Determination of the relative molecular mass of a gas Chapter 1 section 1.3 page 43 pages 41 45 Pearson Baccalaureate HL Chemistry 2014 Catrin Brown AIM To calculate the relative molecular mass of carbon dioxide, CO2, by application of the ideal gas equation. INTRODUCTION The ideal gas equation can be used to calculate the molar mass (M) of a gas when P, V, T and the mass of the gas sample are known; M = mrt PV In this experiment we will heat a known mass of copper carbonate in a test tube. The solid will undergo thermal decomposition, releasing carbon dioxide and leaving copper oxide. CuCO3(s) à CuO(s) + CO2(g) The carbon dioxide gas given off will be collected by displacement of water from an inverted glass tube. After heating the test tube and contents are reweighed. We can determine the molar mass of carbon dioxide from the following data collected in this experiment. Mass (m) of gas: calculated from the loss in mass on heating the copper carbonate Volume (V) of gas: measured from the volume of water displaced Temperature (T) of gas: measured room temperature Pressure (P): measured room pressure Pre-lab questions: Ø What colour change do you expect to occur as the copper carbonate is heated? Ø The equation used to determine the molar mass of CO2 is known as the ideal gas equation. Under which conditions would you expect it to be most accurately followed by a real gas?
METHOD 1. Set up the apparatus as shown below. Try to ensure that the inverted glass tube is full of water with no air trapped. <Insert Fig 1.18 without caption> 2. Put 2 spatula loads of copper carbonate, CuCO3, into the boiling tube and weigh the tube with contents carefully. 3. Gently heat the sample until the graduated tube is about 3/4 full of gas. 4. Be very careful to avoid suck-back by taking the delivery tube out of the trough of water as soon as you stop heating. Allow time for the boiling tube to cool. 5. Measure the volume of gas in the inverted tube. 6. Reweigh the boiling tube and contents. 7. Carry out repeat trials. RESULTS Record all quantitative data in a table see page 43 for a sample. Record all qualitative data. ANALYSIS Process your data to calculate the molar mass of CO2 for each trial, showing all units through your calculations. Take an average value of your results to give your experimental result. CONCLUSION and EVALUATION Compare your experimental result with the theoretical value for the molar mass of CO2 and calculate the percentage error. Calculate the % error arising from random errors in the experiment and compare this with the figure above. There are many possible systematic errors in this experiment, and some of them may cause errors in opposite directions and so partly cancel each other out. Consider the possible errors, assumptions and limitations in this experiment, in which direction they would influence the result, and possible modifications to the procedure that would reduce the impact of these errors. Use a table with the following columns: systematic error influence on result (increase or decrease the value of M(CO2) possible modification to reduce impact
For consideration: 1. This method could not be used to determine the molar mass of a gas that is soluble in water. Suggest an alternative approach for such a gas. 2. Explain what is meant by suck back and why it occurs.
Equipment list Chemicals/ material CuCO3(s) Apparatus (per group of students) Balance Boiling tubes x 3 Stand and clamp Delivery tube with attachment for boiling tube Calibrated glass tube / inverted burette Thermometer Barometer
CALCULATION Use your results above and the gas constant R = 8.3 Jmol - 1 K - 1 to calculate Mr for CO2. Convert the units as appropriate and be sure to show all the steps in your calculation. If you do the experiment more than once, calculate the average of the readings which you consider to be the most accurate. CONCLUSION AND DISCUSSION - Compare the Mr of carbon dioxide that you obtained with the accepted value. - Calculate the percentage error. - Itemise the sources of error in this experiment.