C ONCENTRATION AND D ILUTION

D EFINITIONS Solute -the minor component of a solution; dissolved in the solvent; usually solid or liquid Solvent -the component of a solution that dissolves the solute; much larger amount than the solute; typically a liquid Concentration -amount of solute in a certain amount of solvent or solution Molarity (M)- one way to describe the concentration of a solution molarity = moles of solute liters of solution molarity = moles of solute liters of solution

D EFINITIONS C ONTINUED Dilution – lab procedure for preparing a less concentrated solution from a more concentrated one. **Does not alter the number of moles present moles of solute after dilution = moles of solute before dilution M 1 V 1 = M 2 V 2 Standard Solution – a solution whose concentration is definitively known Concentrated Make dilutions from this standard

S AMPLE P ROBLEM 1: M OLARITY Tyipcal blood serum is about 0.14 M NaCl. What volume of blood contains 1.0 mg NaCl?

S AMPLE P ROBLEM 2: C ONCENTRATION OF I ONS Give the concentration of each type of ion in the following solution: 0.50 M Co(NO 3 ) 2

S AMPLE P ROBLEM 3: C ONCENTRATION OF I ONS Calculate the number of moles of Cl - ions in 1.75 L of 1.0 x M ZnCl 2.

S AMPLE P ROBLEM 4: D ILUTION What volume of 16M sulfuric acid must be used to prepare 1.5 L of a 0.10 M H 2 SO 4 solution? **Remember: always add acid to water! Think about what this would look like in the lab…

S PECTROPHOTOMETERS -D EFINITIONS Transmittance-fraction or percentage of light that passes through the solution relative to the light that passes through the solvent (incident light) Absorbance-the amount of light that is absorbed by the solution. Blank-Sample that contains everything in the solution except the solute to be measured. (Therefore the concentration of the solute is 0.)

S PEC -20 I NSTRUCTIONS Instructions : 1) Plug in turn on machine, and allow to warm up for at least 10 minutes. 2) Set the wavelength to the pre-determined wavelength for the particular sample using the wavelength knob. This may be constant for the entire lab or may change depending on what you are measuring. 3) Fill a cuvette with a portion of the solvent. Wipe the sides of the cuvette with a Kim wipe. 4) Place the cuvette in the sample holder and close the cap. Adjust the light control knob so the transmittance is 100% (and the absorbance is 0). 5) For each sample, place a portion in a cuvette, wipe the cuvette with a Kim wipe, and place in the sample holder, close the cap, and take measurement.

I MPORTANT THINGS TO REMEMBER : Avoid touching the sides of the cuvette and always wipe the sides before placing in the spec 20 Make sure to blank the machine before taking measurements, and when you change wavelengths or solutions measured. Always take measurements with the cap down over the sample.

B EER ’ S L AW P LOT States a liner relationship between the absorbance and concentration of a solute It is no longer linear at high concentrations. WHY?? The goal is to create a plot by analyzing a series of dilutions. Then, test an unknown to determine the concentration. A = ebc where: A = absorption at a given wavelength of light, e = molar absorptivity, unique to each molecule and varying with wavelength, b = the path length through the solution that the light has to travel, and c = the concentration of the solution in moles per liter (molarity). A = ebc where: A = absorption at a given wavelength of light, e = molar absorptivity, unique to each molecule and varying with wavelength, b = the path length through the solution that the light has to travel, and c = the concentration of the solution in moles per liter (molarity).