A sample of lanthanum is found to have 99.91% atoms with a mass of 138.9 amu and 0.09% atoms with a mass of 137.9 amu. what is the average atomic mass of lanthanum

The average atomic mass of the lanthanum is 137.63 amu

Explanation:

Average atomic mass is defined as the sum of masses of it's isotope each multiplied by the it's natural abundance .

The formula of calculating average atomic mass is= (f= fraction of the natural abundance of the isotope,M=mass number)

Given,

Lanthanum contain  99.91% atom with mass 138.9 amu

and, 0.09% atoms with mass of 137.9 amu

(Converting the percentage into decimal form we will get,

99.91%=  and 0.09%=

   Then, the average atomic mass of lanthanum is, amu

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.

answer; talc and gypsum;