Silicon dioxide reacts vigorously with magnesium powder to form magnesium silicide, a dark blue solid - Leaving Cert Chemistry - Question c - 2015

To determine the number of moles of magnesium silicide produced from 7.6 g:

1. Calculate the molar mass of magnesium silicide (Mg₂Si):

   • Molar mass of Mg = 24.31 g/mol
   • Molar mass of Si = 28.09 g/mol
   • Molar mass of Mg₂Si = $2(24.31) + 28.09 = 76.71$ g/mol

2. Calculate the moles of magnesium silicide:

   $ext{Moles of Mg}_2Si = \frac{7.6 ext{ g}}{76.71 ext{ g/mol}} \\ \approx 0.099 ext{ mol}$

3. From the balanced equation, $1 ext{ mol of Mg}_2Si$ reacts with $4 ext{ mol of Mg}$:

   • Moles of magnesium required: $0.099 imes 4 = 0.396$ mol.

Using the earlier result of 0.099 moles of magnesium silicide:

1. From the balanced equation, $1 ext{ mol of Mg}_2Si$ reacts with $4 ext{ mol of HCl}$:
   • Moles of hydrogen chloride required:
   $ext{Moles of HCl} = 0.099 imes 4 \\ = 0.396 ext{ mol}$

To find the mass of magnesium chloride produced per mole of magnesium silicide:

1. The molar mass of magnesium chloride (MgCl₂) is:

   • Molar mass of Cl = 35.45 g/mol
   • Molar mass of MgCl₂ = $24.31 + 2(35.45) = 95.21$ g/mol

2. From the balanced equation, $1 ext{ mol of Mg}_2Si$ produces $2 ext{ mol of MgCl}_2$:

   • Hence, from 0.099 moles of magnesium silicide:

   $ext{Moles of MgCl}_2 = 2 imes 0.099 = 0.198 ext{ mol}$

   $ext{Mass of MgCl}_2 = 0.198 imes 95.21 \\ ext{g} = 18.86 ext{ g}$

1. From the reaction of silane (SiH₄) with oxygen:

   • The balanced equation indicates:
   • $2 ext{ mol of SiH}_4$ reacts with $4 ext{ mol of O}_2$

2. Moles of silane produced from magnesium silicide:

   • Moles of SiH₄ = $0.099 ext{ mol}$ (as shown in part (ii))

   $ext{Moles of O}_2 = 0.099 imes 2 = 0.198 ext{ mol}$

3. Using the ideal gas law at room temperature and pressure (22.4 L/mol):

   $ext{Volume of O}_2 = 0.198 imes 24 ext{ L/mol} \\ ext{Volume} \\ = 4.75 ext{ L}$