C1 : 1.2-RELATIVE ATOMIC MASS (R.A.M)

RELATIVE ATOMIC MASS (R.A.M) 

A mass of one atom of an element compared to 1/12 mass of one atom of 12C with the mass 12.000

Example : Ar H = 1, Ar O = 16, Ar Na = 23, Ar F = 19, Ar Mg = 24, Ar Br =80

EXAMPLE 1:

Average atomic mass 16O is 15.995 amu. Calculate RELATIVE ATOMIC MASS 16O .?

RELATIVE ATOMIC MASS (RAM) 16O;

EXAMPLE 2:

Ans:

Relative Atomic Mass, Ar Ag 

= [(51.84% X 106.91) + (48.16% X 108.90)] / 100 

= 107.87

EXAMPLE 3:

What is the relative atomic mass of Boron if it exists as 19.90% ¹⁰B (10.013 amu) and 80.10%¹¹B (11.009 amu)?

Ans:

1st step : Calculate average atomic mass Boron

Average Atomic Mass Boron

= [(19.90% X 10.013amu) + (80.10% X 11.009amu)] / 100 

= 10.811 amu

2nd step : Calculate relative atomic mass, Ar Boron

Relative Atomic Mass, Ar Boron

=     10.811 amu  

      1/12 x 12 amu

= 10.811

EXAMPLE 4:

Determine the relative atomic mass, A_r  of an element Z if the atomic mass ratio of Z to carbon-12 is 1.17

Ans:

EXAMPLE 5:

Copper consists of two isotopes, copper-63 and copper-65. 

Its relative atomic mass is 63.62.

Calculate the % abundance of each isotope.

Ans:

% copper-63 + % copper-65 = 100%
assume the abundance of copper-63= y% 

thus, the abundance of copper-65 = (100 - y) 

63.62 = (y/100 x 63) + [(100 - y)/100 x 65]
63.62 = 63y/100 + 6500/100 - 65y/100
6362 = 63y + 6500 - 65y
-135 = -2y
y = 69

Abundance of copper-63 = 69/100 = 69%

Abundance of copper-65 = 100 - 69 = 31%

EXAMPLE 6:
An atom of element X is 19.6 times heavier than one carbon-12 atom.  What  is the relative atomic mass of X?

Ans:
Relative atomic mass of X 
= 19.6 x mass C-12 
    1/12 x mass C-12
= 235.2

EXAMPLE 7:

Given that the percentage abundance of is 75% and that of  is 25%, calculate the Ar of chlorine.

Solution:

EXAMPLE 8:

Bromine has two isotopes, Br-79 and Br-81. Both exist in equal amounts. Calculate the relative atomic mass of bromine.

Solution:

EXAMPLE 9:

The neon element has three isotopes. They are 90.92% of , 0.26% of  and 8.82% of 

Solution:

EXAMPLE 10:

Ans:
1st step : Calculate average atomic mass Iron

2nd step : Calculate relative atomic mass, Ar Iron

Relative Atomic Mass, Ar Boron

=     55.85 amu  

      1/12 x 12 amu

= 55.85

EXAMPLE 11:

Ans: