KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
a) Identify the solids
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Molten M | Conduct electric current and is not decomposed |
Molten N | Conduct electric current and a gas is formed at one of the electrodes |
Suggest the type of bonding present in;
a) Substance M (1mk)
b) Substance N (1mk)
- Empirical formula shows the simplest whole number ratio of the moles of atoms of different elements in a compound while the molecular formula shows the actual number of atoms of various elements present in one molecule of the compound.
(a) Calculate the mass of phosphorus in a 20 kg packet 14.0; H = 1.0; P = 31.0; 0 16.0)
(b) State one advantage of this fertilizer, (NH4)2HPO4 over urea CO(NH2)2
- The composition of commercial indicators remains constant. hence gives consistent results.
- The composition of flower extracts change with time giving inconsistent results.
- A solution of sodium chloride contains ions which are free to conduct electricity while that of sugar contains molecules hence cannot conduct electricity.
(b) Describe how the amount of iron in a sample of iron(III) oxide can be determined.
Magnetite;
Siderite.
(b) Weigh the iron (III) oxide together with a crucible;
Heat the Iron(III) oxide and coke to a constant mass;
Cool and re-weigh residue and crucible
The difference in mass is weight of the iron.
- A non -luminous flame is invisible, hence it should be put off to avoid being accidentally burned.
(b) Write an equation for the reaction which occurs in tube Q in the first few minutes of the experiment.
(c) Give a suitable Conclusion for the experiment in the set-up.
(b) Iron sheets are dipped in molten zinc to prevent rusting. Name this process.
atomic number:
(b)The following ionic equation represents the reaction between metal Y and an aqueous solution of Z2+.
(b) On the grid provided, show with a tick (✓) the position of element D whose atomic number is 18.
(c) Element E is more reactive than A. Explain.
(b) period 3, group 8
(c) E has a bigger atomic radius than A / the valence electrons of element E are further from the nucleus, hence loosely held by the positive nucleus and requires less energy to be removed during reaction.
OR
A has a smaller atomic radius than E / the valence electrons of element
A are closer to the nucleus, hence strongly held by the positive nucleus and requires more energy to be removed during a reaction.
(b) State one functi0 of the part labelled Z.
(c) Calculate the e.m.f. of the cell.
Add potassium hydrogen carbonate little by little as the mixture is stirred until effervescence stops;
Evaporate the solution to saturation and allow to cool for crystals to form;
Dry the crystals in between filter papers.
- One reagent is missing, hence reagents provided cannot produce chlorine;
- Wrong drying agent — Calcium oxide will react with the chlorine gas;
- Incorrect method of gas collection
- No gas will be collected / chlorine is denser than air.
(b) Explain why it is not advisable to use temperatures higher than 773 K in the Haber process.
(b)Forward reaction is exothermic, excessive temperatures would favour the backward reaction therefore lowering the yield of ammonia.
(a) Give the name of the solid.
(b) Explain why it is not advisable to allow the solid to accumulate in the environment.
(b)It is non-biodegradable, hence pollutes the environment.
- The oxide of U has higher melting point than the oxide of V;
- -This is because the oxide of U is a solid with ionic bonds hence requires a lot of energy to melt;
- -The oxide of V is a gas hence requires little energy to melt/break molecular forces of attraction.
- Wet a measuring cylinder/gas jar and sprinkle some iron filings on the wet surface. Remove the excess iron filings;
- Invert the measuring cylinder in a trough of water;
- Take the reading of the air column in the measuring cylinder. Leave the set-up for 2 days;
- Read and record the volume of the air column;
- The volume of the air reduced and the grey filings changed to a brown substance;
- The brown substance is a compound of iron and oxygen;
- The oxygen in the air reacted with the iron fillings to form rust.
(b) Select:
(i) one Substance Which Contains a Component not present in T.
(ii) a substance which is least Soluble in the solvent used.
(b)(i)Q
(ii)S
(a) Name the anion present in the potassium salt.
(b) Write an ionic equation for the formation of solid M.
(c) Give one use of gas N.
(b) Explain why a balloon filled with helium gas deflates faster than a balloon of the same size filled with argon gas.
The rate of diffusion of a gas is inversely proportional to the square root of its density at constant temperature and pressure.
(b)Helium is less dense than argon hence it diffuses out of the balloon faster than argon.
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