waec model questions vol1 2018 chemistry | Practical

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Question 1 View Details
A student carried out an experiment to determine the molar mass of magnesium using the reaction: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The following data were recorded: - Mass of dry magnesium ribbon: 0.150 g - Volume of hydrogen gas collected over water: 156.0 mL - Temperature of the gas: 25 °C - Atmospheric pressure: 760 mmHg - Vapour pressure of water at 25 °C: 23.8 mmHg Using the ideal gas equation (R = 0.0821 L·atm·mol⁻¹·K⁻¹), answer the questions below.
Question Parts
(a)
Calculate the pressure of the dry hydrogen gas collected (in mmHg and in atm).
(b)
Determine the number of moles of hydrogen gas collected and from this calculate the experimental molar mass of magnesium.
(c)
State two possible sources of error that could affect the result obtained.
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Question 2 View Details
A student was given an unknown metal oxide sample (MO) weighing 0.500 g. The sample was placed in a crucible and heated strongly until a constant mass was obtained. The final mass of the crucible with the residue (metal) was 0.300 g. The atomic mass of oxygen is 16.00 g·mol⁻¹. Using this information, answer the questions below.
Question Parts
(a)
Calculate the mass of oxygen that was lost on heating and determine the number of moles of oxygen removed.
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Question 3 View Details
A sample of an unknown metal oxide was analysed by heating in a crucible. The mass of the sample before heating was 1.456 g and after heating the mass of the residue (metal) was 1.018 g. The loss in mass is due to the removal of oxygen. Using the data provided, answer the following:
Question Parts
(a)
Calculate the mass of metal and the mass of oxygen that was lost on heating.
(b)
Determine the number of moles of metal and oxygen and deduce the empirical formula of the oxide. (Atomic weight of the metal is 55.85 g mol⁻¹.)
(c)
From the empirical formula, calculate the oxidation state of the metal in the oxide.
(d)
Based on the physical properties of the oxide (high melting point, insoluble in water, non‑conducting as a solid but conducts when molten), state the type of chemical bond present and give a brief justification.
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Question 4 View Details
A 0.250 g sample of an unknown monoprotic acid (HX) was dissolved in distilled water and titrated against a standard sodium hydroxide solution of concentration 0.1000 mol L⁻¹. The volume of NaOH required to reach the endpoint was 20.00 mL. Answer the following questions:
Question Parts
(a)
Calculate the number of moles of NaOH used in the titration.
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