Quantitative Chemistry

Question 1

Si₂H₆ reacts with oxygen.
The equation for the reaction is:
2Si₂H₆(g) + 7O₂(g) → 4SiO₂(s) + 6H₂O(g)
30 cm³ of Si₂H₆ is reacted with 150 cm³ (an excess) of oxygen.
Calculate the total volume of gases present after the reaction.
All volumes of gases are measured at the same temperature and pressure. [4 marks]

Answer 1

(volume of oxygen for 30 cm³ Si₂H₆ = 3.5 × 30) = 105 (cm³)
(volume of excess oxygen = 150 – 105) = 45 (cm³)
(volume of water (vapour) = 3 × 30) = 90 (cm³)
(volume of gases = 45 + 90) = 135 (cm³)

allowed alternative approach:
(moles Si₂H₆ = 0.03/24 =) 0.00125 (1)
(moles water vapour formed = 3 × 0.00125 =) 0.00375
and
(moles oxygen used = 3.5 × 0.00125 =) 0.004375 (1)
(moles excess oxygen = 0.15/24 - 0.004375 =) 0.001875 (1)
(volume of gases = 24 × (0.00375 + 0.001875) = 0.135 dm³ =)
135 (cm³) (1)