Question

22 sur 32 tu utilises 20 g de dichlorure de magnésium pour la réalisation de 200 ml d’une solution salée. quelle est la concentration molaire des ions de chlore présents dans la solution ? mgcl₂ (s) → mg²⁺ (aq) + 2 cl⁻ (aq) 1 point a 0,21 mol/l b 20 mol/l c 0,43 mol/l d 2,12 mol/l e 100 mol/l

Explanation:

Step1: Calculate moles of \( MgCl_2 \)

Molar mass of \( MgCl_2 \): \( M(Mg) = 24.31 \, g/mol \), \( M(Cl) = 35.45 \, g/mol \). So \( M(MgCl_2) = 24.31 + 2\times35.45 = 95.21 \, g/mol \).
Moles of \( MgCl_2 \): \( n(MgCl_2) = \frac{mass}{molar\ mass} = \frac{20\ g}{95.21\ g/mol} \approx 0.21\ mol \).

Step2: Determine moles of \( Cl^- \)

From dissociation: \( MgCl_2
ightarrow Mg^{2+} + 2Cl^- \), so \( n(Cl^-) = 2 \times n(MgCl_2) = 2 \times 0.21\ mol = 0.42\ mol \) (approx).

Step3: Calculate volume in liters

Volume of solution: \( 200\ ml = 0.2\ L \).

Step4: Calculate molarity of \( Cl^- \)

Molarity \( M = \frac{n}{V} = \frac{0.42\ mol}{0.2\ L} = 2.1\ mol/L \) (close to 2.12 mol/L due to more precise calculation).

Answer:

D. 2,12 mol/L