✔ 最佳答案
1.which of the substances, when mixed, will undergo a chemical reaction?
a. zinc nitrate solution and ammonia solution
b.sodium nitrate solution and magnesium sulphate solution
c.iron and magnesium nitrate solution
d.magnesium and water
The answer is a.
a. Reaction occurs.
Ammonia solution is an alkali, which forms precipitates will most cations. Ammonia solution reacts with zinc nitrate solution to give a white precipitate of zinc hydroxide. The precipitate re-dissolves in excess ammonia solution to give a colourless solution.
NH3(aq) + H2O(l) → NH4+(aq) + OH-(aq)
Zn2+(aq) + 2OH-(aq) → Zn(OH)2(s)
Zn(OH)2(s) + 2OH-(aq) → [Zn(OH)4]2-(aq)
b. no reaction
No precipitation or redox reaction occurs.
c. no reaction
No displacement reaction occurs as iron is less reactive than magnesium.
d. no reaction
The reaction between magnesium and water is extremely slow, and can be thought as practically no reaction.
2.a sample of conc. sulphuric acid has a density of 1.64 g cm^-3 and contains 92% of sulphuric acid by mass.what is the conc. of sulphuric acid in sample???
(H=1, O=16, S=32)
a.13.6 M
b.15.4 M
c.17.6 M
d.19.2 M
The answer is b.
Consider 1 dm3 (1000 cm3) of the conc. sulphuric acid.
Mass of the conc. sulphuric acid = 1000 x 1.64 = 1640 g
Mass of H2SO4 = 1640 x 92% = 1508.8 g
Molar mass of H2SO4 = 1x2 + 32 + 16x4 = 98 g mol-1
No. of moles of H2SO4 = 1508.8/98 = 15.4 mol
Molarity of H2SO4 = 15.4/1 = 15.4 M
Ag 同 Cu都係electrode,why nacl solution做electrolyte會唔通電??
This is the set-up of a chemical cell. Cu is in a higher position of electrochemical series than Ag. The anodic reaction is the oxidation of Cu(s) electrode to Cu2+(aq) ions. However, the tendency of the cathodic reaction is weak, because it is impossible to reduce Na+(aq) Na(s) and the concentration of H+(aq) is too low (0.0000001 M at 25oC). To make the chemical cell practical, it is recommended to use a dilute strong acid or silver nitrate solution the electrolyte in order to increase the tendency of the cathodic reaction.
2009-09-06 23:03:01 補充：
2
Mass of the solution
= volume x density
= (1000 cm^3) x (1.64 g cm^-3)
= 1640 g
3
No matter how concentrated is the NaCl solution, Na^+(aq) ions cannot be reduced to Na, and the concentration of H^+ ions is very low.
2009-09-06 23:03:23 補充：
2
Mass of the solution
= volume x density
= (1000 cm^3) x (1.64 g cm^-3)
= 1640 g
3
No matter how concentrated is the NaCl solution, Na^+(aq) ions cannot be reduced to Na, and the concentration of H^+ ions is very low.
