Chem CE calculations+acid

2010-10-31 6:24 am

1 (1989-9)
Which of the following metals can liberate hydrogen from both dilute hydrochloric acid and dilute sodium hydroxide solution?
A.Aluminium
B.Iron
C.Lead
D.Magnesium Why?

2(1990-22)
X is a white solid. When dilute hydrochloric acid is added to X, a colourless gas is liberated. An aqueous solution of X gives a white precipitate with silver nitrate solution. X is probably
A.Ammonium chloride
B.Sodium ethanoate
C.Sodium carbonate
D.Calcium carbonate.
Answer: C (Why not D?)

3.Which of the following reagents is/are commonly stored in brown bottles? (1999-38)
(1)Potassium permanganate solution
(2) Concentrated sulphuric acid
(3) Concentrated nitric acid
A.(1) only
B.(2) only
C.(1) and (3) only
D.(2) and (3) only
Answer: C (Why?)

4.7.5g of calcium carbonate is added to 50.0cm3 of 2M hydrochloric acid. What is the volume of carbon dioxide liberated at room temperature and pressure? (1998-28)
(Relative atomic masses: C=12.0, O=16.0, Ca = 40.0;
Molar volume of gas at room temperature and pressure = 24.0
dm3)
A.0.9 dm3
B.1.2 dm3
C.1.8 dm3
D.2.4 dm3 The answer is B instead of C. Why?

5.Which of the following products are formed when nitrogen dioxide reacts with sodium hydroxide solution? (1992-32)
(1) Sodium nitrite
(2) Sodium nitrate
(3) Nitrogen monoxide
A.(1) and (2) only
B.(1) and (3) only
C.(2) and (3) only
D.(1), (2) and (3)
What are the reactions involved? Where can I find relevant information in CE syllabus?

6. What happens when concentrated sulphuric acid is heated with copper?

回答 (1)

己式庚辛

2010-10-31 8:14 am

✔ 最佳答案

1. A
2Al + 6H(+) ------> 3H2 + 2Al(3+)
2Al + 6H2O + 2OH(-) ------> 2[Al(OH)4](-) + 3H2
the second equation can be tricky.
first write equation between aluminium and water to give aluminium oxide and hydrogen; followed by reaction between Al2O3 and hydroxide to give aluminate ion.

2.
X should be soluble in water. sodium carbonate is, but calcium carbonate isn't.

3.
potassium permanganate will break down into manganese dioxide and oxygen, speed-up by light;
nitric acid will undergo photo-decomposition to give nitrogen dioxide and oxygen.
sulphuric acid is stable under light.

4.
CaCO3 + 2HCl ------> CaCl2 + H2O + CO2
no. of mole of HCl = (50/1000)x2 = 0.1 mole
no. of mole of calcium carbonate = 7.5 / (40+12+16x3) = 0.075 mole
mole ratio of CaCO3 : HCl = 1:2
thus hydrochloric acid is IN EXCESS!!
no. of mole of CO2 produced = 0.05 mole
volume = 0.05 x 24 = 1.2 dm^3

ALWAYS check if one or more reactants are in excess!
which one is limited reagent?

5.
it's related to Redox reaction ---- REDuction-OXidation reaction. 氧化還原作用
oxidation number of N in NO2 is +4, N in nitrite is +3, N in nitrate is +5.
nitrogen dissolves in water to give nitrous acid and nitric acid.

6.
copper dissolves to give blue solution. colorless gas is given out.
HOWEVER, the gas is not hydrogen; it's sulphur dioxide.
2Cu + 2H2SO4 ------> 2CuSO4 + SO2 + 2H2O
don't worry. it'll be included in Redox.

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