✔ 最佳答案
33.
The answer is B.
According to Avogadro’s law, under constant temperature and pressure, the volume ratio of gases is equal to their mole ratio.
2CO(g) + O2(g) → 2CO2(g)
Volume ratio (CO used) : (O2 used) = 2 : 1
When 50 cm3 of CO is completely reacted,
volume of O2 used = 50 x (1/2) = 25 cm3
Hence, CO is the limiting reactant (completely reacted).
Volume of O2 unreacted = 50 - 25 = 25 cm3
Volume ratio (CO used) : (CO2 formed) = 2 : 2 = 1 : 1
Volume of CO reacted = 50 cm3
Volume of CO2 formed = 50 cm3
Final volume at the end of the combustion
= (Volume of O2 unreacted) + (Volume of CO2 formed)
= 25 + 50
= 75 cm3
==========
38.
The answer is D.
A is incorrect.
Pb(s) + H2SO4(aq) → PbSO4(s) + H2(g)
Since Pb is unreactive, the reaction is very slow. Furthermore, the PbSO4(s) formed will coat on the surface of Pb(s), and this will stop the reaction soon after the reaction starts.
B is incorrect.
Pb(s) + CuSO4(aq) → PbSO4(s) + Cu(s)
The PbSO4(s) are mixed with Cu(s), and it is difficult to separate them. Further, PbSO4(s) and Cu(s) formed will coat on the surface of Pb(s), and this may stop the reaction soon after the reaction starts.
C is incorrect.
PbO(s) + H2SO4(aq) → PbSO4(s) + H2O(l)
The PbSO4(s) formed will coat on the surface of PbO(s), and this will stop the reaction soon after the reaction starts.
D is correct.
Pb(NO3)2(aq) + H2SO4(s) → PbSO4(s) + 2HNO3(aq)
or Pb2+(aq) + SO42-(aq) → PbSO4(s)
When the solutions are mixed, PbSO4(s) is precipitated out. It is easy to obtain PbSO4(s) by filtration of the reaction mixture, followed by washing and drying the residue.
