Question

How many moles of water are required to be dissociated to produce oxygen which is just sufficient to form 800 g of magnesium oxide by burning magnesium? (atomic weight of Mg is 24)

Answer 1

Step-1: Amount of oxygen required to form 800 g of magnesium oxide

$$\begin{gathered} 2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }2\mathrm{MgO}\left(\mathrm{s}\right) \\ \left(\mathrm{Magnesium}\right)\left(\mathrm{Oxygen}\right)(\mathrm{Magnesium}\mathrm{oxide}) \end{gathered}$$

Gram molecular weight of magnesium oxide= 2x24+16= 40 g mol^-1

From the balanced chemical equation,

80 g of Magnesium oxide requires= 32 g of oxygen (O₂)

1 g of Magnesium oxide requires=$\frac{32}{80}$ g of oxygen (O₂)

800 g of Magnesium oxide requires= $\frac{32}{80}\times 800=320$ g of oxygen (O₂)

Step-2: Calculation of the number of moles required to produce 320 g of oxygen

$$\begin{gathered} 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ \left(\mathrm{Water}\right)\left(\mathrm{Hydrogen}\right)\left(\mathrm{Oxygen}\right) \end{gathered}$$

From the balanced chemical equation,

32 g of oxygen is produced by= 2 moles of water

1 g of oxygen is produced by= $\frac{2}{32}$ moles of water

320 g of oxygen is produced by= $\frac{2}{32}\times 320=20$ moles of water