Chemistry/Analysis of Limestone

INTRODUCTION
Limestone is usually described as rock made from calcium carbonate, CaCO3, but in fact most limestone rock contains significant amounts of magnesium, silicates, manganese, iron, titanium, aluminum, sodium, potassium, sulphur (as sulphides or sulphates) and phosphorus. In this experiment, you will analyze a sample of limestone to find how much calcium it contains.

The reactions you will carry out are as follows. First, the carbonates present are reacted with hydrochloric acid. This procedure releases the calcium and magnesium ions present into solution. Any manganese, iron, titanium, aluminum, sodium, potassium and phosphates present will also go into solution. CaCO3(s) + 2 HCl(aq) ===>   Ca2+(aq) + CO2(g) + H2O(l) + 2 Cl–(aq)

MgCO3(s) + 2 HCl(aq) ===>   Mg2+(aq) + CO2(g) + H2O(l) + 2 Cl–(aq)

Some impurities, such as silicates, sulphides and sulphates, remain unreacted and do not go into solution.

Adding substantial amounts of both ammonia and ammonium ions to the solution creates a “buffer” solution which is slightly basic: NH3(aq) + H2O(l) ===>  NH+4 (aq) + OH–(aq)

The resulting buffer solution does not contain sufficient concentrations of OH– to precipitate the calcium, magnesium, sodium and potassium ions from solution. However, the concentration of OH– is sufficient to precipitate all the iron, manganese, aluminum, titanium and phosphates from solution. For example: Fe3+(aq) + 3 OH–(aq) ===> Fe(OH)3(s) Fe3+(aq) + PO3-4 ===> FePO4(s)

Filtration at this point completes the removal of everything except the calcium, magnesium, sodium and potassium from the solution.

The solution containing the remaining ions is acidified with hydrochloric acid. The addition of oxalate ion, in the form of ammonium oxalate, prepares the solution to precipitate calcium oxalate. However, no precipitate forms because all oxalates are soluble in acidic solutions. By neutralizing the acid with ammonia the resulting solution becomes slightly basic and a precipitate of calcium oxalate monohydrate forms. Since magnesium, sodium and potassium oxalates are soluble in basic solutions, only calcium oxalate precipitates.

Ca2+(aq) + C2O2-4aq) + H2O(l) ===>   CaC2O4•H2O(s)

CALCULATIONS AND ANALYSIS

 * 1) Calculate the mass of the dried precipitate, CaC2O4•H2O.
 * 2) Calculate the moles of CaC2O4•H2O you produced.
 * 3) Since each molecule of CaC2O4•H2O contains one atom of calcium, the moles of calcium contained in your sample of limestone equals the moles of CaC2O4•H2O produced.  Use this fact to calculate the mass of calcium contained in your sample.
 * 4) Use the mass of calcium found in Calculation 3 and your mass of limestone to calculate the percentage of calcium present in your sample. This result is called the ACTUAL PERCENTAGE OF CALCIUM in your sample of limestone.
 * 5) Calculate the percentage composition of CaCO3.  The percentage of Ca found in your percentage composition is the THEORETICAL PERENTAGE OF CALCIUM in pure limestone.
 * 6) Calculate the percentage purity of your limestone sample as follows.