moles of oxygen gas contained in a sample of 120 cm3 of the gas at 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-

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(2 x 0.2 x 6.02 x1023)

6. Calculate the number of hydrogen molecules contained in 6 dm3 of hydrogen gas at room 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 nitrogen molecules.[Molar volume: 22.4 dm3 mol-1 at STP. Avogadro constant, NA: 6.02 x 1023 mol-1]

(Ans: 8.96 dm3 )

E CHEMICAL FORMULAE

Learning OutcomesYou should be able to

state the meaning of chemical formula state the meaning of empirical formula

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state the meaning of molecular formula determine empirical and molecular formula of substances compare and contrast empirical formula with molecular formula solve numerical problems involving empirical and molecular formula. write ionic formula of ions construct chemical formulaf ionic compounds state names of chemical compounds using IUPAC nomenclature. use symbols and chemical formula for easy and systematic communication in the field of

chemistry.

ACTIVITY 9 (Refer text book pg 40)

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 ……………………………………………

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

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H2

Subscript shows 2 hidrogen atoms in a molecule

The letter H shows …………….…………….


** 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 - Molecular formula -

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

Activity 10

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 : ……………

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)

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Molecular formula = (Empirical formula)n


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.

Activity 11 :Chemical Formula for ionic compounds:

Complete the table below :

Cation Formula Anion Formula

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Hydrogen ion Flouride ion

Lithium ion Chloride ion

Sodium ion Bromide ion

Potassium ion Iodide ion

Magnesium ion Hydroxide ion

Calcium ion Nitrate ion

Barium ion Manganate(VII) ion

Copper(II) ion Ethanoate ion

Iron(II) ion

Iron (III) ion Sulphate ion

Lead (II) ion Sulphide ion

Zinc ion Carbonate ion

Chromium (III) ion Dichromate (VI) ion

Aluminium ion

Ammonium ion Chromate (VI) ion

Avtivity 12

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

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(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

Learning Outcomes

You should be able to

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1. state the meaning of chemical equation identify the reactants and products of a chemical equation

2. write and balance chemical equations3. interpret chemical equations quantitatively and qualitatively4. solve numerical problems using chemical equations5. identify positive scientific attitudes and values practiced by scientist in doing research 6. justify the need to practice positive scientific attitudes and good values in doing researsh7. use chemical equations for easy and systematic communication in the field of chemistry.

Activity 13 (refer text book pg 48)

Example: C (s) + (g) (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 (g) + (g) 2 (l)

(Interpreting): 2 molecules (2 mol) of react with 1 molecule (1 mol) of 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 ……………………………

……….. + ……….. …………………………..

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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:

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

Activity 14

** Numerical Problems Involving Chemical Equations

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Hydrogen peroxide decomposes according to the following equation:

2 (l) 2 (l) + (g)

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

decomposition of 34 g of hydrogen peroxide, .

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

(Ans: 11.2 dm3)

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

2 Ag2CO3(l) 4 (s) + 2 (g) +

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, is reacted with excess methane, . Using the equation

below, find the mass of copper that is produced.

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

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4 (s) + (g) 4 (s) + (g) + 2 (l)

(Ans : 12.8 g)

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

equation below:

(s) (s) + (g).

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

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

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

24 dm3 at room conditions]

(Ans : (a). 4.8 dm 3 (b) 11.2 g)

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3 Chemical Formulae and Equations

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