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� IMPORTANT TERMS, FORMULAE AND FACTS

The measurement of a physical quantity is based upon certain fixedstandards. It is guided by skill and accuracy of the observer as well as bythe limitation of the measuring scale. The accuracy of the measured quantityis generally expressed in terms of significant figures.

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The significant figures in a number are all the certain digits plus one

doubtful digit.

Rules for Counting Significant Figures

1. All non-zero digits are significant.

2. The zeros at the beginning of a number are not significant.

3. The zeros appearing in the middle of a number are significant.

4. All zeros placed at the end or to the right of a number are significantprovided they are on the right side of the decimal point.

5. Zeros at the end of a number without a decimal point are ambiguous.For example, 2600 may have two, three or four significant figures dependingupon whether the precision in measurements is 100, 10 or 1 respectively.Therefore, if no uncertainty is mentioned, the zeros at the end are regardedas non-significant. But in 2600., the zeros are significant (four significantfigures) because the number includes decimal point.

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Very large and very small numbers are generally expressed in exponentialform known as scientific notation. In this notation, a number is written as

To count the significant digitsin a number, start countingfrom left to right with first non-zero digit and ending with thedigit that has uncertain value.

1/1

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1/2 MODERN’S abc OF OBJECTIVE CHEMISTRY (NEET)

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Rounding off to 4 significant figures2.1267 → 2.1272.1264 → 2.1262.1245 → 2.124

(preceding digit is even)2.1275 → 2.128

(preceding digit is odd)

N × 10n , where N is a number with a single non-zero digit to the left of thedecimal point and n is an integer called exponent.

The numerical portion in scientific notation i.e. N represents the numberof significant figures.

The results obtained by counting are exact numbers i.e. numbers withoutany uncertainty.

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123 ← Significant

0.000 260 3 Significant digits

↑Not significant

1230.0300 3 Significant figures

12 34525.000 5 Significant figures

1

0.000005 1 Significant figure12 1234

2600 2600.2 or more 4 Significant figuressignificant figures0.00000362 3.62 × 10–6 3 Significant figures

Scientific notation

560000000000 5.6 × 1011 2 Significant figures

Scientific notation

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To retain the required number of significant digits, rounding offprocedure is applied.

Rules for Rounding Off Digits1. If the digit coming after the desired number of significant figures

happens to be more than 5, the preceding digit is increased by one.2. If the digit coming after the desired number of significant figures

happens to be less than 5, the preceding digit is not changed.3. If the digit coming after the desired number of significant figures

happens to be 5, the preceding digit is increased by 1 if it is odd and thepreceding digit is not changed if it is even.

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Addition and subtractionWhen addition or subtraction is to be carried out in the numbers having

different precisions, the final result should be reported upto the samenumber of decimal places as are present in the number having the leastnumber of decimal places.

32.0 (upto 1 decimal place)– 0.0664

31. 9336 = 31.9 (rounded off)

3.262517.390

+ 1.1 (upto 1 decimal place)

21.7525 = 21.8 (rounded off)

It may be noted that incalculations, the exact numberswill not have any effect on thenumber of significant figures. Thenumber of significant figures ina calculation result, dependsonly on the number ofsignificant figures in quantitieshaving uncertainties. 1 2

e.g. 3.0 × 8 = 24(upto two significantfigures as in 3.0)

SOME BASIC CONCEPTS OF CHEMISTRY 1/3Multiplication and divisionIn multiplication and division, the final result should be reported up to

the number of significant figures equal to the minimum significant figurespresent in the numbers multiplied or divided.

12 123

0.096 × 10.1 = 0.9696or = 0.97 (rounded off upto 2 significant figures)

1234 12

0.4962 + 8.1 = 0.061259

= 0.061 (rounded off upto 2 significant digits)Arithmetics using Scientific numbersTo add or subtract numbers in scientific notations, the exponent (n)

must be made same in both the numbers.e.g. 4.238 × 104 + 6.562 × 103

4.238 × 104 + 0.6562 × 104 = (4.238 + 0.6562) × 104

= 4.892 × 104

8.264 × 104 – 3.692 × 103

8.264 × 104 – 0.3692 × 104 = (8.264 – 0.3692) × 104

= 7.8948 × 104

For multiplication and division, we use the following relations:(10)x × (10)y = 10(x + y)

and 10

10

x

y= 10(x – y)

e.g. (4.2 × 103) × (3.6 × 105) = (4.2 × 3.6) × 103 + 5

= 15.12 × 108

××

10

64.76 10

3.20 10= −⎛ ⎞

⎜ ⎟⎝ ⎠10 64.76

103.20

= 1.4875 × 104

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The term precision refers to the closeness of the set of values obtainedfrom identical measurements of a quantity. It is expressed as the differencebetween a measured value and the arithmetic mean value for a series ofmeasurements.

