KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
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.
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(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.
When solid magnesium carbonate was added to a solution of hydrogen chloride in methylbenzene, there was no apparent reaction. On addition of water to the resulting mixture, there was vigorous effervescence. Explain these observations
One of the fuels associated with crude oil is natural gas. Name the main constituent of natural gas and write an equation for its complete combustion
Name: Equation:
When excess carbon monoxide gas was passed over heated lead (II) oxide in combustion tube, lead (II) oxide was reduced
(a) Write an equation for the reaction, which took place (b) What observation was made in the combustion tube when the reaction was complete? (c) Name another gas, which could be used to reduce lead (II) oxide
Expected Response
(a) Pb(s) + CO(g) →Pb(s) + CO2(g)
(b) Silvery white or grey metal/ shiny grey metal (c) Hydrogen gas / ammonia
Compound L reacts with hydrogen bromide gas to give another compound whose structure is
(a) Give the structural formula and name of compound L
(b) Write an equation for the reaction, which takes place between ethane and excess chlorine gas
State and explain one disadvantages of using hard water in boilers
Expected Response
On boiling hard water deposits CaCO3 which forma fur/ scales in the boiler. This reduces conductivity
Graphite is one of the allotropes of carbon.
(a) Name one other element which exhibits allotropy. (b) Explain why graphite is used in the making of pencil leads.
ANSWERS
(a) Sulphur or phosphorus.
(b) Carbon atoms in graphite are arranged in layers of hexagons which are held by weak van der waal forces. The layers slide over each other when some force is applied on them; hence suitable in making pencil leads.
Describe how the PH of anti-acid (Actal) powder can be determined in the laboratory
ANSWERS
50kg of ammonium sulphate (NH4)2SO4and 30kg of urea CO(NH2)2 fertilizers were applied in two equal sizes of plots A and B to enrich their nitrogen content.Show by working, which plot was more enriched with nitrogen. (N =14;S = 32; O = 16; C = 12; H = 1)
Under certain conditions, chlorine gas reacts with sodium hydroxide to form sodium hypochlorite.
(a) Name the conditions under which sodium hydroxide reacts with chlorine to form sodium hypochlorite. (b) State two uses of sodium hypochlorite.
ANSWERS
(a) Cold and dilute sodium hydroxide.
(b) Used in sterilizing of water. Used as a bleaching agent.
The set up below was used to prepare a gas and study some of its properties. Study it and answer the questions that follow:
(a) State and explain the observations made in the:
(i) tube labelled A; (ii) beaker labelled B. (b) State one precaution that should be taken when carrying out this experiment.
ANSWERS
(a) (i) Black solid is deposited.
(ii) The indicator turns red. (b) The experiment should be done in fume chamber or in open air.
Ethanoic acid and ethanol react as shown in the equation below:
Other than warming, how would the state of equilibrium be established within a short time?
Soap dissolves in water according to the equation below;
NaSt(aq) —> Na+(aq) + St- where St- is the stearate ion. (a) Write the formula of the scum formed when soap is used in hard water. (b) Write the ionic equation for the reaction that occurs when sodium carbonate is used to remove hardness in water.
Two organic compounds P and Q decolourise acidified potassium manganate (VII) solution but only P reacts with sodium metal to give a colourless gas. Which homologous series does compound P belong? Give a reason.
ANSWERS
P is in alkanol R - OH. The alkanol reacts with sodium metal to produce the colourless gas.
Distinguish between the terms deliquescent and efflorescent as used in chemistry
ANSWERS
Sodium hydroxide can be prepared by the following methods; I and II.
(a) Name the precaution that needs to be taken in method I.
(b) Give the name of process A. (c) Give one use of sodium hydroxide,
ANSWERS
(a) Small piece of sodium metal (pea size) with a lot of water.
Perform the experiment wearing goggles. (b) Electrolysis. (c) Manufacture of soap.
Exhaust fumes of some cars contain carbon(II)oxide and other gases.
(a) Explain how carbon (II) oxide is formed in the internal combustion engines. (b) Name two gases other than carbon (II) oxide that are contained in exhaust fumes and are pollutants.
ANSWERS
(a) Carbon (II) oxide is formed in the internal combustion engines when fuel burns under limited oxygen.
(b) Pollutant gas - Carbon (IV) oxide, Nitrogen (1V) oxide and Sulphur (IV) oxide.
During a class experiment, chlorine gas was bubbled into a solution of potassium iodide.
(a) State the observations made. (b) Using an ionic equation, explain why the reaction is redox .
State two reasons why hydrogen is not commonly used as a fuel.
ANSWERS
(a) What is meant by a strong acid?
(b) In an experiment 40cm3 of 0.5M hydrochloric acid was reacted with excess sodium carbonate and the volume of carbon (IV) oxide produced recorded with time. In another experiment, the same volume and concentration of ethanoic acid was also reacted with excess sodium carbonate and the volume of carbon (IV) oxide produced recorded with time. On the grid below, sketch and label the curves if the volumes of carbon (IV) oxide were plotted against time.
When lead(II) nitrate is heated, one of the products is a brown gas.
(a) Write the equation of the reaction that occurs . (b) If 0.290dm3 of the brown gas was produced, calculate the mass of the lead(II) nitrate that was heated. (R .F.M of lead (II) nitrate = 331; Molar gas volume = 24dm3 ). |
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