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Chapter 3 : Chemical Formulae and Equations

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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 , A r = The average mass of an atom of the element

1/12 x the mass of an atom of carbon-12

Example: A r of C=12 A r of O=16 A r 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, M r = The Average mass of a molecule of the substance

1/12 x the mass of an atom of carbon-12


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Calculating Relat ive m olecular m ass ,M rMr = The sum of A r of all atoms present in one molecule

Example:

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

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

For ionic substance , Relative formula mass , F r = The sum of A r of all atoms present in the formula

Example:F r of Magnesium oxide, MgO = 24 + 16 = 40F r 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, M r

Hydrogen gas H 2 2(1) = 2Propane C 3H8

Ethanol C 2H5OHBromine gas Br 2

Methane CH 4 Glucose C 6H12 O 6

Ammonia NH 3 [Relative atomic mass : H,1; C,12; O,16; Br,80 ; N,14 ]

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

Substance Compound formula Relative formula mass, F r

Potassium oxide K 2O 2(39) + 16 = 94

Aluminium sulphate Al 2(SO 4)3 2(27)+3[32+4(16)]=342

2 HydrogenatomsMolecular

formula

Relative atomic massfor OxygenRelative atomic mass

for Hydrogen

All A r, M r andFr have no unit


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Zinc nitrate Zn(NO 3)2

Aluminium nitrate Al(NO 3)3

Calcium carbonate CaCO 3

Calcium hydroxide Ca(OH) 2

Hydrated copper(II)sulphate

CuSO 4.5H 2O 64 + 32 + 4(16) + 5[2(1) + 16]=250

Hydrated sodium

carbonate

Na 2CO 3.10H 2O

Sodium hydrogensulphate

NaHSO 4

Aluminium chloride AlCl 3

Copper(II) sulphate CuSO 4

Zinc carbonate ZnCO 3

Potassiumcarbonate

K2CO 3

[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]

B THE MOLE AND THE NUMBER OF PARTICLES


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

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.


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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 10 23 is called (N A)

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

0.5 mol of carbon atoms atoms of carbon

0.2 moles of hydrogen gas ( H 2) (i) .. moleculesof hydrogen gas

(ii) . Atoms of hydrogen

2 mol of carbon dioxide molecules x 10 23 molecules of carbon dioxide gascontains :

. atoms of C and

. atoms of O

number of moles number of particles


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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 ( O 3) . x 10 23 molecules of ozone,

contains :

atoms of Oxygen.

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) contain s .. ions

d) 2 mol of sodium carbonate, (Na 2CO 3) contains .

e) 3 mol of carbon dioxide, (CO 2) cont ains .. molecules

f) 0.5 mol Copper (II) nitrate, Cu(NO 3)2 co ntains .. Cu 2+ ions

and . NO 3- ions

C NUMBER OF MOLES AND MASS OF SUBSTANCES


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 (N A) number of particles

= The mass of ____________________ particles


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x 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 H 2O ?

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

= 36g

3. Calculate the masses of these substances

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 (O 2)Mass =

e) 0.1 moles of sodiumMass =

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

g) 1 mole of carbon dioxide ( CO 2)Mass =

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

i) 2 moles of calcium carbonate (CaCO 3 )Mass =

j) 0.25 moles of calcium chloride (CaCl 2 )Mass =

Number of moles Mass in g Molar mass


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

l) 0.25 moles of sodium carbonate (Na 2CO 3)Mass =

m) 0.5 moles of potassium manganate (VII)

(KMnO 4)Mass =

n) 0.25 moles of hydrated magnesium sulphate

(MgSO 4.7H 2O)Mass =

Activity 5

4. Calculate the Number of Moles from a given Mass

Example : How many moles are there in 88g of CO 2

Number of moles = 88 = 2 moles

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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 (O 2)Number of moles = f) 213g of chlorine molecules (Cl2)Number of moles =

g) 0.56g of nitrogen molecules (N 2)Number of moles =

h) 254g of iodine molecules (I 2)Number of moles =

i) 88g of carbon dioxide (CO 2)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 (H 2SO 4)Number of moles =


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m) 170g of ammonia (NH 3)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 =

q) 15.8g of potassium manganate (VII)KMnO 4

Number of moles =

r) 8g of ammonium nitrate (NH 4NO 3)Number of moles =

s) 0.78g of aluminium hydroxide Al(OH) 3 Number of moles = t) 0.92g of ethanol (C 2H5OH)Number of moles =

Activity 6

5. Complete the following table.

Element/compoundChemicalformulae Molar mass Calculate

Copper Cu RAM= 64 (a)Mas s of 1 mol = g

(b) Mass of 2 mol = . g

(c)Mass of mol = . g

(d)Mass of 3.01x10 23 Cu atoms

=

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

(b) Number of moles of sodium hydroxide in20 g =

Zinc nitrate Zn(NO 3)2 RFM = a) Number of moles in 37.8 g of zinc nitrate :


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D NUMBER OF MOLES AND VOLUME OF GAS

Learn ing Outcom esYou 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 o f a gas is defined as the .

