Chapter 7 Laboratory: Solubility and Solutions 127RNNINGU THE NUmBERS
In this case, the calculation is trivial; we’re making up a
one-tenth of a liter of 1 M solution, so we obviously require
0.1 moles of the solute. For other volumes and molarities,
the calculations may be a bit more complicated. For
example, to determine how much solute is required to
make up 25 mL of a 0.75 M solution of copper sulfate, you
might calculate as follows:
(25 mL) · (1 L/1000 mL) · (0.75 mol/L) · (249.7 g/mol)
= 4.68 g
Milliliters (mL) cancel out between multiplicands one and
two, liters (L) cancel out between multiplicands two and
three, and moles cancel out between multiplicands three
and four, leaving only grams (g).POCEDURER
CUTIOA nS
Copper sulfate is moderately toxic by ingestion,
inhalation, or skin contact. Wear splash goggles, gloves,
and protective clothing at all times.z
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.- Place a weighing paper or boat on the balance and tare
the balance to read 0.00 g. - Transfer copper sulfate pentahydrate crystals to the
weighing paper until the balance reads as close as
possible to 25.22 g. Read the mass to 0.01 g and record
it. (It’s not important to have exactly 25.22 g of copper
sulfate on the weighing paper, but it is important to know
the mass as exactly as possible.) - Use the graduated cylinder to measure about 85 mL of
distilled water, and transfer it to the beaker. - Transfer the copper sulfate pentahydrate from the
weighing paper to the beaker, making sure that all of the
crystals are added to the beaker. - Swirl the beaker gently until all of the copper sulfate has
dissolved. This may take a few minutes, because this
solution is nearly saturated. - Place the funnel in the mouth of the volumetric flask,
and carefully pour the copper sulfate solution into the
volumetric flask. - Use the wash bottle to rinse the inside of the beaker
with 2 or 3 mL of water, and add the rinse water to the
volumetric flask, rinsing the funnel as you do so. (Be
careful not to use too much rinse water; the volumetric
flask already contains about 85 mL of solution, and we
can’t go over 100 mL.) - Repeat the rinse with another 2 or 3 mL of water. This
procedure is called a quantitative transfer, and the goal
is to make sure that all of the copper sulfate has been
transferred to the volumetric flask.
Remove the funnel from the mouth of the volumetric
flask, and use the dropper to add water dropwise until the
level of solution in the flask reaches the reference line.
Insert the stopper in the volumetric flask and invert the
flask several times to mix the solution thoroughly.
Using the funnel, transfer the solution from the
volumetric flask to the labeled storage bottle.
Using the exact mass you recorded in step 3, calculate
the molarity to the correct number of significant figures
and record that molarity on the label. Also, record the
date on the label.
Rinse the beaker, funnel, and volumetric flask and
stopper with tap water and then with distilled water, and
set them aside to dry.
dISpoSAL: The weighing paper can be disposed of with
household waste.