Given a sample of an unknown metal carbonate, MxCO3, and 3.0M hydrochloric acid, HCl(aq), a balloon, — Synthesis and Laboratory Design Chemistry Question
Problem Context
Given a sample of an unknown metal carbonate, MxCO3, and 3.0M hydrochloric acid, HCl(aq), a balloon, and some laboratory equipment, devise and carry out an experiment by combining these two substances to determine the volume of the gas produced and the unknown metal. The possible metals are Ba, Ca, Li, or Na. Room Temp. = 25oC, Standard Pressure = 1 atm
Give a brief description of your experimental plan.
Before beginning your experiment, you must get approval (for safety reasons) from the examiner.
Model Answer
Students needed to figure out a method of gas collection without the volume of HCl(aq) occupying the balloon. One possibility was to insert a measured mass of the solid carbonate into the balloon, then carefully pull the lip of the balloon over the top of the provided graduated cylinder (filled with the HCl. By shaking the solid carbonate into the cylinder with the balloon still over the top of the cylinder, the two chemical reactants can combine to produce CO2 gas to fill the balloon. One method for measuring the volume could have been to wait for the completed reaction, then carefully twist shut the balloon at its lip, tie off the balloon to make a more spherical shape, then use the string and ruler to measure the balloon’s circumference to then calculate its approximate spherical volume using the formula for circumference and volume of a sphere. Students might have also measured the circumference, emptied the balloon of the products, then refilled it with water to the approximate volume when filled with CO2, and finally measure the volume of water using the graduated cylinder.
Record your data and other observations.
Model Answer
Sample Student Data:
General reaction: MxCO3 + HCl → MCly + H2O(l) + CO2(g), where the mol ratio of MxCO3 : CO2 is 1 : 1.
Mass of MxCO3 used = 1.00 g
Circumference of balloon = 23.9 cm
Radius of balloon, using C = 2πr = 3.8 cm
Calculations and Conclusions.
Model Answer
Calculation of volume from a sphere, V = 4/3πr3 = 240 cm3
Use ideal gas law, PV = nRT, to find moles of CO2 = (1)(0.240) = n(.0821)(298), n = 0.00981
Molar mass of MxCO3: 0.00981 mol = 1.00g/mol. mass = 102 or close to 100, mol. mass if M = Ca, therefore unknown compound is CaCO3.
Conclusions: The volume of gas produced: The unknown metal:
Model Answer
The volume of gas produced: 240 cm3 (based on sample data).
The unknown metal: Ca
Sources of Error in this experiment (please number):
Model Answer
Points of error abound, including assuming standard pressure (1 atm), room temperature (25 oC), a perfect sphere, CO2 collected as a 'wet' gas, completed reaction, and some air and uncaptured CO2 gas remaining in the graduated cylinder.