KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Explain the change in mass that occurs when the following substances are separately heated in open crucibles
(a) Copper metal (b) Copper (II) nitrate
Expected Response
a) Mass increases because oxygen combine with copper metal
b) Mass decreases it decomposes into gases that escape.
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The empirical formula of A is CH2Br. Given that 0.470g of A occupies a volume of 56cm3 at 546K and 1 atmospheric pressure, determine its molecular formula.(H = 1.0, C = 12.0, Br = 80.0, molar gas volume at STP = 22.4 dm3).
(a) Name two cations that are present in hard water.
(b) Explain how the ion exchange resin softens hard water.
120g of iodine - 131 has a half life of 8 days and decays for 32 days. On the grid provided, plot a graph of the mass of iodine - 131 against time.
10cm3 of concentrated sulphuric (VI) acid was diluted to 100cm3. 10cm3 of the Resulting solution was neutralised by 36cm3 of 0.1M sodium hydroxide solution. Determine the mass of sulphuric (VI) acid that was in the concentrated acid (S = 32.0; H = 1.0; O = 16.0).
Ammonium ion has the following structure:
Label on the structure:
(a) covalent bond;
(b) coordinate (dative) bond.
Study the information in the table below and answer the questions that follow: A mixture containing 35g of CuS04 and 78g of Pb(N03)2 in l00g of water at 60°C was cooled to 40°C.
(a) Which salt crystallised out? Give a reason
(b) Calculate the mass of the salt that crystallised out.
A sample of water in a beaker was found to boil at 101.5°C at 1 atmospheric pressure.
Assuming that the thermometer was not faulty, explain this observation.
ANSWERS
The table below shows properties of some elements A, B, C and D which belong to the same period of the periodic table. The letters are not the actual symbols of the elements.
(a) Arrange the elements in the order they would appear in the period. Give a reason.
(b) Select the metallic element which is the better conductor of electricity. Give a reason.
ANSWERS
(a) BDACAcross the period the atomic radius decreases.
(b) D Across the period the conductivity increase due to increase in delocalised electrons.
In an experiment, dry hydrogen gas was passed over heated Lead (II) Oxide as shown in the diagram below.
State and explain the observations made in the combustion tube.
ANSWERS
Iron (III) oxide was found to be contaminated with copper (II) sulphate. Describe how a pure sample of iron (III) oxide can be obtained.
Charcoal is a fuel that is commonly used for cooking. When it burns it forms two oxides.
(a) Name the two oxides. (b) State one use of any of the two oxides.
ANSWERS
(a) Carbon (IV) Oxide and Carbon (II) Oxide
(b) CO2- Fire extinguishers Fizzy drinks Food preservative . Solvay process . CO -Manufacture of fuel (water gas) Reduction in the extraction of metals Manufacture of methanol The table below shows the PH values of solutions I, II, III and IV.
(a) Which solution is likely to be that of calcium hydroxide?
(b) Select the solution in which a sample of aluminium oxide is likely to dissolve. Give reason for your answer
Bromine reacts with ethane as shown below
(a) What condition is necessary for this reaction to occur?
(b) Identify the bonds which are broken and those that are formed
The graph below shows the mass of a radioactive isotope plotted against time
(a) Using the graph, determine the half – life of the isotope
(b) Calculate the mass of the isotope present after 32 days
What name is given to elements which appear in group (II) of the periodic table?
ANSWER
A sample of river water is suspected to contain zinc and sulphate ions.
Describe how the presence of zinc ions and sulphate ions can be established.
ANSWERS
Explain why the following substances conduct an electric current.
(a) Magnesium metal. (b) Molten magnesium chloride.
The diagram below shows the bonding between aluminium chloride and ammonia.
(a) Name the types of bonds that exist in the molecule.
(b) How many electrons are used for bonding in the molecule?
The flow chart below shows some processes involved in the extraction of zinc metal:
a) Name oneore from which zinc is extracted.
b) Write the equation of the reaction taking place in unit II c) Name two uses of zinc metal.
The data given below was recorded when Metal M was completely burnt in air. M is not the actual symbol of the metal. (RA.M; M = 56, O ==16)
Mass of empty crucible and lid = I0.240g Mass of crucible, ;lid and metal M = 10.352g Mass crucible, lid and metal oxide = 10.400g (a) Determine the mass of: (i) Metal M (ii) oxygen. (b) Determine the empirical formula of the metal oxide.
(a) State the Gay Lussac's Law.
(b) 10cm3 of a gaseous hydrocarbon, C2HX required 30cm3 of oxygen for complete combustion. If steam and 20cm3 of carbon (IV) oxide were produced, what is the value of X?
The table below gives the number of electrons, protons and neutrons in substances X, Y and Z. Study it and answer the questions that follow.
(a) Which letter represents an ion?
(b) Which of the substances are isotopes? Give a reason.
ANSWERS
(a) Y
(b) Y and Z They have the same number of protons (8) but different atomic masses.
The thermal chemical reaction between carbon and sulphur is as shown by the equation below:
The table below gives some properties three elements in group (VII) of the periodic table .Study it and answer the questions that follow:
(a) Which element is in liquid form at room temperature? Give a reason.
(b) Explain why the boiling point of iodine is much higher than that of chlorine.
ANSWERS
(a) Bromine M
At room temperature (25°C), Bromine is liquid since its MP and BP is between -7 and 59 (b) Atomic mass of iodine is higher than that of chlorine. Van der waal’s forces are stronger in iodine than chlorine hence iodine’s BP is higher than that of chlorine. |
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