You have learnt about atomic weight in the previous posts. And we can calculate molecular weight by simple addition of atomic weights of bonded atoms so why do we need another measuring unit?
For chemical recipes scientists need a common measuring unit like we use ‘Dozen’ for 12 items. For those they defined a new unit and named it ‘Mole’.
A mole of any substance contains similar number like 1 dozen apples contains 12 apple, 1 dozen bananas contains 12 bananas. A mole is equal to 6.022×1023 items. It is a magical number and you are going to be amazed by its magic.
Can you answer these questions?
- How many atoms of carbon are present in a mole of carbon?
- How many molecules are present in a mole of water?
Do you know the answer of the above questions is the magical number 6.022×1023. It is the master key for chemists and it is called the Avogadro constant and is denoted as N.
You have understood that one mole is equal to 6.022×1023. But how do you measure a mole of a substance? We can't count those tiny atoms or molecules, can we? Don't worry, we don't have to.
One mole is equal to the atomic weight of any element in grams. For examples: 1 mole Carbon is equal to the 12g, 1 mole Oxygen is equal to the 16g and 1 mole Hydrogen is equal to the 1g.
Similarly for molecules one mole is equal to their formula mass in grams. For example 1 mole water H2O is equal to 2(atomic wt of Hydrogen) + atomic wt of Oxygen= 2g+16g=18g.
Now try to solve these puzzles:
- How many molecules of water are present in 18g of water?
- How many atoms are present in 16g of Oxygen?
- Is the number of atoms present in 1g Hydrogen equal to the number of atoms present in12g of Carbon?
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