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Question

Calculate the formula unit mass of ZnO, Na2O and K2CO3. Give atomic mass of Zn = 65u Na = 23u K = 39u C = 12u O = 16u.


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Solution

Definition.

  1. The formula unit mass is the sum of the atomic masses of all the ions present in the formula of the compound multiplied by the number of atoms involved in the compound.
  2. It is an important formula used in physical chemistry and used for doing the stoichiometric measurements of ionic as well as covalent solids.
  3. Let us suppose a compound AxBy, the formula unit mass for this compound is

FormulaunitmassofAXBY=(atomicmassofAion×X)+(atomicmassofBionxY)

Given data

  1. Atomic mass of Zn = 65u
  2. Atomic mass of Na = 23u
  3. Atomic mass of K = 39u
  4. Atomic mass of C = 12u
  5. Atomic mass of O = 16u

i) ZnO

FormulaunitmassofZnO=(atomicmassofZincionx1)+(atomicmassofOxygenionx1)

=(65x1)+(16x1)

=81u

ii) Na2O

FormulaunitmassofNa2O=(atomicmassofsodiumionx2)+(atomicmassofOxygenionx1)

=(23x2)+(16x1)

= 62u

iii) K2CO3

Formula unit mass of K2CO3 =(atomicmassofPotassiumionx2)+(atomicmassofCarbonx1)+(atomicmassofOxygenx3)

=(39x2)+(12x1)+(16x3)

= 78 + 12 + 48

= 138 u


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