What is the molarity of a solution containing 100 g of NaCl made up to 500 mL with distilled water? Assume a gram molecular weight of approximately 58 g/mol.

• A. 3.45 M
• B. 1.72 M
• C. 290 M
• D. 5.27 M

Answer: (A)

Molarity is the amount of solute in moles per liter of solution. First find the moles of NaCl: 100 g divided by its molar mass of about 58 g/mol gives roughly 1.72 moles. The solution is made up to 500 mL, which is 0.500 L. So, M = moles / liters = 1.72 / 0.500 ≈ 3.44 M, rounding to 3.45 M. This value is the only one consistent with both the mass of NaCl and the final volume. The other numbers arise from using the wrong volume (for example, 1 L instead of 0.500 L) or other miscalculations.