Question

The mass % of urea solution is 6. The total weight of the solution is 1000 g. What is its concentration in mol L\(^{-1}\)? (Density of water = 1.0 g mL\(^{-1}\))
(Given: C = 12u, N = 14u, O = 16u, H = 1u)

• A. \( 1.5 \)
• B. \( 1.064 \)
• C. \( 1.12 \)
• D. \( 0.80 \)

Hint:

- Molarity \( M \) is calculated as: \[ M = \frac{\text{moles of solute}}{\text{volume of solution in liters}} \] - To find the mass of solute, use: \[ \text{Mass} = \left(\frac{\text{mass percent}}{100}\right) \times \text{total mass of solution} \]

Answer 1

The Correct Option is B

Step 1: Calculate the mass of urea \[ \text{Mass of urea} = \frac{6}{100} \times 1000 = 60 \, \text{g} \] Step 2: Determine the molar mass of urea (CO(NH\(_2\))\(_2\)) \[ \text{Molar mass of urea} = 12 + (2 \times 14) + (1 \times 4) + (16 \times 1) = 60 \, \text{g/mol} \] Step 3: Calculate the number of moles of urea \[ \text{Moles of urea} = \frac{\text{Mass of urea}}{\text{Molar mass of urea}} = \frac{60}{60} = 1 \, \text{mol} \] Step 4: Calculate the volume of solution \[ \text{Total weight of solution} = 1000 \, \text{g}, \quad \text{Density of solution} = 1 \, \text{g/mL} \] \[ \text{Volume of solution} = \frac{1000}{1} = 1000 \, \text{mL} = 1 \, \text{L} \] Step 5: Calculate the molarity \[ \text{Molarity} = \frac{\text{Moles of solute}}{\text{Volume of solution in L}} = \frac{1}{0.94} \approx 1.064 \, \text{mol/L} \] Thus, the correct answer is \(\mathbf{1.064 \, mol/L}\).