Chap-1 Mole Concept
- Chemistry is basically an experimental science.
- In it we study physical and chemical properties of substance and measure it upto possibility.
- The results of measurement can we reported in two steps,
(i) Arithmetic number
(ii) Unit of measurement.
- Every experimental measurement vary slightly from one another and involves
some error or uncertainty depending upon the skill of person making the
measurements and measuring instrument.
- The closeness of the set of values obtained from identical measurement called
precision and a related term, refers to the closeness of a single measurement
to its true value called accuracy.
Significant figures
In the measured value of a physical quantity, the digits about the correctness of which we are surplus the last digit which is doubtful,
are called the significant figures. Number of significant figures in a physical quantity depends upon the least count of the instrument used
for its measurement.
(1) Common rules for counting significant figures
Following are some of the common rules for counting significant figures in a given expression
Rule 1. All non zero digits are significant.
Example : x 1234 has four significant figures. Again x 189 has only three significant figures.
Rule 2. All zeros occurring between two non zero digits are significant.
Example : x 1007 has four significant figures. Again x 1 . 0809 has five significant figures.
Rule 3. In a number less than one, all zeros to the right of decimal point and to the left of a non zero digit are not significant.
Example : x 0 .0084 has only two significant digits. Again, x 1 . 0084 has five significant figures. This is on account of rule 2.
Rule 4. All zeros on the right of the last non zero digit in the decimal part are significant.
Example : x 0 . 00800 has three significant figures 8, 0, 0. The zeros before 8 are not significant again 1.00 has three significant
figures.
Rule 5. All zeros on the right of the non zero digit are not significant.
Example : x 1000 has only one significant figure. Again x 378000 has three significant figures.
Rule 6. All zeros on the right of the last non zero digit become significant, when they come from a measurement.
Example : Suppose distance between two stations is measured to be 3050 m. It has four significant figures.
The same distance can be expressed as 3.050 km or 3. 050 10 5 cm . In all these expressions, number of significant figures
continues to be four.
(2) Rounding off :
While rounding off measurements, we use the following rules by convention
Rule 1. If the digit to be dropped is less than 5, then the preceding digit is left unchanged.
Example : x 7. 82 is rounded off to 7.8, again x 3 . 94 is rounded off to 3.9.
Rule 2. If the digit to be dropped is more than 5, then the preceding digit is raised by one.
Example : x = 6.87 is rounded off to 6.9, again x = 12.78 is rounded off to 12.8.
Rule 3. If the digit to be dropped is 5 followed by digits other than zero, then the preceding digit is raised by one.
Example : x = 16.351 is rounded off to 16.4, again x 6 .758 is rounded off to 6.8.
Rule 4. If digit to be dropped is 5 or 5 followed by zeros, then preceding digit is left unchanged, if it is even.
Example : x = 3.250 becomes 3.2 on rounding off, again x 12 .650 becomes 12.6 on rounding off.
Rule 5. If digit to be dropped is 5 or 5 followed by zeros, then the preceding digit is raised by one, if it is odd.
Example : x = 3.750 is rounded off to 3.8, again x 16 .150 is rounded off to 16.2.
(3) Significant figure in calculation
(i) Addition and subtraction : In addition and subtraction the following points should be remembered
(a) Every quantity should be changed into same unit.
(b) If a quantity is expressed in the power of 10, then all the quantities should be changed into power of 10.
(c) The result obtained after addition or subtraction, the number of figure should be equal to that of least, after decimal point.
Example : 10.11 kg + 3.6 kg = 13.71 kg , but it should be written as 13.7 (as the least decimal point is 1 in 3.6 kg)
, MD’s Chemistry
(ii) Multiplication and division:
(a) The number of significant figures will be same if any number is multiplied by a constant.
(b) The product or division of two significant figures, will contain the significant figures equal to that of least.
Example : 2.8723 x 1.6 = 4.59568 , but it should be written as 4.6 (as the least significant figures are 2 in 1.6)
Units for measurement
The chosen standard of measurement of a quantity which has essentially the same nature as that of the quantity is called the unit
of the quantity. Following are the important types of system for unit,
(1) C.G.S. System : Length (centimetre), Mass (gram), Time (second)
(2) M.K.S. System : Length (metre), Mass (kilogram), Time (second)
(3) F.P.S. System : Length (foot), Mass (pound), Time (second)
(4) S.I. System : The 11th general conference of weights and measures (October 1960) adopted International system of units, popularly
known as the SI units. The SI has seven basic units from which all other units are derived called derived units. The standard prefixes which helps to
reduce the basic units are now widely used.
Table 1.1 Seven basic S.I. units
Length Mass Time Temperature Electric Current Luminous Amount of
Intensity substance
metre (m) Kilogram (kg) Second (s) Kelvin (K) Ampere (A) Candela (Cd) Mole (mol)
Table 1.2 Derived Units
Physical quantity Unit Symbol
Area square metre m2
Volume cubic metre m3
Velocity metre per second ms–1
Acceleration metre per second square ms–2
Density kilogram per cubic metre kg m–3
Molar mass kilogram per mole kg mol–1
Molar volume cubic metre per mole m3 mol–1
Molar concentration mole per cubic metre mol m–3
Force newton (N) kg m s–2
Pressure pascal (Pa) N m–2
Energy work joule (J) kg m2 s–2, Nm
- Chemistry is basically an experimental science.
