124 DIY Science: Illustrated Guide to Home Chemistry Experiments
mISCIBILITy
Some liquid solutes and solvents can form solutions in any
proportion. For example, acetone is freely soluble in water.
(Another way of saying this is that water is freely soluble
in acetone.) That means a water-acetone solution may
contain anything from just under 100% water to just under
100% acetone. Such liquids are referred to as miscible.Standard solutions
Standard solutions are used in quantitative analyses and
are made up to whatever level of accuracy is required.
Typically, standard solutions are accurate to four or five
significant figures (e.g., 1.002 M or 1.0008 M) and are
standardized against a reference standard (a solution of
which the concentration is known very accurately).
For example, to make up 1 L of a nominal 1.0000 M solution
of sodium carbonate requires one mole (105.99 g) of sodium
carbonate. You begin by weighing an amount of sodium
carbonate as close to 105.99 g as possible, but the exact
amount is less important than recording the actual mass to
within 0.01 g. You might, for example, end up with 106.48 g
of sodium carbonate in the balance pan (or 104.37 g) but
the important thing is that you know the mass to within
0.01 g. After recording the actual mass, you transfer the
sodium carbonate to a 1 L volumetric flask that contains
perhaps 800 mL of distilled water and swirl or shake the flask
to dissolve the solute. You then rinse any small amount of
sodium carbonate that remains in the weighing boat into the
flask, fill the flask to within a centimeter of the reference line,
and mix the contents again by swirling or shaking the flask.
Finally, you fill the flask exactly to the reference line using
a dropper.At this point, you have 1 L of sodium carbonate solution
with the concentration known to a high degree of accuracy.
Depending on how much sodium carbonate you actually
used, that concentration may be (for example) 1.0046 M,
and the storage container can be labeled accordingly. To
verify the exact concentration, you can titrate the solution
against a reference standard.Stock solutions
Stock solutions are concentrated solutions of stable
chemicals, often saturated or nearly saturated, that are
normally diluted before use. Stock solutions may be
purchased or made up. The most common examples of
purchased stock solutions are concentrated acids (such as
acetic, hydrochloric, nitric, and sulfuric acids), some bases
(such as aqueous ammonia), and other common liquid
laboratory chemicals, such as concentrated hydrogen peroxide
and formalin.Many chemists keep on hand a variety of made-up stock
solutions of solid chemicals that they use frequently. Stock
solutions minimize required storage space and make it easy
and convenient to produce more dilute bench solutions simply
by diluting the stock solution in some proportion. For example,
a 12 M stock solution of hydrochloric acid can easily be diluted
with an equal part of water to provide 6 M HCl, with three parts
of water to provide 3 M HCl, or with 11 parts of water to provide
1 M HCl. Stock solutions can also be standardized and diluted
more accurately to provide more dilute standard solutions.When you’re making up solutions, it’s important to consider the
solubility of the solute, which specifies the maximum amount of
solute that dissolves in the solvent at a particular temperature.
For example, you might decide to make up 100 mL of a 1.00 M
potassium permanganate stock solution. You look up the formula
weight of potassium permanganate and find that it is 158.04 g/
mol, which means you need 15.8 g to make up 100 mL of 1.00 M
solution. So far, so good.But if you start weighing and dissolving without further
checking, you’re going to have an uh-oh moment. Why?
Because, according to the CRC Handbook, at 20°C, 100 mL of
water dissolves only 6.38 g of potassium permanganate and the
other 9.42 g of potassium permanganate will remain undissolved
in the bottom of your 100 mL volumetric flask. Oops. Time for a
bit more figuring.If the solubility of potassium permanganate in water at 20°C is
63.8 g per liter and the formula weight of potassium
permanganate is 158.04 g/mol, that means that a saturated
solution of potassium permanganate is [63.8 g] / [158.04
g/mol] = 0.40+ M. Because you don’t want potassium
permanganate crystallizing out of the stock solution if the
temperature in your lab happens to fall below 20°C, make
up your potassium permanganate stock solution to 0.3 M by
dissolving 4.74 g of potassium permanganate and making the
solution up to 100 mL.