Several different salts of calcium are available in nature. Carbonate, chloride and sulfate salts ar — Physical Chemistry — Kinetics Chemistry Question
Analysis of calcium salts
Several different salts of calcium are available in nature. Carbonate, chloride and sulfate salts are among the most common ones. Unlike the carbonate and sulfate salts, calcium chloride is soluble in water. Calcium carbonate reacts with Brönsted acids releasing carbon dioxide:
CaCO3(s) + 2 H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
In this experiment, the composition of a mixture of calcium salts will be determined based on solubility, and reaction of CaCO3 with a strong acid.
Chemicals and reagents
* A mixture of calcium salts (% by mass): 40.0 % of CaCO3, 5.0 % of CaCl2 and 55.0 % of CaSO4. (Any mixture can be used. The amount of sample and the hydrochloric acid can be adjusted for a desired volume of carbon dioxide gas.)
* Hydrochloric acid, HCl(aq), c = 3.0 mol dm-3
* Sodium chloride, NaCl
* Acetone, CH3COCH3
Apparatus and glassware
* Apparatus shown in Figure 32-1 [VISUAL]
* Ice bath
* Pipette, 10 cm3
* Graduated cylinder, 25 cm3
* Funnel
* Filter paper (Whatman 42)
* Stirring bar
* Stirrer
* Septum
A. Preparation of ice-bath
Add sufficient amount of NaCl in an ice-bath containing about 300 cm3 of ice-water mixture to obtain a solution at a temperature below –5 ºC. Use ice at a temperature below –10 ºC.
B. Reaction with hydrochloric acid
1. Assemble the experimental setup as shown in Figure 32-1 [VISUAL] in a hood. Check that the experimental set up is held on a support and the graduated tube is connected to the Schlenk tube by a Tygon tubing.
Figure 32-1 Apparatus for measuring the volume of gas evolved from the reaction.
2. Fill the graduated glass tube with water by pouring water through the bulb opening.
3. Transfer exactly 1.00 g of the salt mixture from the glass vial to the Schlenk tube through the funnel. Put a magnetic stirring bar into the Schlenk tube.
4. Rinse the funnel with about 3 - 4 cm3 water to get all the sample down to Schlenk tube.
5. Freeze the water by immersing the bottom of the Schlenk tube into the ice-bath.
6. Add 10.0 cm3 of HCl solution with a concentration of 3.00 mol dm-3 into the Schlenk tube and close the Schlenk tube with a septum.
7. By changing the bulb height adjust the water level in the graduated tube to zero.
8. Open the stopcock connecting the Schlenk tube to the graduated tube and remove the ice bath.
9. When the ice in the Schlenk tube melts, the acid rapidly reacts with CaCO3 liberating carbon dioxide gas. Stir the solution vigorously.
10. Wait until no more change in the level of water inside the graduated tube takes place. When no more gas evolution is observed, record the volume of the gas evolved.
11. Open the Schlenk tube and filter the solution. Wash the solid with distilled water and then rinse with acetone. Decant the filtrate into the waste acid container.
12. Record the mass of calcium sulfate dried.
Calculate the number of moles of carbon dioxide gas liberated. (Vapor pressure of the acid solution at the temperature of experiment has to be considered).
Model Answer
In the experiment 1.0 g of calcium salts mixture was used containing:
40.0 % CaCO3, i.e. 400 mg,
5.0 % CaCl2, (50 mg)
55.0 % CaSO4, (550 mg)
10.0 cm3 of HCl solution (2.0 mol dm-3) were prepared from concentrated HCl solution (12.06 mol dm-3) by diluting: 1.66 cm3 of concentrated HCl solution to 10 cm3 with water.
Atmospheric pressure = 0.908 atm; vapor pressure of HCl(aq) 0.043 atm at 25.0 °C
The volume of CO2 gas evolved = 112.3 cm3
CaCO3(s) + 2 H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
pCO2 = patm – pHCl(aq) = 0.908 – 0.043 = 0.865 atm
Assuming ideal behavior for CO2 gas, from PV = n RT
0.865 atm × 0.1123 L = n × 0.082 atm·L·mol-1·K-1 × 298 K, ⇒ n = 3.98 mmol CO2
Calculate the amount of CaCO3 reacted.
Model Answer
1 mol CO2 ⇔ 1 mol CaCO3
1 mol CaCO3 = 100.0 g
m (CaCO3) reacted = 3.98 ⋅ 10-3 mol CaCO3 × 100.0 g mol-1 = 0.398 g
Calculate the weight percentage of CaSO4 present in the sample.
Model Answer
The precipitate CaSO4 is filtered from the solution and weighed.
m(CaSO4) = 0.453 g
% of CaSO4 (by mass) = 0.453 /1.000 × 100 = 45.3 %
Calculate the weight percentages of CaCO3 and CaCl2 present in the salt mixture.
Model Answer
Mass of CaCl2 in the mixture is calculated by subtracting the masses of CaCO3 and CaSO4 from the total mass of the mixture (1.000 g):
1.000 – 0.398 – 0.453 = 0.149 g CaCl2
% CaCO3 (by mass) = (0.398 / 1.000) × 100 = 39.8 %
% CaCl2 (by mass) = (0.149 /1.000) × 100 = 14.9 %
Discuss the possible sources of experimental error.
Model Answer
For CaCO3 = (400 – 398) / 400 = 0.5 % (neg. error)
for CaSO4 = (550 – 453) / 550 = 17.6 % (neg. error)
for CaCl2 = (149 – 50) / 50 = 198 % (positive error)
Filtration of CaSO4 is quite difficult due to the size of the particles. Thus, the amount of CaSO4 determined is less than the actual value. Since the amount of CaCl2 is found from the amounts of CaCO3 and CaSO4, the greatest error is observed in the calculation of CaCl2. Possible errors in CO2 measurement and filtration of CaSO4 affect the amount of CaCl2 excessively.