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500 mL of 0.2 M NaCl solution is mixed with 1500 mL of 0.4 M MgCl2 solution (volumes are additive). Which of the following statements about the final ionic concentrations is incorrect?

1. [Na+] = 0.2 M
2. [Mg2+] = 0.3 M
3. [Cl-] = 0.65 M
4. [Mg2+] = 7.2 g/L

Correct answer: [Cl-] = 0.65 M

Solution

Total volume = 2000 mL = 2 L. Moles Na+ = 0.5 L * 0.2 mol/L = 0.1 mol. [Na+] = 0.1/2 = 0.05 M (not 0.2 M). Moles Mg2+ = 1.5 * 0.4 = 0.6 mol. [Mg2+] = 0.6/2 = 0.3 M. [Mg2+] in g/L = 0.3 * 24 = 7.2 g/L. Moles Cl- from NaCl = 0.1 mol. Moles Cl- from MgCl2 = 1.5 * 0.4 * 2 = 1.2 mol. Total Cl- = 1.3 mol. [Cl-] = 1.3/2 = 0.65 M. So [Na+] = 0.05 M (option A says 0.2 M which is wrong), [Mg2+] = 0.3 M (option B correct), [Cl-] = 0.65 M (option C correct), [Mg2+] = 7.2 g/L (option D correct). The only incorrect statement is option A. But option C is listed as the intended answer in many sources. Let me recheck [Cl-]: 0.1 (from NaCl) + 2*0.6 (from MgCl2) = 0.1 + 1.2 = 1.3 mol. [Cl-] = 1.3/2 = 0.65 M - this is actually CORRECT. So option C is correct and option A is incorrect. However, the question asks for the only incorrect statement. Both A and C need checking. [Na+] = 0.1/2 = 0.05 M, so option A claiming 0.2 M is wrong. The question asks which is the only incorrect one. Option A is incorrect.

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