Make a 100 mL volume of 100 mM acetic acid (HAc) HAc (glacial, 100%, 17.4 M, 1.05 g/mL, MW 60.05) ddH2O Add to 100 mL Measure mL of ddH2O using a 100-mL graduated cylinder and transfer this volume to a 250 mL beaker. Add L of 100 mM HAc to the water: Mix with a stir bar.
The Correct Answer and Explanation is:
Correct Answer:
To make 100 mL of 100 mM acetic acid (HAc) from glacial acetic acid (17.4 M):
- Calculate the volume of glacial acetic acid needed using the dilution formula:
C1V1=C2V2C_1V_1 = C_2V_2C1V1=C2V2
Where:
- C1=17.4 MC_1 = 17.4 \, \text{M}C1=17.4M (stock concentration)
- V1=?V_1 = ?V1=? (volume of glacial acetic acid needed)
- C2=0.1 M=100 mMC_2 = 0.1 \, \text{M} = 100 \, \text{mM}C2=0.1M=100mM (final concentration)
- V2=0.1 L=100 mLV_2 = 0.1 \, \text{L} = 100 \, \text{mL}V2=0.1L=100mL (final volume)
(17.4)(V1)=(0.1)(0.1)⇒V1=0.0117.4≈0.0005747 L=0.5747 mL(17.4)(V_1) = (0.1)(0.1) \Rightarrow V_1 = \frac{0.01}{17.4} \approx 0.0005747 \, \text{L} = 0.5747 \, \text{mL}(17.4)(V1)=(0.1)(0.1)⇒V1=17.40.01≈0.0005747L=0.5747mL
- Measure 99.43 mL of distilled water (ddH₂O) using a 100-mL graduated cylinder and transfer it to a 250-mL beaker.
- Add 0.5747 mL of glacial acetic acid to the water in the beaker. Mix thoroughly with a stir bar.
Explanation
To prepare a 100 mL solution of 100 mM acetic acid, we must dilute a concentrated stock solution — in this case, glacial acetic acid, which is pure and has a molarity of 17.4 M. Using the dilution equation C1V1=C2V2C_1V_1 = C_2V_2C1V1=C2V2, we calculate the exact amount of glacial acetic acid needed to reach the desired final concentration and volume.
Our goal is to make 100 mL (0.1 L) of a 0.1 M (100 mM) solution. Plugging these values into the formula gives us the required volume of glacial acetic acid: 0.5747 mL. It is important to be precise here, as acetic acid is a strong-smelling and corrosive chemical in its concentrated form.
Next, we measure 99.43 mL of ddH₂O using a graduated cylinder and pour it into a 250-mL beaker. Water is added first for safety reasons—this avoids the exothermic reaction that might occur if water were added to a small amount of acid. Then, using a micropipette or accurate small-volume pipette, 0.5747 mL of glacial acetic acid is added to the water.
Once combined, the solution is stirred using a magnetic stir bar to ensure even distribution of the acetic acid molecules throughout the water. The result is 100 mL of 100 mM acetic acid, ready for use in experiments or titrations.
Care must be taken with measurements and handling to ensure both safety and accuracy in preparation.
