Mass of NaCl residue: 0.523 g Molar Mass of NaCl: 58.44 grams/mol Mass of NaCl solution: 10.00 g Volume of NaCl solution: 10.00 mL a. Calculate the mass % (or mass fraction) of the NaCl solution given above. Mass % (mass of solute/mass of solution) * 100 b. Calculate the molarity (M) for the NaCl solution given above. Molarity = (moles of solute)/(liters of solution)
The Correct Answer and Explanation is:
Given:
- Mass of NaCl (solute) = 0.523 g
- Mass of NaCl solution = 10.00 g
- Volume of NaCl solution = 10.00 mL = 0.01000 L
- Molar mass of NaCl = 58.44 g/mol
a. Mass percent (%):
Formula: Mass percent=(Mass of soluteMass of solution)×100\text{Mass percent} = \left( \frac{\text{Mass of solute}}{\text{Mass of solution}} \right) \times 100Mass percent=(Mass of solutionMass of solute)×100 =(0.523 g10.00 g)×100=5.23%= \left( \frac{0.523\ \text{g}}{10.00\ \text{g}} \right) \times 100 = 5.23\%=(10.00 g0.523 g)×100=5.23%
b. Molarity (M):
Step 1: Calculate moles of NaCl: Moles of NaCl=Mass of NaClMolar mass of NaCl=0.523 g58.44 g/mol≈0.00895 mol\text{Moles of NaCl} = \frac{\text{Mass of NaCl}}{\text{Molar mass of NaCl}} = \frac{0.523\ \text{g}}{58.44\ \text{g/mol}} \approx 0.00895\ \text{mol}Moles of NaCl=Molar mass of NaClMass of NaCl=58.44 g/mol0.523 g≈0.00895 mol
Step 2: Use the molarity formula: Molarity (M)=Moles of soluteLiters of solution=0.00895 mol0.01000 L=0.895 M\text{Molarity (M)} = \frac{\text{Moles of solute}}{\text{Liters of solution}} = \frac{0.00895\ \text{mol}}{0.01000\ \text{L}} = 0.895\ \text{M}Molarity (M)=Liters of solutionMoles of solute=0.01000 L0.00895 mol=0.895 M
Final Answers:
- Mass percent: 5.23%
- Molarity: 0.895 M
Explanation
When working with solutions, two important ways to describe concentration are mass percent and molarity. Mass percent tells you how much of the solution’s total mass is made up of the solute—in this case, sodium chloride (NaCl). You calculate it by dividing the mass of the solute (0.523 grams of NaCl) by the total mass of the solution (10.00 grams), then multiplying by 100. This gives a result of 5.23%, which means 5.23% of the solution’s mass is pure NaCl.
On the other hand, molarity focuses on the number of moles of solute per liter of solution. To compute molarity, you must first convert the given mass of NaCl into moles. Since the molar mass of NaCl is 58.44 grams per mole, you divide 0.523 grams by this value to get approximately 0.00895 moles. Next, you take into account the volume of the solution, which is 10.00 milliliters. Converting milliliters to liters is essential because molarity uses liters in its definition. So, 10.00 milliliters equals 0.01000 liters. Finally, dividing the moles by the volume in liters gives a molarity of 0.895 M.
Understanding both measurements provides flexibility. Mass percent is often used in fields like food science and pharmacology, where weights are easier to measure. Molarity is more common in chemistry labs, especially when working with reactions that require precise mole ratios. Both measurements help scientists and students prepare and analyze solutions with accuracy.
