2. Measurementand Units
A lot of experimental chemistry relies on
accurate measurement which should allow the
scientific community to communicate
effectively across the disciplines and border.
Scientist have developed the SI system
SystemeInternational
3. SI UNITS
Mass _ kg
1000 g = 1 kg
1000 mg = 1 g
Time – s
60 sec = 1 min
60 min = 1 hr
Temperature – K
Based on thermal motion
Absolute zero (all thermal motion ceases)
K = ˚C + 273
4. Volume –m3 (cubic metre)
1 L = 1 dm3= 1000 mL
Pressure –Pa (Pascal)
1 atmosphere = 1.01 x 105Pa = 101 kPa
thesequantitiesarehowevernotalwaysconvenientforthe
quantitiestypicallyusedinlaboratory
In a lab: Mass in g
volume in cubic centimetre
Pressure in atm.
5. Amounts of Substances
Chemist need to measure quantities for certain purposes.
As we know that, all matter composed of varying types of
substances which are
Atoms–building blocks of matter; extremely small; exist as
varieties in universe (elements)
Compounds–substances composed of atoms joined together in
whole-number ratios
Because of size, atoms and compounds cannot be counted
directly
Counted based on mass and using mole concept
6. THE MOLE
WHAT IS A MOLE ?
it is the standard unit of amount of a substance -
it is just a number, a very big number
it is a way of saying a number in words, just like...
DOZEN for 12 SCORE for 20 GROSS for 144
HOW BIG IS IT ?
602200000000000000000000 (Approximately)... THAT’S BIG !!!
It is a lot easier to write it as... 6.022 x 1023
It doesn’t matter what the number is as long as everybody sticks to the same
value
7. Avogadro No. or MOLE
The ratios with which elements combine depend on the
number of atoms not on their mass.
Individual atoms and molecules are extremely small.
Hence a larger unit is appropriate for measuring
quantities of matter.
A mole is the amount of substance which
contains the same no. of chemical sps as there
are atoms in exactly 12 g of the C-12 . Its S.I. unit
is mol.
This number is known as Avogadro’s constant.
1 mole is equal to 6.02 x 1023 particles.
One gram of H atoms contains 6.02 x 1023
atoms.
8. THE MOLE
WHY USE IT ?
Atoms and molecules don’t weigh much so it is easier to count large
numbers of them. In fact it is easier to weigh substances.
Using moles tells you... how many particles you get in a certain mass
the mass of a certain number of particles
DO I NEED TO KNOW ANYTHING ELSE ?
Yes, it would help if you can balance equations
AND Keep trying, you will get the idea ... EVENTUALLY!
9. THE MOLE
CALCULATING THE NUMBER OF MOLES OF A SINGLE SUBSTANCE
moles = mass/ molar mass
mass = moles x molar mass
molar mass = mass / moles
UNITS
mass g or kg
molar mass g mol-1 or kg mol-1
MOLES(n) = MASS(m)
MOLAR MASS
MASS
__________
MOLES x
MOLAR MASS
10. Molar Mass
The mass of 1 mole of a species is c/d the molar mass. It
is the relative mass expressed in g and has unit g/mol.
The relative molecular mass(Mr) is defined as the sum of
the relative atomic masses of the atoms in the molecular
formula.it is expressed in g.
Q-Calculatethe relative molecular mass ofethanol C2H5OH
The relative molecular mass of the molecule
={relative atomic mass* no. of atoms}C,H,O
= ( 2×12)+(6×1.01)+16.00
= 46.08
11. The term Relative molecular mass is used in case of
molecules ,but in case of ions relative formula mass is
used.
Calculatetheno. ofmoles in4.00gof sodium hydroxide,NaOH.
Sol:Relative atomic masses are Na=22.99,O=16,H=1.01
Relative formula mass =2.99+16.00+1.01
=40.00g/mol
No. of moles(n)= m =4.00 =0.100mol
M 40.00
12. MOLES OF A SINGLE
SUBSTANCE
1. Calculate the number of moles of oxygen molecules in 4g
oxygen molecules have the formula O2
relative mass will be 2 x 16 = 32 ; molar mass will be 32g mol-1
moles = mass = 4g = 0.125 mol
molar mass 32g mol -1
2.What is the mass of 0.25 mol of Na2CO3 ?
Relative Molecular Mass of Na2CO3 = (2x23) + 12 + (3x16) = 106
Molar mass of Na2CO = 106g mol-1
mass = moles x molar mass = 0.25 x 106 = 26.5g
13. Counting Particles
Mass of an individual atom is measured by using
Mass Spectrometer.
From it: mass of 1 mole of12 C = 1.99252 x10-23
No. of atoms in one mole (12 g of12 C ) = 12
1.99252×10-23
= 602000000000000000000000
This is a big no. and is c/d Avogadro’s no.
No. of Particles(N)= n x Avogadro's constant(L)
N =n x L
14. Calculate the amount of water,H2O that
contains 1.80 x 1024.
N=n x L
= 1.80 x 1024 =2.99 mol
6.02x1023
(the answer should be given to 3 significant figure- the same precision as the data given in the
question. If the amount given was1.8x 1024 the correct answer would be 3.0
15. Calculate how many hydrogen atoms are present in
3.0 moles of ethanol( C2H5OH).
n= In 1 molecule of ethanol there are 6 H atoms
In 1 mole of ethanol ther are 6 moles of H atoms
In 3 moles of ethanol there are 6×3 =18 moles of
H atoms
N = nL
N =18 ×6.023×1023
= 1.08×1025
16. REACTING MASS
CALCULATIONS
CaCO3 + 2HCl ———> CaCl2 + CO2 + H2O
1. What is the relative formula mass of CaCO3? 40 + 12 + (3 x 16) = 100
2. What is the mass of 1 mole of CaCO3 100 g
3. How many moles of HCl react with 1 mole of CaCO3? 2 moles
4. What is the relative formula mass of HCl? 35.5 + 1 = 36.5
5. What is the mass of 1 mole of HCl? 36.5 g
6. What mass of HCl will react with 1 mole of CaCO3 ? 2 x 36.5g = 73g
7. What mass of CO2 is produced ? moles of CO2 = moles of CaCO3
moles of CO2 = 0.001 moles
mass of CO2 = 0.001 x 44 = 0.044g
Be the first to comment