2 HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + H2CO3 Analysis of Soda Ash Hand in: Pre-Lab and Open Acetic Acid spreadsheet Today: Determine the amount of Na2CO3 in a sample of Soda Ash Soda Ash is made up of Na2CO3 and other stuff We can use the fact that Na2CO3 is a base that will react with standardized HCl to calculate % in the original Ash sample: 2 HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + H2CO3
Analysis of Soda Ash 2 HCl + Na2CO3 → 2NaCl + H2CO3 Is actually a two-step process: H+ + CO3-2 (aq) →HCO3- (step 1) And then H+ + HCO3- → H2CO3 (step 2) So when you titrate: you will see two plateaus and two inflection points Today, we will determine the 2nd endpoint to determine how much HCl is required to completely titrate all the Na2CO3
HCO3- → H+ + CO32- Ka2 = 4.7 x 10-11 H2CO3 → H+ + HCO3- Ka1 = 4.5 x 10-7
Analysis of Soda Ash So Today: Set up Excel with active titration graph – use similar format as last week: Column A – mL of HCl Column B is pH Column C is derivative Set up Apparatus Similar to last week including: ring stand, magnetic stirrer, burette holder, burette, pH meter, magnetic stir bar. Acid (HCl) goes in burette soda ash (about .2 grams) goes in beaker (WRITE VIAL # on data sheet as soon as you begin) add 3-4 drops of methyl orange indicator Calibrate the pH meter probe with 7 & 4 buffers
Analysis of Soda Ash Use Indicator Methyl Orange [Yellow(5), Red(3)] to help with the endpoint – see examples for color change to expect You will ultimately do 5-6 titrations of the soda ash (enough in the vial) Trial 0 Add HCl carefully in order to graph 2 plateaus and find the 2nd endpoint Determine the pH at the endpoint Trails 1-4 or 5 Simply titrate to THAT pH at the 2nd equivalence point Record the volume of HCl to reach that pH May want to do One or Two titrations to find EXACT pH at 2nd Equivalence Point At least four titrations to get %Na2CO3 “Save as” with different file names for first titration curve(s)
Analysis of Soda Ash
SIMILAR TO PRE-LAB QUESTION #2 Calculations 2 HCl + Na2CO3 → H2CO3 + 2 NaCl SIMILAR TO PRE-LAB QUESTION #2 .108 moles
Lab Report Due next Lab Period Title Page Purpose Results - One Titration/Derivative Graph - Data Sheet (with sample calculations) - Present/Staple in order data/calculations were obtained/performed - Significant Figures and Units - Precision and Accuracy (the vials have true values you will be graded against) Conclusion [ Errors , Improvements ] Individual Checked-Pre-labs