1.2 Flashcards
1.2 Determination of the relative atomic mass of magnesium aims
Aims
• To determine the relative atomic mass of magnesium using two different methods and using a range of practical skills.
• Method 1: By measuring the volume of hydrogen produced when a weighed mass of magnesium reacts with excess sulfuric acid.
• Method 2: By evaporating the solution of magnesium sulfate to dryness and weighing the mass of product obtained
Equipment
- safety spectacles (or goggles)
- access to balance accurate to two decimal places
- Measuring cylinder (10 cm3)
- Conical flask
- Bung with delivery tube (see notes below)
- 250 cm3 measuring cylinder
- Evaporating basin
- Bunsen
- tripod
- heatproof mat
- pipeclay triangle
- trough (small washing up bowls could be used)
Prodedure
- Set up the apparatus as shown. Using a measuring cylinder, add 10.0 cm3 of the 1.00 mol dm–3 sulfuric acid (an excess) to the conical flask.
- Weigh accurately between 0.12 g and 0.16 g of magnesium. Record the exact mass of magnesium using an appropriate format.
- Remove the stopper, add the magnesium to the flask and quickly replace the stopper.
- Collect the gas and record the final volume of hydrogen.
- Keep the solution in the conical flask for method 2.
- Assume room temperature and pressure.
Other prodedure
Read the information below and decide how you will show your results.
Then record your measurements of mass in a suitable format.
1. Weigh a clean, dry evaporating basin.
2. Pour the contents of the conical flask into the evaporating basin. Wash the evaporating basin with a little distilled water and add to the evaporating basin
3. Evaporate the solution of magnesium sulfate to dryness. Allow the basin to cool and reweigh the evaporating basin and solid magnesium sulfate.
4. Record your results.
5. Calculate the amount, in mol, of magnesium sulfate, MgSO4, formed.
6. Deduce the amount, in mol of magnesium, Mg, in the MgSO4.
7. Using the original mass of Mg that was reacted, calculate the relative atomic mass of magnesium.
- When you add the magnesium, you are introducing two errors: one results in a smaller volume of gas and the other a larger volume of gas. What are these errors?
- Smaller volume. There is a time lag between adding the magnesium and replacing the bung. Some gas is lost during this time, decreasing the volume.
Larger volume. When the bung is replaced, the volume of the bung displaces the same voljume of air into the measuring cylinder, incresaing the volume.
- Modify your experiment so that the magnesium and acid could be mixed without needing to remove the stopper.
- The magnesium could be suspended on some cotton about the acid. The bung could be loosened just enough to release the cotton and drop the magnesium into the acid. The bung would then be resealed, minimising any effect from a time lag and volume of bung (See 1 above)
- You have assumed room temperature and pressure. Repeat the experiment, recording the actual temperature and pressure. Then repeat your analysis of your results, using the Ideal Gas Equation.
- After measuring the gas volume, temperature and pressure, volume must be converted into m3 (from cm3, 10–6), temperature must be converted into K (+ 273), pressure must be converted into Pa (any conversion will depend on units of pressure used for measuring pressure)
The ideal gas equation is rearranged to find the amount of H2: n = pV/RT
The remainder of the analysis is the same as outlines above.
- Calculate the percentage uncertainties in the measurements that you have made. Identify the most significant uncertainty and how this could be reduced.
- Uncertainty in a 2 place balance is ±0.005 g
% uncertainty = 0.005/mass of Mg 100
Uncertainty in a 250 cm3 measuring cylinder is ±1 cm3 (assuming half division)
% uncertainty = 1/gas volume 100
Uncertainty in a mass measurements of MgSO4 ±0.01 g
(as there are two weighings, each with an uncertainty of ±0.005 g)
For 0.15 g Mg and 148 cm3,
% uncertainty of Mg mass = 0.005/0.15 100 = 3%
Uncertainty in a 250 cm3 measuring cylinder is ±1 cm3
(assuming half division)
% uncertainty = 1/148 100 = 0.7%
For 0.74 g MgSO4,
% uncertainty of MgSO4 mass = 0.01/0.74 100 = 1.4%
Therefore mass of Mg is the most significant uncertainty.
Hazard information h2so4
Causes skin irritation
Causes serious eye irritation
Hazard information mg
Highly Flammable
extra
. A no 31 bung with one hole usually fits into wide-necked conical flasks. The student may wish to clamp the flask neck as well.
Alternatively, a Buchner flask can be used with rubber tube in place of glass delivery tube and rubber bung for the top. This is far easier to handle, less likely to fall over and removes need for a glass delivery tube.
