Question 3
- An impure sample of zinc metal weighing 1.70 g reacted completely with 25.0 cm3 of 1.50 moldm-3 hydrochloric acid.
- Write a balanced chemical equation for the reaction.
- Determine the:
- mass of pure zinc;
- percentage purity of the sample. [Zn = 65.0]
- A white curdy precipitate of silver chloride is obtained by mixing aqueous solutions of sodium chloride and silver trioxonitrate (V). State what would happen when a portion of the precipitate is:
- exposed to light.
- shaken with excess aqueous ammonia solution;
- shaken with dilute trioxonitrate (V) acid.
- Consider the following table:
- has the strongest binding force;
- is likely to have hydrogen bonding;
- could be a solvent for grease;
- is very compressible;
- requires heating to melt.
-
- Name the process involved in the conversion of a gas into its liquid state.
- Using the kinetic theory, explain briefly how a given volume of a gas could be converted to its liquid state.
[4 marks]
- Define the basicity of an acid.
- Determine the basicity of each of the following acids:
[8 marks]
[4 marks]
Substance X |
Substance Y |
Substance Z |
No fixed shape |
Fixed shape |
No fixed shape |
No fixed volume |
Fixed volume |
Fixed volume |
Low boiling point |
High melting point |
Low boiling point |
Insoluble in water |
Soluble in water |
Miscible in water |
State the substance which:
[5 marks]
- H2PO4-;
- CH3COOH.
[4 marks]
Observation
Majority of the candidates responded to this question, and their performance was average.
In part (a), majority of the candidates wrote a balanced chemical equation for the reaction zinc metal and hydrochloric acid. However, few of them were able to determine the mass of pure zinc.
In part (b), majority of the candidates could not give the correct observation a white curdy precipitate of silver chloride is exposed to light, shaken with excess aqueous ammonia solution and shaken with dilute trioxonitrate (V) acid.
In part (c), based on the information in the table, majority of the candidates were able to state the substances which have the strongest bonding force, is likely to have hydrogen bonding and could be a solvent for grease.
In part (d), majority of the candidates named the process involved in the conversion of a gas into its liquid state.
In part (e), majority of the candidates defined the basicity of an acid.
The expected answers include:
:
(a)
(b)
(i) The precipitate turns black / dark / grey
(ii) The precipitate dissolves, it forms colourlesssolution
(iii) The precipitate does not dissolve / insoluble
(c)
(i) Y
(ii) Z
(iii) Z
(iv) X
(v) Y
(d)(i) Condensation / Liquefaction
(ii) As the temperature decreases, there is a decrease in the kinetic energy of the gas molecules, there will be an increase in the intermolecular forces of attraction the molecules then come closer to each other resulting in the formation of the liquid
(e) (i) The basicity of an acid is the number of ionizable / displaceable hydrogen atoms in one molecule of the acid
(ii)
I. 2 /dibasic
II. 1 / monobasic