A simple diagram that relates Avogadro's number, moles, and number of atoms / molecules.

No abstract

Recently a student asked, "Why do we use both density and specific gravity? They have about the same value". The answer can be found in our inability to measure volumes as effectively as we can measure mass. A milliliter sample of liquid can be weighed to four decimals on an ordinary analytical balance but to only one decimal for the volume using a small graduated cylinder. This disparity between mass and volume can be eliminated by a pycnometer, which allows very small samples to be analyzed and requires only weighings. In essence a pycnometer provides density comparisons between a standard liquid, usually distilled water, and an unknown sample.Mass of X Specific gravity = Volume Mass of Water Volume Since the volume is the same for both density measurements, it will cancel out leaving only the more valuable mass data. To convert specific gravity to density, simply multiply the specific gravity of the sample by the density of the standard at the temperature of use. A compromise is often encountered by the use specific gravity with exponential notation, which indicates the temperatures at which the two weighings were made, for example, 0.901 25/25.

Mast laboratary experiments in undergraduate chemistry begin with a 8ottle or two, bottles containing solutions and solid reagents needed to carry out one sort of reaction or another. This is quite normal and expected for it has been going on for years with generally suitable results. But some experiments don't have to start with a bottle at all.