Chemistry Design Lab 724w53

Chemistry Design lab

Date: 07/10/2011

Name: Siddharth Mahesh

Class: IB year 1

Title: Back Titration Aim: To find the percentage purity of a given mass of sea shells (CaCo3 – Calcium Carbonate) using the process of back titration. Background: Back titration is basically, an analytical technique in chemistry, which is performed backwards in method. That is, a needs to find the concentration of a reactant of a given unknown concentration by reacting it with an excess volume of another reactant of a known concentration. Once these mixtures are reacted with each other, there will be a resulting mixture, which is titrated back in the process. The molarity of the excess, which was added in the process, is taken under consideration too. Seashells are primarily made of calcium carbonate (CaCo3). In this experiment we are to find the percentage purity of a fixed given mass of seashells (calcium carbonate) using hydrochloric acid, and sodium hydroxide. The reactions take place as given: CaCo3 + 2HCl

CaCl2 + H2O + CO2

HCl + NaOH

NaCl + H2O

Apparatus required: 1. 250 cm3 beaker 2. Electronic balance (± 0.01 g) 3. 250 cm3 volumetric flask (± 0.2 cm3) 4. 50.0 cm3 burette (± 0.1 cm3) 5. 25. 0 cm3 pipette 6. conical flask 7. Clamp and retort stand 8. 1.0 mol dm-3 hydrochloric acid, HCL 9. 1.5 g of limestone 10. Phenolphthalein indicator 11. mol dm-3 Variables to be controlled:  Volume of acids, and bases  Temperature, pressure Independent variables:  Mass of the seashells.

Procedure: 1. Measure accurately the given sample of seashells on an electronic balance. This can be done as follows: a. Take a dish. b. Measure the mass of the dish on the electronic balance and name this M1. c. Put the seashells in the dish and measure the new mass of the entire apparatus. Name this M2. d. Calculate the mass of the seashells by subtracting M1 from M2: M2 – M1. 2. Crush the shells. 3. Dissolve the crushed shells in excess 1.0 mol dm-3 HCl (hydrochloric acid) solution. 4. Use a burette to add the HCl till all of the CaCO3 has reacted and dissolved. Record this volume and name it V1. Stir constantly to make sure everything dissolves evenly. 5. Add the solution into the volumetric flask and top it up till 250 cm3. 6. Mount the burette onto the retort stand. 7. Fill it with known concentration of sodium hydroxide solution till the 50cm3 mark. 8. Using a suction and a pipette, accurately take 25cm3 of the HCl and seashell solution and pour it in a conical flask. 9. Add up to two drops of phenolphthalein indicator to the acid solution. Titrate the solution of the seashells against the sodium hydroxide in the burette. 10. This can be done by slowly adding drops of NaOH from the burette into the acid solution till a pink solution is achieved. Keep stirring in between to make sure that the base is evenly dissolved. 11. Repeat the titration sequence a minimum of 3 times to obtain accurate results. Raw data table: No. Mass of Sea Shells +/- 0.01 grams

1.

Volume of HCl +/- 0.1 cm3

Concentration of HCl solution.

Volume of CaCO3 and HCl solution for titration. (Fixed) 25 cm3

Concentration of sodium hydroxide solution

Data table for Titration: No. Volume of Sodium Hydroxide solution needed to neutralized acid solution/cm3 1. 2.

V2 V3

3.

V4

Average Volume of NaOH needed to neutralize acid solution = (V2 + V3 + V4)/3