To investigate the percentage of CaCO3 - Sample Essay

Materials used :  Beaker Conical Flask Measuring Cylinder of 15 cc Burette  Indicator  Clamp Stand  Pipette Beam Balance  HNO3 of 2M concentration OH of 2M concentration Procedure :  Crush the sample of marble in to small pieces, and take 1 gram of it in a beaker  Add excess of acid into the beaker, allow to react completely Pipette 25 cc of the mixture into the conical flask Add 2 drops of indicator (phenolphthalein) in the conical flask  Rinse the burette with NaOH, then fill it with the latter up to the 0 cc mark.

Titrate the mixture with the base until color changes to purple. In the process, shake the conical flask thoroughly  Calculate the amount of base reacted  Repeat the titration again to improve accuracy. Data Collection : Burette reading (NaOH) / cc HNO3 + marble used (cc) Initial Final Used 0 19 19 25 0 18. 8 18. 8 25 0 18. 5 18. 5 25 Mass of marble used = 3 grams Amount of acid used to dissolve the marble, 100 cc = 0. 1 dm3 Volume of mixture taken = 0. 025 dm3 Mean volume of NaOH used = 18. 76 cm3 = 0. 01876 dm3.

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CaCO3(s) + 2HNO3(aq) Ca(NO3)2(aq) + H2O(l) + CO2(g) Mol of HNO3 used = 0. 1 x 2M = 0. 2 mol Molar ratio of NaOH ~ 0. 01876 x 2 = 0. 03752 mol Mol in 0. 1 dm3 of NaOH = 0. 03752 x 4 = 0. 15008 mol Mol reacted with HNO3 = 0. 2 – 0. 15008 = 0. 04992 mol Mol of CaCO3 = 0. 04992 = 0. 02496 mol Mass of CaCO3 = 0. 02496 x 100 = 2. 496 grams % of CaCO3 = 2. 496/3 x 100% = 83. 2 % Precautions taken : Make sure that the marble reacted completely with the acid, it may take quite a long time, but we are not allowed to proceed with the titration.

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