140 DIY Science: Illustrated Guide to Home Chemistry Experiments
LABORATORY 7 .4:
mAkE Up A mASS-To-voLUmE pERCENTAGE SoLUTIoN
Mass-to-volume percentage solutions are
more commonly used by pharmacists than by
chemists, but these solutions still have a place
in chemistry labs. The advantage of a mass-
to-volume percentage solution is that it makes
it easy to transfer a precise amount of solute
by measuring the necessary volume of the
solution. For example, a pharmacist may make
up a solution that contains 3.00 g of a drug
per 100 mL of solution. If the proper dosage of
the drug is 150 mg, the pharmacist can direct
the patient to take one teaspoon (5.0 mL).
Similarly, if the proper dosage is 1.5 mg, the
pharmacist can direct the patient to take one
drop (0.05 mL).
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance and weighing papers£ graduated cylinder, 100 mL£ beaker, 150 mL£ funnel£ labeled storage bottle£ labeled small dropper (Barnes) bottle£ ethanol (100 mL)£ phenolphthalein powder (1 g)CUTIOA nS
Although none of the chemicals used in this lab
are hazardous, it’s good practice to wear splash
goggles, gloves, and protective clothing at all times.
(Phenolphthalein, formerly widely used as a laxative, was
withdrawn from the market because of concerns about
possible links with cancer, but the small amounts used as
an indicator solution are not ingested and are no cause
for concern.)z
dISoALp S : The weighing paper can be disposed of with
household waste.SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute drug-store rubbing alcohol
(ethanol or isopropanol) for the ethanol.
The most common use of mass-to-volume solutions in
chemistry labs is for indicators and similar solutions. For
example, a chemist might keep on hand a 1% ethanol solution
of phenolphthalein, a 0.2% aqueous solution of phenol red, and
0.4% aqueous solutions of bromocresol green and bromothymol
blue. For most laboratory uses, the exact concentrations
of these solutions is unimportant. For example, rather than
weighing 1.000 g of phenolphthalein powder, dissolving it in
ethanol, and making it up to 100 mL in a volumetric flask, most
chemists would make up the solution by dissolving about a gram
of the indicator powder in about 100 mL of ethanol, measured
roughly in a beaker. We’ll take a bit more care in this lab, but
not much more.