neco 2017 chemistry | Practical

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Question 1 View Details
A student carries out an experiment to determine the molar mass of magnesium by reacting a weighed piece of magnesium ribbon with excess hydrochloric acid. The procedure is as follows: 1. A clean, dry magnesium ribbon is weighed and its mass recorded as 0.150 g. 2. The ribbon is placed in a conical flask containing excess 1.0 M HCl and the gas evolved is collected over water. 3. The volume of hydrogen gas collected is 156.0 mL at a temperature of 25 °C and an atmospheric pressure of 760 mmHg. 4. The vapour pressure of water at 25 °C is 23.8 mmHg. Assume the reaction: Mg + 2HCl → MgCl₂ + H₂ and that the gas behaves ideally. Answer the following:
Question Parts
(a)
Calculate the number of moles of hydrogen gas collected.
(b)
Determine the experimental molar mass of magnesium from the data.
(c)
Suggest two possible sources of error that could cause the experimental value to differ from the accepted molar mass of magnesium (24.31 g·mol⁻¹).
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Question 2 View Details
A student carries out a flame‑test experiment to determine the concentration of an unknown sodium ion solution. The following standard solutions are prepared and their flame‑emission intensities (arbitrary units) are recorded using a calibrated photometer: | Standard concentration (M) | Intensity (units) | |----------------------------|-------------------| | 0.10 | 12 | | 0.20 | 22 | | 0.30 | 31 | The unknown solution gives a flame intensity of 25 units. Answer the following questions:
Question Parts
(a)
Plot a calibration graph of intensity (y‑axis) against concentration (x‑axis) and obtain the equation of the best‑fit straight line (express the equation in the form I = mC + b).
(b)
Using the equation from part (a), calculate the concentration of the unknown sodium solution.
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Question 3 View Details
A sample of an unknown metal oxide, MxOy, is analysed by the gravimetric method. The procedure and data are as follows: - Mass of clean crucible: 1.200 g - Mass of crucible containing the weighed oxide sample: 5.203 g - The oxide is heated strongly in the crucible until a constant mass is obtained. The mass of the crucible containing the reduced metal is recorded as 4.000 g. Using the data above, determine the empirical formula of the metal oxide.
Question Parts
(a)
Calculate the mass of the oxide sample that was originally weighed.
(b)
Calculate the mass of the metal obtained after heating.
(c)
Using the masses obtained, determine the empirical formula of the oxide. (i) Find the mass of oxygen present. (ii) Convert the masses of metal and oxygen to moles. (iii) Obtain the simplest whole‑number mole ratio and write the empirical formula.
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Question 4 View Details
A piece of magnesium ribbon is used to determine its purity by reacting it with excess hydrochloric acid and collecting the hydrogen gas over water. Data obtained are: - Mass of dry magnesium ribbon before the experiment: 0.1500 g - Volume of hydrogen gas collected over water: 112.0 mL - Atmospheric pressure: 760 mmHg - Temperature of the gas: 25 °C - Vapor pressure of water at 25 °C: 23.8 mmHg Assuming the reaction proceeds according to Mg + 2HCl → MgCl₂ + H₂, calculate the percent purity of the magnesium sample.
Question Parts
(a)
Correct the pressure of the collected hydrogen for the water vapour and calculate the pressure of dry H₂ in atmospheres.
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