Limestone consists mainly of the mineral calcite, CaCO 3 . The carbonate content

Question

Limestone consists mainly of the mineral calcite, CaCO3. The carbonate content of 0.5413 g of powdered limestone was measured by suspending the powder in water, adding 10.00 mL of 1.339 M HCl, and heating to dissolve the solid and expel CO2.

The excess acid required 39.96 mL of 0.1059 M NaOH for complete titration to a phenolphthalein end point. Find the weight percent of calcite in the limestone.
%

CaCO3(s) + 2 H+ Ca2+ + CO2 + H2O calcium carbonate FM 100.087 Limestone consists mainly of the mineral calcite, CaCO3. The carbonate content of 0.5413 g of powdered limestone was measured by suspending the powder in water, adding 10.00 mL of 1.339 M HCl, and heating to dissolve the solid and expel CO2. The excess acid required 39.96 mL of 0.1059 M NaOH for complete titration to a phenolphthalein end point. Find the weight percent of calcite in the limestone.

Explanation / Answer

HCl + NaOH => NaCl + H2O

Moles of excess HCl = moles of NaOH = volume x concentration of NaOH

= 39.96/1000 x 0.1059 = 0.0042318 mol


Initial moles of HCl = volume x concentration of HCl

= 10.00/1000 x 1.339 = 0.01339 mol


Moles of reacted HCl = 0.01339 - 0.0042318 = 0.0091582 mol


CaCO3 + 2 HCl => CaCl2 + CO2 + H2O

Moles of CaCO3 = 1/2 x moles of reacted HCl

= 1/2 x 0.0091582 = 0.0045791 mol


Mass of CaCO3 = moles x molar mass of CaCO3

= 0.0045791 x 100.087 = 0.45831 g


Weight% = mass of CaCO3/mass of sample x 100%

= 0.45831/0.5413 x 100%

= 84.67%

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