- In it we study physical and chemical properties of substance and measure it upto possibility.
- The results of measurement can we reported in two steps,
(i) Arithmetic number
(ii) Unit of measurement.
- Every experimental measurement vary slightly from one another and involves
some error or uncertainty depending upon the skill of person making the
measurements and measuring instrument.
- The closeness of the set of values obtained from identical measurement called
precision and a related term, refers to the closeness of a single measurement
to its true value called accuracy.
Significant figures
In the measured value of a physical quantity, the digits about the correctness of which we are surplus the last digit which is doubtful,
are called the significant figures. Number of significant figures in a physical quantity depends upon the least count of the instrument used
for its measurement.
(1) Common rules for counting significant figures
Following are some of the common rules for counting significant figures in a given expression
Rule 1. All non zero digits are significant.
Example : x 1234 has four significant figures. Again x 189 has only three significant figures.
Rule 2. All zeros occurring between two non zero digits are significant.
Example : x 1007 has four significant figures. Again x 1 . 0809 has five significant figures.
Rule 3. In a number less than one, all zeros to the right of decimal point and to the left of a non zero digit are not significant.
Example : x 0 .0084 has only two significant digits. Again, x 1 . 0084 has five significant figures. This is on account of rule 2.
Rule 4. All zeros on the right of the last non zero digit in the decimal part are significant.
Example : x 0 . 00800 has three significant figures 8, 0, 0. The zeros before 8 are not significant again 1.00 has three significant
figures.
Rule 5. All zeros on the right of the non zero digit are not significant.
Example : x 1000 has only one significant figure. Again x 378000 has three significant figures.
Rule 6. All zeros on the right of the last non zero digit become significant, when they come from a measurement.
Example : Suppose distance between two stations is measured to be 3050 m. It has four significant figures.
The same distance can be expressed as 3.050 km or 3. 050 10 5 cm . In all these expressions, number of significant figures
continues to be four.
(2) Rounding off :
While rounding off measurements, we use the following rules by convention
Rule 1. If the digit to be dropped is less than 5, then the preceding digit is left unchanged.
Example : x 7. 82 is rounded off to 7.8, again x 3 . 94 is rounded off to 3.9.
Rule 2. If the digit to be dropped is more than 5, then the preceding digit is raised by one.
Example : x = 6.87 is rounded off to 6.9, again x = 12.78 is rounded off to 12.8.
Rule 3. If the digit to be dropped is 5 followed by digits other than zero, then the preceding digit is raised by one.
Example : x = 16.351 is rounded off to 16.4, again x 6 .758 is rounded off to 6.8.
Rule 4. If digit to be dropped is 5 or 5 followed by zeros, then preceding digit is left unchanged, if it is even.
Example : x = 3.250 becomes 3.2 on rounding off, again x 12 .650 becomes 12.6 on rounding off.
Rule 5. If digit to be dropped is 5 or 5 followed by zeros, then the preceding digit is raised by one, if it is odd.
Example : x = 3.750 is rounded off to 3.8, again x 16 .150 is rounded off to 16.2.
(3) Significant figure in calculation
(i) Addition and subtraction : In addition and subtraction the following points should be remembered
(a) Every quantity should be changed into same unit.
(b) If a quantity is expressed in the power of 10, then all the quantities should be changed into power of 10.
(c) The result obtained after addition or subtraction, the number of figure should be equal to that of least, after decimal point.
Example : 10.11 kg + 3.6 kg = 13.71 kg , but it should be written as 13.7 (as the least decimal point is 1 in 3.6 kg)
, MD’s Chemistry
(ii) Multiplication and division:
(a) The number of significant figures will be same if any number is multiplied by a constant.
(b) The product or division of two significant figures, will contain the significant figures equal to that of least.
Example : 2.8723 x 1.6 = 4.59568 , but it should be written as 4.6 (as the least significant figures are 2 in 1.6)
Units for measurement
The chosen standard of measurement of a quantity which has essentially the same nature as that of the quantity is called the unit
of the quantity. Following are the important types of system for unit,
(1) C.G.S. System : Length (centimetre), Mass (gram), Time (second)
(2) M.K.S. System : Length (metre), Mass (kilogram), Time (second)
(3) F.P.S. System : Length (foot), Mass (pound), Time (second)
(4) S.I. System : The 11th general conference of weights and measures (October 1960) adopted International system of units, popularly
known as the SI units. The SI has seven basic units from which all other units are derived called derived units. The standard prefixes which helps to
reduce the basic units are now widely used.
Table 1.1 Seven basic S.I. units
Length Mass Time Temperature Electric Current Luminous Amount of
Intensity substance
metre (m) Kilogram (kg) Second (s) Kelvin (K) Ampere (A) Candela (Cd) Mole (mol)
Table 1.2 Derived Units
Physical quantity Unit Symbol
Area square metre m2
Volume cubic metre m3
Velocity metre per second ms–1
Acceleration metre per second square ms–2
Density kilogram per cubic metre kg m–3
Molar mass kilogram per mole kg mol–1
Molar volume cubic metre per mole m3 mol–1
Molar concentration mole per cubic metre mol m–3
Force newton (N) kg m s–2
Pressure pascal (Pa) N m–2
Energy work joule (J) kg m2 s–2, Nm
