A sample of cesium is 75.00% Cs-133, 20.00% CS-132, and 5.000% C$-134. What is the

average atomic mass of cesium? Round your answer to four significant figures.

A

132 amu

B

132.8 amu

C

132.9 amu

D

133 amu

C) 132.9 amu

Explanation:

Data Given:

Abundance of Cs-133 = 75 % = 0.75

mass of Cs-133 = 133 amu

Abundance of Cs-132 = 20 % = 0.20

mass of Cs-132 = 132 amu

Abundance of Cs-134 = 5 % = 0.05

mass of Cs-134 = 134 amu

Average atomic mass of cesium = ?

Solution:

formula used to check average atomic mass is as under

Average atomic mass = m₁A₁ + m₂A₂ + m₃A₃

Where

m₁, m₂ and m₃ = mass of isotope 1, 2 and 3 respectively

A₁, A₂ and A₃ = abundance or fraction of isotope 1, 2 and 3 respectively

Put values in above formula

Average atomic mass = (133 amu) (0.75) + (132 amu) (0.20) + (134 amu) (0.05)

Average atomic mass = 99.75 + 26.4 + 6.7

Average atomic mass = 132.85 amu

So

Round to four significant figures

Average atomic mass of Cs = 132.9 amu

option C is correct