i.e., Precision = Individual value – Arithmetic mean value.Accuracy refers to the closeness of a single measurement to its true

value. It is a measure of the difference between the experimental value orthe mean value of a set of measurements and the true value.

i.e., Accuracy = Mean value – True value.Smaller the difference between the mean value and the true value, the

larger is the accuracy.

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The value of a physical quantity is expressed as its mathematical valueand a unit. There are three basic units of mass, length and time. These arecalled fundamental units. The units of other quantities can be derivedfrom these units and hence are called derived units.

In recent years, scientists have agreed to use SI units. The designationcomes from its French name Systeme Internationale.

The SI system has seven basic units from which all other units arederived.

Before adding or subtractingscientific numbers, it isimportant to make bothnumbers to the same power of10.

1/4 MODERN’S abc OF OBJECTIVE CHEMISTRY (NEET)

It may be noted that the unitof temperature according to S.I.system is Kelvin but stillcelsius scale (°C) oftemperature is commonly usedin our daily life. These two unitsare related as :Kelvin temperature (K)

= °C + 273.15

Different scales of temperature

°F = ( )932

5C° +

°C = [ ]5F 32

9° −

Also, K = °C + 273.15°C = K – 273.15

e.g.� Room temperature

25°C = 77°F = 298.15 K� Human body temperature

37°C = 98.6°F = 310.15 K

These are given below :Table 1.1. Seven Basic S.I. Units.

Quantity Unit Symbol

Length metre mMass kilogram kgTime second sTemperature kelvin KAmount of substance mole molElectric current ampere ALuminous intensity candela cd

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The SI units of some of the physical quantities are either too small or toolarge. To change the order of magnitude, these are expressed by using prefixesbefore the name of the units. The various prefixes are given below :

Table 1.2. List of Common prefixes

Multiple Prefix Symbol

10–1 deci d10–2 centi c10–3 milli m10–6 micro μ10–9 nano n10–12 pico p10–15 femto f10–18 atto a10–21 zepto z10–24 yocto y

Multiple Prefix Symbol

101 deka da102 hecto h103 kilo k106 mega M109 giga G1012 tera T1015 peta P1018 exa E1021 zetta Z1024 yotta Y

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The units of different physical quantities can be derived from the basicseven systems. These are called derived units. For example,

Area is equal to Area = Length × Breadth = m × m = m2

Density is mass per unit volume

Density =−= = 3

3kgMass

kg mVolume m

Velocity is change in distance per unit time

Velocity = Distance

Time =

ms

= m s–1

Acceleration is change in velocity per unit time

Acceleration = Velocity

Time =

−1m ss

= m s–2

SOME BASIC CONCEPTS OF CHEMISTRY 1/5

� Law of conservation of masswas proposed by Lavoisier.

� Law of definite proportionswas proposed by JosephProust.

� Law of multiple proportionswas proposed by Dalton.

� Law of reciprocal proportionswas proposed by Richter.

Force is mass multiplied by acceleration. Force = Mass × Acceleration = (kg) × (m s–2) = kg m s–2

Some common derived units are given below :

Table 1.3. Some common derived units.

Physical quantity Relation with other basic quantities S.I. units

Area Length square m2

Volume Length cube m3

Density Mass per unit volume kg/m3 or kg m–3

Speed Distance travelled per unit time m/s or m s–1

Acceleration Speed change per unit time m/s × 1/s or m s–2

Force Mass × acceleration kg m/s2 or kg m s–2 (= newton, N)Pressure Force per unit area kg m–1 s–2 (= pascal, Pa)Energy Force × distance travelled kg m s–2 × m or kg m2 s–2 (= Joule, J)

All the objects around us are composed of matter. Matter is defined asanything that occupies space and has mass. The matter can be felt by one ormore of our senses. The study of matter is the fundamental study of science.

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According to physical classification, matter may be classified into threestates : solid, liquid and gaseous state.

On the basis of chemical composition, matter may be classified aselements, compounds or mixtures.

Element. The simplest pure form of matter which cannot be decomposedby ordinary chemical means nor it can be formed by the combination ofother substances.

Compound. A substance made up of two or more elements chemicallycombined in a fixed proportion by weight.

Mixture. A material obtained by mixing two or more substances(elements or compounds) in any proportion is called a mixture.

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1. Law of conservation of mass. This law states that during any physicalor chemical change, the total mass of the products is equal to the total massof the reactants. This law is also known as law of indestructibility of matter.

2. Law of constant composition. This law states that a pure chemicalcompound always contains the same elements combined together in thesame proportion by weight.

3. Law of multiple proportions. According to this law, when twoelements combine to form two or more than two compounds, then the weightsof one of these elements which combine with the fixed weight of other elementbear a simple ratio to one another.

4. Law of reciprocal proportions. This law states that weights of twoelements A and B, which separately combine with a third element C, areeither the same or simple multiple of the weights of A and B which combinewith each other. This is also called law of equivalent proportion.

5. Gay Lussac's law of combining volumes. This law states that undersimilar conditions of temperature and pressure, whenever gases reacttogether, the volumes of the reacting gases as well as products (if gases)bear a simple whole number ratio.

1/6 MODERN’S abc OF OBJECTIVE CHEMISTRY (NEET)

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The main postulates of Dalton's atomic theory are :(i) All matter is made up of very tiny particles which cannot be further

broken. These tiny particles are called atoms.(ii) Atoms can neither be created nor destroyed during chemical

reactions.(iii) Atoms of a particular element are identical in all respects, i.e., have

the same size, mass and other properties.(iv) Atoms of different elements are different.(v) Atoms combine to form molecules.(vi) Atom is the smallest unit that takes part in chemical combination.

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� According to Berzelius hypothesis equal volumes of all gases containequal number of atoms under similar conditions of temperature andpressure.

� Avogadro’s law states as :Under similar conditions of temperature and pressure, equal volumesof all gases contain equal number of molecules.Mathematically, we can say V � Nwhere N is the number of molecules.For example, if we enclose equal volumes of three gases hydrogen(H2), oxygen (O2) and chlorine (Cl2) in different flasks of the samecapacity under similar conditions of temperature and pressure, wefind that all the flasks have the same number of molecules. However,these molecules may differ in size and mass.Avogadro’s law helped to explain :(i) Atomicity of elementary gases. The number of atoms present

in one molecule of the substance is called atomicity.e.g. atomicity of oxygen is 2

atomicity of ozone is 3(ii) Relationship between vapour density and molar mass of a

gas. These are related as :Molar mass = 2 × Vapour density

(iii) Relationship between mass and volume of gas. 22.4 L of anygas at S.T.P. weigh equal to molar mass expressed in grams i.e.gram molecular mass.

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Scientists’ quest to know the ultimate particle of a substance has led tothe discovery of atoms and molecules.

Atom. The smallest particle of an element which may or may not haveindependent existence is called an atom.

Molecule. The smallest particle of a substance (element or compound)which is capable of independent existence is called a molecule.

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Atomic mass. Atoms are extremely small particles and it is very difficultto determine their actual masses. For example, mass of one atom ofhydrogen has been found to be = 0.00000000000000000000000166 g or1.66 × 10–24 g. Obviously, such a small mass cannot be determined evenwith the help of a very sensitive balance. Moreover, this value is not veryconvenient for calculations. To overcome this problem, the atomic massesare expressed as relative masses, i.e. with reference to mass of a standard

For a universally acceptedatomic mass unit in 1961,carbon - 12 isotope was selectedas standard. Today, this scaledepends on the measurement ofatomic mass by a massspectrometer. We can makeaccurate measurement of masson this instrument bycomparing mass of an atom withthe mass of a particular atomselected as standard. On thepresent atomic mass scale, thecarbon – 12 is chosen as thestandard and is arbitrarilyassigned a mass of exactly 12atomic mass unit. One atomicunit (amu ) is thus the massequal to exactly 1/12th of themass of a carbon – 12 atom.

MOD ABC Of Objective Chemistry ForNEET Part-1

Publisher : MBD GroupPublishers

ISBN : 9789351846772Author : Dr. S. P. Jauhar,Dr. Sheenu Jauhar

Type the URL : http://www.kopykitab.com/product/11994

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