KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
The diagram below represent a set-up that was used to show that part of air is used during burning.
a) Given that phosphorus used was was in excess, draw a diagram of the set-up at the end of the experiment (when there was no further observable change).
b) Suggest one modification that should be made on the apparatus if the percentage of the air used is to be determined.
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Expected Response
- Iron wool turns or rusts due to formation of hydrated iron (III) oxide
- Level of water inside the tube rises to occupy the space left by oxygen - Level of water in the beaker will fall
In an experiment, a piece of magnesium ribbon was cleaned with steel wool. 2.4 g of the clean magnesium ribbon was placed in a crucible and completely burnt in oxygen. After cooling, the product weighed 4.0 g
(a) Explain why it was necessary to clean the magnesium ribbon (b) What observation was made in the crucible after burning (c) Why was there an increase in mass? (d) Write the equation for the reaction which took place in the crucible (e) The product in the crucible was shaken with water and filtered. Explain the observation which was made when blue and red litmus papers were dropped into the filtrate.
(a) The diagram below shows spots of pure substance A,B, and C on a chromatography paper. Spot D is that of a mixture
After development, A, B and C were found to have moved 8cm, 3cm and 6 cm respectively. D has separated into two spots which had moved 6cm and 8 cm
(i) On the diagram I Label the baseline ( origin) II Show the positions of all the spots after development (ii) Identify the substances present in the mixture D (b) Describe how solid ammonium chloride can be separated fro a solid mixture of ammonium chloride and anhydrous calcium chloride (c) The table shows liquids that are miscible and those that are immiscible
Use the information given to answer the questions that follow
(i) Name the method that can be used to separate L1 and L3 from a mixture of two (ii) Describe how a mixture of L2 and L4 can be separated
ANSWERS
(ii) A and C b)Since NH4CL4 sublimes but CaCl2 does not; sublimation process would do. Heat the mixture. Ammonium chloride sublimates into vapour and condenses on the cooler part of the heating tube. Calcium chloride will remain on the bottom of the heating tube. c)i)Fractional distillation ii)Separating funnel method Since the tow liquids are immiscible, pour both the liquids in a separating funnel and allow to settle, the denser liquid will settle down and the less dense will form a second layer on top. Open the tape and run out the liquid in the bottom layer leaving the liquid in the second layer in the funnel. a) Give the name of each of the processes described below which takes place when salts are exposed to air for sometime. i) Anhydrous copper sulphate becomes wet (1mk) ii) Magnesium chloride forms an aqueous solution (1mk) iii) Fresh crystals of sodium carbonate, Na2CO3. 10H2O (1mk) b) Write the formula of the complex ion formed in each of the reactions described below. (i) Zinc metal dissolves in hot alkaline solution (1mk) (ii) Copper hydroxide dissolves in excess ammonia solution. (1 mk) (c) A hydrated salt has the following composition by mass. Iron 20.2% Oxygen 23.0%, sulphur 11.5%, water 45.3 %. Its relative formula mass is 278. (i) Determine the formula of the hydrated salt.. (3mks) (Fe=56, S=32; O = 16, H =1) (ii) 6.95gm of the hydrates salt were dissolved in distilled water and the total volume made to 250 cm3 of solution. Calculate the concentration of the salt solution in moles per litre.
Apiece of chromatography paper was spotted with coloured inks obtained from pens labeled 1 to 6. The diagram below shows the spots after the chromatogram was developed.
a) Which two pens contained in the same pigment?
b) Which pens contained only one pigment c) According to the chromatogram, which pigments are present in the ink of pen number 6.
ANSWERS
(a) 4 and 5 blue and Green (full) H2SO4 (aq) is on electrolyte
(b)2 and 3 yellow and red (c)Yellow and red 4 – Blue 5 – Green 2 – Yellow 3 – Red award it the colour is tied to the number
Write an equation for the reaction that takes place when carbon monoxide gas is passed over heated lead (II) oxide. (1mks)
When wood is burnt, a grey powder called ash remains. The ash is stirred with water and filtered, a colourless solution is obtained.
a) What is the main component of the colourless solution? b) Explain your answer in (a) above
answers
Explain how you would obtain solid carbonate from a mixture of lead carbonate and sodium carbonate powders. (3mks)
Expected Response
Dissolve in water, filter to remove lead carbonate as a residue, evaporate filter to saturation and allow to cool. Crystallization to take place. Filter the crystals and dry. Evaporate to dryness
The diagram below represents a set-up that was used to react lithium with water study it and answer the questions that follow:
On strong heating, sodium nitrate oxygen gas. In the spaces provided below, draw a labeled diagram of a set-up that could be used for heating sodium nitrate and collecting the oxygen gas liberated. (3mks)
When a candle was brunt completely. The total mass product was found to be greater than the original mass of the candle. Explain
ANSWER
When magnesium metal is burnt in air, it reacts with both oxygen and nitrogen gases giving a white ash. Write two equations for the reactions that take place. (2mks)
answers
(a) Candle wax is mainly a compound consisting of two elements.
Name the two elements (b) The set- up below was used to investigate the burning of a candle study it and answers the questions that follow
(i) What would happen to the burning candle if the pump was turned off? Give reasons
(ii) State and explain the changes in mass that are likely to occur in tube N by the end of the experiment (iii) Name two gases that come out through tube M (iv) Name another substance that could be used in the place of calcium oxide in tube N (a) The table below gives information about the major constituents of crude oil. Study it and answer the questions that follow. (i) Which one of the constituents or crude oil has molecules with the highest number of carbon atoms? (2mks) (ii) Name the process you would use to separate a mixture of petrol and diesel and explain how the separation takes place. (2mks) (iii) Explain why the constituent of crude oil and write its formula (1mk) (iv) Name one gas that is likely to be a constituent of crude oil and write its formula. (1mk) (b) What condition could cause a poisonous gas to be formed when Kerosene is burnt? Explain (2mks) (c) Give one use of bitumen (1mk) Describe how the following reagents can be used to prepare lead sulphate solid potassium sulphate, solid lead carbonate, dilute nitric acid and distilled water.
Expected Response
Dissolve the potassium sulphate (1/2) in water, dissolve (1/2) the lead carbonate in the nitric acid, mix the two solutions (1/2) and filter (1/2) off the lead sulphate precipitate//Dissolve lead carbonate in nitric acid add solid pbSO4 and filter off (max1 ½)//Dissolve this in HNO3 and add solid pbCO3 and filter off the precipitate.
Expected Response
W because its solubility decreases with increase in temperature
Explain how you would separate mixture of nitrogen and oxygen gases given that their boiling points are –1960C and 1830C respectively (2mks)
Answer
Cool the mixture to a temperature below – 1960C to form a liquid then start warming, Nitrogen distils off a gas at – 1960 (cool first)
KCSE PAST PAPERS - CHEMISTRY FORM 1 TOPICAL QUESTIONS AND ANSWERSQUESTIONS![]()
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With reference to atomic number of one, explain why hydrogen can be placed in either group I and VII of the periodic table. K.N.E.C Marking Scheme
Hydrogen forms compounds by losing one electron like group I elements or by gaining one electron like group VII element /Hydrogen has one electron in outermost shell. |
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