AP Lab Day (Sept 20th) Get out your laptops and lab books! Ready to go at 3:30 on the dot! Don’t forget for tomorrow HW: Webassign 2 and peer edit the lab

Lab 3: Kool Aid Concentration Goal: What is the concentration (in mg/mL) of the unknown Kool Aid solution?

Research (10 min) with partners How can we assay (test) for an unknown concentration of a colored solution? What materials do we need? What methods do we need to use? What else do we need to know?

Serial Dilutions Why do we use them? 1) To make very small molarity samples more accurately 2) To make a range of known concentrations to use for color analysis (Lab 2)

Lab 3: Kool Aid Concentration Analytical Balance Training (W) Serial dilution recommendations (Th?) Tips on using the colorimeter (Th) REMEMBER: This is sticky, sugary stuff! Clean WELL with soap and water! Notes: stock solution should be about 15-20 g Kool Aid in a 50 mL VF, do serial dilutions of 25 ml using VP. ( in total gets a nice light color at the end.

Warm-up (Sept 21st) Laptop out, please! If a 1.00 liter stock solution of HCl was prepared at 2.40 molar, and you took a 10.00 mL aliquot out, added it to a 100 mL volumetric flask, and then filled it up to the line, what is the concentration of the new diluted solution? If you carried out these dilutions nine total times (serial dilutions), what would the final concentration of the most dilute solution be? 0.240 M – then show excel for serial dilutions

Notes Make sure you have installed Logger Pro on your computer! You can use it for this lab or excel/Google Sheets to make your graph. Logger Pro is more “scientific”  Use 470nm (at least that’s the plan!) 430 if it doesn’t work

Remember – transmittance and absorbance A = Absorbance of a sample (related to Io and I) C = concentration of the sample B = path length E = a constant (to make both sides equal)

What does it mean? http://jennarocca.com/beers-law-equation/

Errors Ms Klug Observed last year Filling up the VF to the 50.00 mL line before the Kool Aid was dissolved (yesterday) Careless use of volumetric and graduated pipettes (line to line, water in there from cleaning, etc) Always rinse a little bit of the new solution in the pipet and discard Transfer was pretty good – from boat and funnel. Always rinse! DO NOT TOUCH WATER BOTTLE TO GLASSWARE! Contamination galore!

Lab 3: Write up Requirements – all in Lab Book for Monday class Purpose Data Table (including masses, dilutions, and absorbance values) Calculations and Calibration Curve Graph Don’t forget to add 0,0 as a point (0 concentration should be 0 absorbance, right?) With Linear trendline, equation to fit data, and R2 Short statement of final answer and an analysis of your results based on your graph: did you have more systematic or random error? Justify your answer.

Homework Day 7 Due Monday in lab book: Lab 3 all in lab book (see previous slide with expectations) Don’t forget your calibration curve with equation and R2 value PASTED or TAPED IN – not just flying free!