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Mole vs Number of Particles

7:52 AM by Unknown

1 mole of chemical substance = 6.02 x 10^{23 of particles}

Avogadro constant = 6.02 x 10²³

Example 1
Find the number of atoms in 2.5 mol of gold.

Answer:
Number of atoms
= Number of mole x Avogadro constant
= 2.5 x 6.02 x 10²³ = 1.505 x 10²⁴

Example 2
How many moles of magnesium that contain 2.76 x 10²³ of magnesium atom?

Answer:
Number of mole
= Number of atoms ÷ Avogadro constant
= 2.76 x 10²³÷ 6.02 x 10²³
= 0.46 mol

Example 3:

How many hydrogen molecules can be made with 20 mol of hydrogen atoms?
How many hydrogen atoms are there in 20 mol of hydrogen molecules?

Answer:
a.
Each hydrogen molecules (H₂) consist of 2 hydrogen atoms.
Therefore, 20 mol of hydrogen atoms can make 10 mole of hydrogen molecules.

b. Number of mole of hydrogen atoms in 20 mole of hydrogen molecules
= 2 x 20 mole = 40 mole.

Example 4:

Find the number of carbon atom in 0.75 mol of carbon dioxide.
Find the number of oxygen atom in 0.75 mol of carbon dioxide.

Answer:
a.
Each carbon dioxide molecules (CO₂) consist of 1 carbon atom and 2 oxygen molecules.
Therefore, 0.75 mole of carbon dioxide contain 0.75 mole carbon atoms.

b.
Number of mole of oxygen atoms in 0.75 mole of carbon dioxide
= 2 x 0.75 mole = 1.50 moles.

Relative Atomic Mass & Relative Molecular Mass

10:44 PM by Unknown

RAM:

Based on Hydrogen Scale:

Therefore, there is weakness of this method:

Not many elements can react readily with hydrogen or are unable to displace hydrogen.
it was found that the relative masses of some elements were not accurate.
hydrogen exists as a gas at room temperature, it is difficult to handle.
it has a number of isotopes with different masses

At present, Carbon-12 is used as the standard atom of comparison internationally.