Relative Mass of Atom and Molecule

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Card 1: Define relative atomic mass.

Relative Atomic Mass (RAM)

Relative atomic mass is the mass of one atom of an element compared to 1/12 that of the mass of an atom of Carbon-12. The relative atomic mass of carbon-12 is taken to be 12.

Example 1
The mass of sodium atom  is 23 times greater than 1/12 of  the mass of carbon-12 atom. What is the relative atomic mass of sodium?

Answer

Relative Atomic Mass of Sodium
= 23

Example 2
An atom of element X is 13 times heavier than one atom of helium. Calculate the relative atomic mass of X.( Ar: He = 4 )

Answer

Relative Atomic Mass of X
= 13 x 4 = 52

Example 3
How many times that 2 bromine atoms are greater than 4 neon atoms. (Ar: Ne = 20; Br = 80 )

Answer

Relative mass of 2 bromine atom = 2 x 80 = 160.
Relative mass of 4 neon atom = 4 x 20 = 80.

The mass of 4 neon atom is 160/80 = 2 times greater than 2 bromine atom.

Card 2: Define relative molecular mass.

Relative Molecular Mass (RMM)

Relative molecular mass is the mass of one molecule of an element or compound compared to that of the mass of an atom of Carbon-12. The relative atomic mass of carbon-12 is taken to be 12.

Card 3: Finding relative molecular mass.

Finding Relative Molecular Mass

A relative molecular mass can be calculated easily by adding together the relative atomic masses of the constituent atoms.

Example 1
What is the relative formular mass of  aluminium sulphate [Al2(SO4)3]?
( Mr: O = 16, S = 32; Al = 27 )

Answer

Relative Formular Mass of Aluminium Sulphate
= 2(RAM of Aluminium) + 3[RAM of Sulphur + 4(RAM of Oxygen)]
= 2(27) + 3[32 + 4(16)] = 327

 

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