neco model questions vol1 2025 chemistry | Practical

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Question 1 View Details
A student carried out an experiment to determine the relative atomic mass of magnesium by converting a known mass of magnesium ribbon into magnesium oxide. Procedure (summarised): 1. A clean, dry crucible with lid was weighed and the mass recorded as 0.200 g. 2. A clean piece of magnesium ribbon was cut, dried and weighed together with the crucible; the combined mass was 0.700 g. 3. The crucible was heated strongly in a Bunsen‑flame until the magnesium ceased to glow and the mass became constant. The product obtained was magnesium oxide (MgO). 4. After cooling, the crucible with its lid was weighed again; the combined mass was 0.842 g. 5. The atomic mass of oxygen is taken as 16.00 g mol⁻¹. Using the data above, answer the following questions.
Question Parts
(a)
Calculate the mass of magnesium used in the experiment.
(b)
Determine the mass of oxygen that combined with the magnesium to give MgO.
(c)
Calculate the experimental relative atomic mass of magnesium obtained from the data.
(d)
Suggest two possible sources of error that could have caused the experimental value to differ markedly from the accepted value (24.31 g mol⁻¹).
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Question 2 View Details
Three metal strips – magnesium (Mg), zinc (Zn) and copper (Cu) – were each placed in separate 250 mL conical flasks containing 100 mL of 0.500 M dilute hydrochloric acid (HCl) at 25 °C. The flasks were fitted with inverted measuring cylinders to collect the hydrogen gas evolved at normal temperature and pressure (NTP). Each reaction was allowed to proceed for exactly 2.00 min, after which the volume of gas collected was recorded. Experimental data obtained: | Metal | Mass of metal (g) | Volume of H₂ collected (mL) | |-------|-------------------|-----------------------------| | Mg | 0.250 | 48.0 | | Zn | 0.300 | 30.0 | | Cu | 0.250 | 0.0 | Using the information above, answer the following questions.
Question Parts
(a)
Calculate the rate of hydrogen evolution (in mL min⁻¹) for each metal.
(b)
Arrange the three metals in order of decreasing reactivity towards dilute HCl.
(c)
Explain the observed order of reactivity using periodic trends (atomic radius and position in the periodic table).
(d)
State two precautions that should be observed to minimise experimental error in this type of gas‑collection experiment.
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Question 3 View Details
A sample of an unknown metal oxide (formula MxOy) weighing 2.00 g is heated strongly in a crucible until no further change in mass occurs. After heating, the mass of the remaining metal is 1.21 g. The oxide is insoluble in water, but when the metal obtained is dissolved in dilute hydrochloric acid, the resulting solution conducts electricity readily. The metal chloride formed is soluble in water. Using the data provided, answer the following questions.
Question Parts
(a)
Calculate the empirical formula of the oxide.
(b)
Determine the percentage by mass of the metal in the oxide.
(c)
Based on the observations (insolubility of the oxide, solubility of the metal chloride, and the conductivity of its aqueous solution), state the type of chemical bonding present in the oxide and justify your answer.
(d)
Suggest one reliable experimental method that could be used to confirm the oxidation state of the metal in the oxide.
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Question 4 View Details
A 0.500 g sample of an unknown metal is placed in excess dilute hydrochloric acid in a gas‑collection‑over‑water apparatus. The volume of gas collected is 112.0 mL at 25 °C and a barometric pressure of 760 mmHg. The vapour pressure of water at 25 °C is 23.8 mmHg. The gas is identified as hydrogen. Using the data supplied, answer the following questions.
Question Parts
(a)
Calculate the number of moles of hydrogen gas collected, correcting for the water vapour pressure.
(b)
Assuming the reaction proceeds according to M + 2HCl → MCl₂ + H₂, determine the number of moles of metal that have reacted.
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