Ap Chemistry - Gravimetric Lab

Willy Rupprecht
Date(s) performed: October 2 - 3, 2012
Gravimetric Analysis of an Unknown Group 1 Metal Carbonate
Purpose:
The purpose of this lab is to determine the identity of a Group 1 metal carbonate compound by gravimetric analysis.
Procedure:
The unknown carbonate is weighed and then dissolved in water. A solution of CaCl2 (calcium chloride) is added to the metal carbonate solution to precipitate the carbonate ions as CaCO3 (calcium carbonate). The precipitate is then filtered, dried, and weighed again.
Data:
Equation 1: M2CO3 (s) 2M+ (aq) + CO32- (aq) Equation 2: Ca2+ (aq) + CO32- (aq) CaCO3 (s) Equation 3: CaCl2 (aq) + M2CO3 (aq) CaCO3 (s) + 2MCl (aq) Mass of Crucible | 25.721 g | Mass of M2CO3 | 2.031 g | Mass of Filter Paper | 0.969 g | Mass of CaCO3 | 1.5782 g | Moles of CaCO3 | 0.01577 moles | Molar Mass of M2CO3 | 128.79 g/mol | Identity of M2CO3 | K2CO3 (potassium carbonate) | Percent error | 6.8% |

Calculations and Graphs:
Mass of M2CO3
Mass of Crucible and M2CO3 minus Mass of Crucible = mass of M2CO3
27.752 g – 25.721 g = 2.031 g Mass of CaCO3 Mass of Filter Paper and CaCO3 minus Mass of Filter Paper = Mass of CaCO3 2.551 g – 0.969 g = 1.5782 g
Moles of CaCO3 1.5782 g CaCO31x 1 mol CaCo3100.08 g CaCO3 = 0.01577 mol CaCO3
Molar Mass of the Unknown Carbonate
2.031 g M2CO30.01577 mol CaCO3 = 128.79 g/mol M2CO3
Conclusions:
By filtering and weighing the carbonate after it has precipitated, the mass and moles of CaCO3 could then be found; with these values, a molar mass of M2CO3 can be found. Using gravimetric analysis, it has been determined that the unknown Group 1 metal carbonate compound is K2CO3 (potassium carbonate).
Experimental Sources of Error: A) The first source of error had to do with the precipitation section of the lab. Not only is possible, but it is almost guaranteed that the CaCO3 did not precipitate to its fullest extent. Attaining a precipitate that is 100% pure and is