A 1.59-g sample of a metal chloride, #MCl_2#, is dissolved in water and treated with excess aqueous silver nitrate. The silver chloride that formed weighed 3.60 g. How do you calculate the molar mass of M?

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Answer 1 · 2016-12-12T15:53:20

#"Molar mass of metal chloride"# #=# #126.2*g*mol^-1#

We need a stoichiometric equation:

#MCl_2(aq) + 2AgNO_3(aq) rarr M(NO_3)_2(aq) + 2AgCl(s)darr#

#"Moles of silver chloride:"#
#=# #(3.60*g)/(143.32*g*mol^-1)=2.51xx10^-2*mol.#

Given the stoichiometry of the precipitation reaction, there were #(2.51xx10^-2*mol)/2# #=# #1.26xx10^-2*mol# #MCl_2#.

So we have a molar quantity, and a given mass, and the #"molecular mass"# is the #"quotient"#, #"mass"/"molar quantity"#

#=(1.59*g)/(1.26xx10^-2*mol)# #=# #126.2*g*mol^-1#.