Chapter 7 Laboratory: Solubility and Solutions 123A Eof EmoL GGS?
A mole is a specific number of objects, much like a dozen
or a gross, but with a much larger numeric value. While
it’s convenient to group eggs by the dozen or small parts
by the gross, atoms and molecules are so unimaginably
tiny that we group them in much larger numbers. Rather
than 12 or 144 objects, one mole (abbreviated “mol”) of a
substance contains 6.0223 · 10^23 (Avogadro’s number) of
the elementary entities (atoms or molecules) that make
up that substance.
One mole of an element or compound is, for all practical
purposes, that quantity whose mass in grams has the
same numeric value as the atomic mass or molecular
mass of the substance. For example, because the atomic
mass of iron is 55.847 unified atomic mass units (u, also
called Daltons, Da), one mole of elemental iron has a
mass of 55.847 grams. Similarly, because the molecular
mass of molecular oxygen is 31.9988, one mole of oxygen
molecules has a mass of 31.9988 grams. (Conversely, one
mole of atomic oxygen, whose atomic mass is 15.9994,
has a mass of 15.9994 grams.)
Moles are often more convenient for chemists to use than
masses, because moles represent specific numbers of
atoms or molecules. For example, one atom of sodium
(Na) reacts with one atom of chlorine (Cl) to form one
molecule of sodium chloride (NaCl). Because one mole
of sodium contains the same number of atoms as one
mole of chlorine, one mole of sodium reacts with one
mole of chlorine to form one mole of sodium chloride.
Or, stated differently, 22.989768 grams (one mole) of
sodium reacts with 35.4527 grams (one mole) of chlorine
to form 58.442468 grams (one mole) of sodium chloride.
Using moles rather than masses makes the proportions of
reactants and products clear.acid molecule. Similarly, dibasic molecules such as barium
hydroxide, Ba(OH) 2 , dissociate in solution to yield two moles
of hydroxide per mole of compound, so for solutions of these
compounds the normality is twice the molarity.mass percentage
Mass percentage, also called weight-weight percentage or w/
w, specifies the mass of the solute as a percentage of the total
mass of the solution. For example, dissolving 20.0 g of sucrose
in 80.0 g of water yields 100 g of a 20% w/w sucrose solution.
Mass percentage is often used to specify concentration for
concentrated acids. For example, a bottle of reagent-grade
hydrochloric acid may specify the contents as 37% HCl, which
means that 100 g of that solution contains 37 g of dissolved
HCl. (Such solutions are often also labeled with the density of
the solution, which allows them to be measured volumetrically
rather than requiring weighing to transfer an accurate amount
of solute.)
mass-volume percentage
Mass-volume percentage, also called weight-volume
percentage or w/v, specifies the mass of the solute as a
percentage of the total volume of the solution. For example,
dissolving 5.0 g of iodine in ethanol and making up the
final volume to 100.0 mL yields 100 mL of a 5% w/v iodine
solution. Mass-volume percentage is often used to specify
concentration for indicators such as phenolphthalein and other
reagents that are typically used dropwise.
volume-volume percentage
Volume-volume percentage, abbreviated v/v, specifies the
volume of the solute as a percentage of the total volume of
the solution, and is often used when mixing two liquids. For
example, a 40% v/v solution of ethanol in water can be made
by measuring 40 mL of 100% ethanol and adding water until
the final volume is 100 mL. I phrased that last sentence very
carefully, because volumes are not necessarily additive. For
example, adding 60.0 mL of water to 40.0 mL of ethanol yields
something less than 100.0 mL of solution.
Keep significant digits in mind when you make up and label
solutions. The necessary accuracy of concentration depends on
the purpose of the solution.
Bench solutions
Bench solutions specify concentration to only one, two,
or three significant figures (e.g., 1 M, 1.0 M, or 1.02 M). For
example, you might make up 500 mL of of a 1 M bench solution
of dilute hydrochloric acid using a graduated cylinder to
measure the concentrated acid and a large beaker to make
up the solution to about 500 mL. The advantage of bench
solutions is that they can be prepared quickly and can be used
when the exact concentration doesn’t matter.