Chapter 11 Laboratory: Acid-Base Chemistry 201SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute foam cups or similar containers for
the beakers. - You may substitute a calibrated beral pipette for
the 1.00 mL transfer pipette. (To calibrate the beral
pipette, count the number of drops required to reach
10.00 mL in a graduated cylinder, and then calculate
the number of drops per mL.) - You can make up 1.0 M acetic acid by diluting 5.7
mL of concentrated (glacial) acetic acid to 100 mL.
Alternatively, you may use distilled white vinegar,
which is close to 1 M straight out of the bottle. - You can make 100 mL of 1.0 M sodium acetate by
dissolving 8.20 g of anhydrous sodium acetate or
13.61 g of sodium acetate trihydrate in some distilled
water and then making up the solution to 100 mL. - You can make 20 mL of 6.0 M hydrochloric acid
by mixing 10.00 mL of concentrated (37%, 12 M)
hydrochloric acid with 10.00 mL of water. If you have
hardware-store muriatic acid (31.45%, 10.3 M), mix
11.65 mL of the acid with 8.35 mL of water. - You can make 20 mL of 6.0 M sodium hydroxide
by dissolving 4.80 g of sodium hydroxide, stirring
constantly, in 90 mL of water and making up the
solution to 100 mL. (Caution: this reaction is extremely
exothermic.)
Conversely, if we add sodium hydroxide to the buffer solution, the
NaOH ionizes completely in solution, yielding hydroxide ions and
sodium ions. According to Le Chatelier’s Principle, the increase
in hydroxide ions forces the acetic acid equilibrium to the right,
decreasing the concentration of hydroxide ions and increasing
the concentration of acetate ions. This equilibrium shift changes
the effective number of moles of acetic acid and acetate ions,
which can be calculated as follows:
final CH 3 Coo– moles =
initial CH 3 CooH moles + initial NaoH moles
final CH 3 CooH moles =
initial CH 3 Coo– moles – initial NaoH moles
In either case, after you calculate the number of moles of acetic
acid and acetate ions, you can use the final volume of the solution
to determine the concentration of the acetic acid and acetate
ions and plug those values into the Henderson-Hasselbalch
equation to determine the new pH.
pH = pka + log 10 ([CH 3 Coo–]/[CH 3 CooH])
CUTIOA nS
Hydrochloric acid and sodium hydroxide are corrosive. Wear
splash goggles, gloves, and protective clothing at all times.z
In this lab, we’ll make up a buffer solution of acetic acid and
sodium acetate and examine the effects of adding hydrochloric
acid and sodium hydroxide to this buffer solution.POCEDURER
1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.- Make up the buffer solution by mixing 100 mL of 1.0 M
acetic acid and 100 mL of 1.0 M sodium acetate in the
250 mL beaker. - Transfer 100 mL of the buffer solution to one of the 150
mL beakers, and 100 mL of distilled water to the second
150 mL beaker. - Read and follow the directions for your pH meter with
respect to calibrating it, rinsing the electrode between
measurements and so on. - Use the pH meter to measure the pH of the buffer
solution, and record the observed value on line A in the
second and fourth columns of Table 11-3. - Use the pH meter to measure the pH of the distilled water,
and record the observed value on line A in the third and
fifth columns of Table 11-3. - Use the 1.00 mL pipette to transfer 1.00 mL of 6.0 M
hydrochloric acid to the beaker that contains the buffer
solution and another 1.00 mL of hydrochloric acid to the
beaker that contains the distilled water. Stir or swirl the
beakers to mix the solutions thoroughly. (If you use only
one stirring rod, rinse it thoroughly before using it in the
other beaker.) - Use the pH meter to determine the pH value for the buffer
solution, and record that value on line B in the second
column of Table 11-3. - Use the pH meter to determine the pH value for the
water solution, and record that value on line B in the third
column of Table 11-3.
Repeat steps 7 through 9, adding 1.00 mL of hydrochloric
acid each time until you have added a total of 10.00 mL
of hydrochloric acid to the buffer solution and water
solution.
Rinse the beakers and pipette thoroughly.
Transfer the remaining 100 mL of buffer solution to one of
the 150 mL beakers, and 100 mL of distilled water to the
second 150 mL beaker.
Repeat steps 7 through 11 using the 6.0 M sodium
hydroxide solution.