Question

What is the molarity of a solution prepared by dissolving 5.85 g of NaCl in water to make 250 mL of solution? (Molar mass of NaCl = \( 58.5 \, \text{g/mol} \)).

• A. \( 0.1 \, \text{M} \)
• B. \( 0.2 \, \text{M} \)
• C. \( 0.4 \, \text{M} \)
• D. \( 1.0 \, \text{M} \)

Hint:

When calculating molarity, convert the volume to liters and ensure the mass is divided by the correct molar mass to find moles. Double-check unit conversions to avoid errors.

Answer 1

The Correct Option is C

Molarity (\( M \)) is defined as the number of moles of solute per liter of solution:
\[ M = \frac{\text{moles of solute}}{\text{volume of solution (in liters)}} \] Given:
- Mass of NaCl = \( 5.85 \, \text{g} \),
- Molar mass of NaCl = \( 58.5 \, \text{g/mol} \),
- Volume of solution = \( 250 \, \text{mL} = 0.250 \, \text{L} \).
Calculate the moles of NaCl:
\[ \text{Moles of NaCl} = \frac{\text{mass}}{\text{molar mass}} = \frac{5.85}{58.5} = 0.1 \, \text{mol} \] Calculate the molarity:
\[ M = \frac{\text{moles of NaCl}}{\text{volume in liters}} = \frac{0.1}{0.250} = 0.4 \, \text{mol/L} = 0.4 \, \text{M} \] Thus, the molarity of the solution is \( 0.4 \, \text{M} \).