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

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.