Question

An oxide of arsenic contains 65.2% arsenic by weight. What will be its simplest formula?

• A. $AsO$
• B. $As2O_3$
• C. $As{O}_2$
• D. $A{s}_2{O}_5$
• E. $A{s}_{2}O$

Answer 1

The correct option is D $A{s}_2{O}_5$

100 g of compound will contain 65.2 g $As$ and $100-65.2=34.8$ g oxygen.
The atomic masses of $As$ and oxygen are 75 g/mol and 16 g/mol respectively.
The number of moles is the ratio of mass to atomic mass.

Number of moles of $As$ $=\frac{65.2}{75}=0.869$ mole

Number of moles of $O$ $=\frac{34.8}{16}=2.175$ mole.

The ratio of the number of moles of $As$$tonumberofmolesof$O\displaystyle = \dfrac {0.869}{2.175} = \dfrac {1}{2.5} = \dfrac {2}{5}$.Hence,thesimplestformulafortheoxidewillbe$\displaystyle As_2O_5$.