Experiment : Solutions Preparation, Part 2 CHE116

Purpose of This Experiment To learn how to prepare solutions from a premade, concentrated solution To practice making a calibration curve Work in groups of 4 (each person must prepare their own calibration curve CHE116

Preparing a Solution by Dilution (Mc) (Vc) = (Md) (Vd) Mc is the molarity of the concentrated solution Vc is the volume of the concentrated solution used Md is the molarity of the diluted or final solution Vd is the volume of the diluted solution CHE116

Preparing a Solution by Dilution To prepare a solution using a concentrated stock solution: You need to know what molarity the new solution will be (Md) You need to know the volume the new solution will be (Vd) You need to know the concentration of the concentrated solution (Mc) You need to calculate the volume needed of the concentrated solution (Vc). This is the volume you will pipet. CHE116

Remember…. Do not put any pipet or other piece of equipment into the stock bottle (reagent bottle). Pour some of the stock solution into a clean, dry beaker and use the stock solution from the beaker. CHE116

Find the Absorbance of Your Prepared Solution Use a disposable transfer pipet (plastic) to almost fill a small test tube with your solution Wipe the outside of the test tube (remove droplets and fingerprints) Place this test tube in the sample compartment of the spectrophotometer (use the same spectrophotometer each time) Record the absorbance readout from the spectrophotometer CHE116

Spectrophotometer An instrument that will indicate the amount of light (of one wavelength) absorbed by a sample Readout detector sample monochromator Light source slit LED readout CHE116

Beer – Lambert Law A = abc A is absorbance of light by the sample a is the molar extinction coefficient b is the solution path length c is the concentration of solute This is a linear relationship; y = mx + b y is absorbance, x is concentration CHE116

Calibration Curves Calibration curves show the linear relationship between absorbance and concentration. Absorbance Concentration, M CHE116

Making the Calibration Curve You will have an absorbance reading for each standard solution (a different concentration from each person in the group) Plot the corresponding concentration and absorbance values as data points on a graph CHE116

Absorbance Concentration, M CHE116

Using the Calibration Curve You will have an absorbance reading for your unknown solution. Find that absorbance reading on the y-axis of the calibration curve. At that point on the y-axis, move horizontally across to the calibration curve line (use a ruler) When you reach the line, drop straight down to the x-axis The value that you are at on the-x axis is the concentration of your solution CHE116

Calibration Curve Absorbance Unknown solution absorbance reading Concentration, (units) Unknown solution absorbance reading Your concentration CHE116