Question

21 pour préparer 25,0 ml d’une solution à 3,5 × 10⁻² mol/l, vous utilisez une solution concentrée à 0,10 mol/l. indiquez comment vous devez vous y prendre.

Explanation:

Step1: Recall dilution formula

The dilution formula is \( C_1V_1 = C_2V_2 \), where \( C_1 \) is the initial concentration, \( V_1 \) is the initial volume, \( C_2 \) is the final concentration, and \( V_2 \) is the final volume.

Step2: Identify known values

We know \( C_2 = 3.5 \times 10^{-2} \, \text{mol/L} \), \( V_2 = 25.0 \, \text{mL} \), and \( C_1 = 0.10 \, \text{mol/L} \). We need to find \( V_1 \).

Step3: Rearrange formula to solve for \( V_1 \)

From \( C_1V_1 = C_2V_2 \), we get \( V_1 = \frac{C_2V_2}{C_1} \).

Step4: Substitute values into the formula

Substitute \( C_2 = 3.5 \times 10^{-2} \, \text{mol/L} \), \( V_2 = 25.0 \, \text{mL} \), and \( C_1 = 0.10 \, \text{mol/L} \) into the formula:
\[
V_1 = \frac{(3.5 \times 10^{-2} \, \text{mol/L}) \times (25.0 \, \text{mL})}{0.10 \, \text{mol/L}}
\]

Step5: Calculate \( V_1 \)

First, calculate the numerator: \( (3.5 \times 10^{-2}) \times 25.0 = 0.875 \, \text{mol/L} \times \text{mL} \) (the units here are just for calculation, we'll handle the final units properly). Then divide by \( 0.10 \, \text{mol/L} \):
\[
V_1 = \frac{0.875}{0.10} \, \text{mL} = 8.75 \, \text{mL}
\]

Step6: Describe the procedure

1. Measure \( 8.75 \, \text{mL} \) of the \( 0.10 \, \text{mol/L} \) concentrated solution using a graduated pipette (or appropriate volumetric glassware).
2. Transfer this measured volume into a \( 25.0 \, \text{mL} \) volumetric flask.
3. Add the appropriate solvent (e.g., water, depending on the solute) up to the calibration mark on the volumetric flask, mixing thoroughly to ensure uniform concentration.

Answer:

To prepare the solution, measure \( 8.75 \, \text{mL} \) of the \( 0.10 \, \text{mol/L} \) concentrated solution, transfer it to a \( 25.0 \, \text{mL} \) volumetric flask, and dilute to the mark with the appropriate solvent. The volume of the concentrated solution required is \( \boldsymbol{8.75 \, \text{mL}} \).