A sulfur and oxygen compound has a molecular mass of $ 80 \frac{g}{mol} $ and contains $ 60 \% \; O $. Determine its molecular formula! First calculate the mass of $ O $ and $ S $ in one mole!
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In one mole = $ 80 \; g $ there is $\frac{100\cdot 60}{80}$ $=$ $48\;g\;O$ et $80-48$ $=$ $32\;g\;S$
Transform these masses into moles of $S$ and $O$!
$n_{S}= \frac{m_{S}}{M_S}$ $=$ $\frac{32}{32}$ $=$ $1\;mol\; S$ $n_{O}= \frac{m_{O}}{M_O}$ $=$ $\frac{48}{16}$ $=$ $3\;mol\; S$
Formula? 1 mole contains 1 mole of $ S $ and 3 moles of $ O $ so: 1 molecule contains 1 atom of $ S $ and 3 atoms of $O$ (Compare: If a dozen bicycles contain 1 dozen frames and 2 dozens of wheels, then a bike contains a frame and two wheels!) Formula: $SO_3$
