2.1: Introduction
A. Volume
We need to be able to measure volume accurately in order to determine the concentration of dissolved substances in our lab. The most common devices used to measure volume are the pipette, graduated cylinder, and volumetric flask.
The Pipette
Examine the pipettes available in our lab today. Determine how the pipettes are calibrated. Both the largest and smallest volumes that a pipette can measure are marked at the top of the pipette. For example, “10 mL in 1/10” means that the pipette can measure a max of 10 mL, each large line marks 1 mL, and each smaller division marks 1/10 mL. We use pipette aids ( propipettes ) to draw liquids into the pipette.
The Micropipette
Micropipettes are used to measure very small volumes accurately and repeatedly. They generally have a continuous range between 1 µL to 100 µL, including fractional volumes. Plastic tips are used to hold the liquid, and the tip can be ejected easily.
The Graduated Cylinder
We use graduated cylinders to measure larger amounts of liquid, typically 5.0 mL or larger. Water sticks more strongly to glass than organic polymers (plastic), so the water will climb up the sides of a glass cylinder to form a bowl-like meniscus. To obtain a precise measurement, we read volume at the bottom of the meniscus.
The Volumetric Flask
Volumetric measuring devices do not have graduations. Instead, they are calibrated to measure a single volume, and should only be used to measure that volume. Volumetric equipment is more accurate than graduated equipment. The narrowness of the neck of the volumetric flask extends the solution into the narrow neck, allowing greater accuracy in matching the meniscus of the solution to the line. Your instructor will demonstrate this activity .
B. Precision versus Accuracy
Accuracy refers to the closeness of an observed value to the true or known value. Precision is the degree to which measurements are reproducible when repeated. A lack of accuracy and/or precision may be a function of the measuring device or the technique used
Preparing and working with Solutions
We measure mass by using a balance or a scale . Containers or weighing papers are often used during measurements of mass. The mass of the container must be subtracted from the total mass to obtain the mass of the material of interest. This is completed by taring the scale. You will need to measure mass when preparing solutions. Solutions are homogenous mixtures of solute (the dissolved substance) and solvent (the dissolving medium). It is most common to use molar concentrations when working in a lab.
A molar (M) solution contains 1 mole of a solute in one liter of solution. A mole of a substance contains Avogadro’s number (6.02 x 10 23 ) of molecules of that substance. For example, the molecular weight of NaCl is 58.5 g/mol. Thus, one mole of NaCl has a mass of 58.5 grams. To prepare one liter of 1 M solution of NaCl, you would add 58.5 g of NaCl to a 1L (1000mL) volumetric flask and then add water until it reaches the 1000 mL mark, meaning a 1M NaCl solution has a total volume (water plus the solute) that is equal to 1L. Thus, the actual volume of water you add is slightly less than 1 L. It is important to realize that if you were to measure 1000 mL of water and then add 58.5 g of NaCl, you would have more than one liter of the solution.
To calculate the amount of solute required to make a solution of a specific molarity, use the following formula:
Grams of solute = [molecular weight (grams)] x [volume (liters)] x [desired molarity].
You will measure the specific gravity of water and different concentrations of NaCl solutions to investigate the relationship between specific gravity and solution concentration. The formula for specific gravity is:
S pecific gravity = (the density of a substance) / (the density of water) .
A hand refractometer is the tool we will use today, however, this is not the only type of tool that can be used to measure specific gravity. You will then prepare a s erial dilution of your NaCl solution. A serial dilution is a stepwise dilution of a substance in solution. The most common dilution is a 1/10 ratio, where you add 1-part of the stock solution, to 9-parts water/solvent. We use serial dilutions to produce low molarity solutions more accurately.