Preparing Solutions of Precise Concentration It is often necessary to prepare solutions of precisely known concentration of a substance If the substance in question is a solid, the first step will generally be to measure a precise weight on the analytical balance (BY DIFFERENCE) The solid is transferred to a container which can be filled to a precise volume. With a precise weight and a precise volume, we can calculate a precise concentration Units of concentration will vary from exercise to exercise

Typical Concentration Measures WEIGHT PER UNIT VOLUME –From the weight and the volume, we can calculate the WEIGHT PER UNIT VOLUME (e.g., g/L or mg/L, etc.) MOLARITY –From the weight, and the molar mass of the substance, we can calculate the number of moles (or millimoles). Combining that with the volume, we can calculate MOLARITY (e.g., mol/L or mmol/mL) WEIGHT PERCENT –From the weight, the volume and the density of the final solution, we can calculate the WEIGHT PERCENT of our substance. (e.g., 4.85% (by weight))

The Apparatus

Establish the range of the desired concentration that can be tolerated. E.g., we wish to prepare mL of a solution of sodium chloride with a concentration between 0.19 M and 0.21 M. I.e., 0.20  0.01 M Calculate the acceptable range of weight of substance to meet the goal. We will need mL X (0.20  0.01) mmol/mL = 50  2.5 mmol NaCl The corresponding weight is (50  2.5) mmol X 58.5 mg/mmol = 2900  146 mg Or, 2.9  0.1 g In the example, our goal is to weigh between 2.8 g and 3.0 g, accurately, into our mL container. Preliminaries

Step 1 - Weigh a Sample of Precise Weight Weigh the vial containing the desired substance. (W i ) Transfer the substance directly from the vial to the volumetric flask – without any intermediate container. When the transferred amount is in the desired range, weigh the vial again. (W f ) The amount transferred, (W i -W f ) should be in the calculated range!

Step 2 - Add Water to the Neck of the Volumetric Flask The level must be NO HIGHER than the BOTTOM OF THE NECK. This insures that the liquid will mix properly when the flask is inverted.

Step 3 - Dissolve the sample and mix thoroughly The dissolution and mixing are accomplished by stoppering the volumetric flask and inverting it a reasonably large number of times. At the conclusion of this step, there must be no visible undissolved substance. It is critical that the solution be of uniform concentration. For colored materials, this can generally be determined by the uniformity of the depth of the color in the entire solution. For colorless substances, one simply must insure that the flask has been inverted back and forth a large number of times.

Step 4 - Add Water to Below the Mark Since a substantial volume remains to be added, we still DO NOT bring the volume of the liquid TO THE MARK on the volumetric flask. We add an amount that brings us just a few drops BELOW that mark so that we can add the final volume in a drop-wise fashion. (Remembering that 1 drop is ~ 0.05 mL, the typical intrinsic precision of a volumetric flask (0.1 mL) represents two drops.)

Step 5 - Bring Volume to the Mark using Dropper Using the dropper to transfer the final volume of solvent minimizes the likelihood of overfilling the flask.

Step 6 - Mix the Final Solution Thoroughly Once the full amount of solvent has been transferred, we again insure that the mixture is homogenous by inverting the volumetric flask several times. Since the amount of unmixed solution is small, the number of inversions at this stage can be fewer than those in the earlier mixing step.