Relative Atomic Mass & Relative Molecular Mass

CHAPTER 3 : CHEMICAL FORMULAE & EQUATIONS 3.1 Relative Atomic Mass & Relative Molecular Mass

3.1 Relative Atomic Mass & Relative Molecular Mass

At the end of this lesson you should be able to:

1.State the meaning of relative atomic mass, Ar based on C-12 scale. 2.State the meaning of relative molecular mass, Mr based on C-12 scale. 3.State why C-12 is used as a standard for determining the Ar and Mr. 4.Calculate the Mr of substances.

THE MEANING OF

Mr

Ar

Why C-12 is used as the standard???

Calculation of Mr

C-12 AS THE STANDARAD

Used as a reference standard in mass spectrometer

Solid at room temperature, easily handled

Easily available, present in many organic substances

RELATIVE ATOMIC MASS, Ar The average mass of one atom of the element when compared with 1/12 of the mass of an atom C-12

Ar =

the average mass (g) of 1 atom 1/12 x mass of an atom C-12

• Ar does not have any unit

EXAMPLE 1

The Ar of a helium atom is 4. This means that one atom of helium is 4 times heavier than 1/12 of one atom of C-12 2

The Ar of an oxigen atom is 16. This means that one atom of oxigen is 16 times heavier than 1/12 of one atom of C-12

RELATIVE MOLECULAR MASS, Mr The average mass of the molecule when compared with 1/12 of the mass of an atom C-12

Mr =

the average mass (g) of 1 molecule 1/12 x mass of an atom C-12 • Mr does not have any unit

EXAMPLE 1 Water, H2O has a Mr of 18. This means that 1 molecule of water is 18 times heavier than 1/12 of one atom of C-12 2 Ammonia, NH3 has a Mr of 17. This means that 1 molecule of ammonia is 17 times heavier than 1/12 of one atom of C-12

CALCULATION OF Mr Mr of a molecule = The total of Ar of all the atoms that are present in the molecule EXAMPLE

Mr for N2 = 2 x Ar of N

Mr for Br2 = 2 x Ar of Br

= 2 x 14

= 2 x 80

= 28

= 160

for ionic substances, the Relative Formula Masses are used in place of Mr EXAMPLE RFM for NaCl = Ar of Na + Ar of Cl = 23 + 35.5 = 58.5 RFM for MgSO4 = Ar of Mg + Ar of S + 4(Ar of O) = 24 + 32 + 4(16) = 120

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