.

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

temperature and pressure.

3. The molar volume of any gas is

24 dm 3 a t or

22.4 dm 3 at .

Example :

1 mol of oxygen gas, 1 mol of ammonia gas, 1 mol helium gas and 1 mol sulphur dioxide gas occupiesthe same volume of 24 dm 3 at room condition

x 22.4 / 24 dm 3

x 22.4/24 dm 3 22.4/24 dm 3

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

Example : Find the volume of 1 mole of CO 2 gas

Volume = number of moles x 22.4 dm 3 = 1 x 22.4 dm 3 = 22.4 dm 3

Number of moles of gas Volume of gas


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a) 3 moles of oxygenVolume =

b) 2 moles of CH 4 Volume =

c) 0.3 moles of ArgonVolume =

d) 0.2 moles of SO 3 Volume =

e) 0.1 moles of N 2 Volume =

f) 1.5 mol of N 2 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 dm 3 mol -1 at STP]

( Ans: 6.72 dm 3)

2. Find the number of moles of oxygen gas contained in a sample of 120 cm 3 of the gasat room conditions.[Molar volume: 24 dm 3 mol -1 at room conditions]

Volume of gas (dm )

Number of molesMass in gram No of particles


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(Ans: 0.005 mol)

3. Calculate the number of water molecules in 90 g of water, H 2O.[Relative atomic mass: H, 1; O, 16. Avogadro constant, N A: 6.02 x 10 23 mol -1]

(Ans; 3.01x 10 24 molecules)

4. What is the volume of 24 g methane ,CH 4 at STP?[Relative atomic mass: H, 1; C, 12. Molar volume: 22.4 dm 3 mol -1 at STP]

(Ans: 33.6 dm 3)5. How many aluminium ions are there in 20.4 g of aluminium oxide, Al 2O 3?

[Relative atomic mass: O, 16; Al, 27. Avogadro constant, N A: 6.02 x 10 23 mol -

(2 x 0.2 x 6.02 x10 23 )

6. Calculate the number of hydrogen molecules contained in 6 dm3 of hydrogen gas at

room conditions.[Molar volume: 24 dm 3 mol -1 at room conditions Avogadro constant, N A: 6.02 x 10 23 mol -1]


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(Ans: 1.505x10 23 molecules)7. Find the volume of nitrogen in cm 3 at STP that consists of 2.408 x 10 23 nitrogen molecules.

[Molar volume: 22.4 dm 3 mol -1 at STP. Avogadro constant, N A: 6.02 x 10 23 mol -1]

(Ans: 8.96 dm 3 )E CHEMICAL FORMULAE

Learning OutcomesYou should be able to

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

H2 Subscript shows 2hidrogen atoms ina molecule

The letter Hshows

. .


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

.. NH 3 . atoms of H combine with 1 atom of N

PropaneC3H8 .. atoms of C combine with . atoms of

H

Magnesium oxide

.. .

.. H2SO 4

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 GlucoseMolecular formula - 42 H C Molecular formula - 6126 O H C

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

Molecular formula = (Empirical formula) n


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Activity 11 :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 2 Ba 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 272 OCr

Aluminium ion 3 Al 34

PO

Ammonium ion Chromate (VI) ion

Avtivity 12

a) Chemical formula of an ionic compound comprising of the ions X m+ and Y n- is constructed

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

Example : Sodium oxide Copper (II) nitrate

Na+

O2-

Cu2+

NO 3-

+1 -2 +2 -1

2 1 1 2

= Na 2O = ....................


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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

Learning Outcomes

You should be able to

1. state the meaning of chemical equationidentify 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 research6. 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.


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

Reactant product1) 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

.


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2) Quantitative aspect of chemical equations

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

equation.

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

(Interpreting): 2 molecules (2 mol) of 2 H 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 ..

+ O 2 ..

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:

.


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Activity 14

** Numerical Problems Involving Chemical Equations

Hydrogen peroxide decomposes according to the following equation:

2 22O H

(l ) 2 O H 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, 22 O H .

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

(Ans: 11.2 dm 3)

2).Silver carbonate Ag 2CO 3 breaks down easily when heated to produce silver metal

2 Ag 2CO 3(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)


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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 O H 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 dm 3 1mol at room conditions]

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

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