A student prepared a stock solution by dissolving 15.0 g of KOH in enough water to make 150. mL of solution. She then took 15.0 mL of the stock solution and diluted it with enough water to make 65.0 mL of a final solution. What is the concentration of the final solution in mol/L?

• A. 0.450 mol/L
• B. 0.300 mol/L
• C. 0.154 mol/L
• D. 0.075 mol/L

Answer: (C)

Verified step by step guidance

1. First, calculate the molar mass of KOH. Potassium (K) has a molar mass of approximately 39.10 g/mol, oxygen (O) is about 16.00 g/mol, and hydrogen (H) is about 1.01 g/mol. Add these values together to find the molar mass of KOH.
2. Next, determine the number of moles of KOH in the stock solution. Use the formula: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \). Substitute the mass of KOH (15.0 g) and the molar mass calculated in the previous step.
3. Calculate the concentration of the stock solution in mol/L. Use the formula: \( \text{concentration} = \frac{\text{moles}}{\text{volume in liters}} \). Convert the volume of the stock solution from mL to L (150 mL = 0.150 L) and substitute the values.
4. Determine the moles of KOH in the 15.0 mL of stock solution that was taken for dilution. Use the concentration of the stock solution and the volume taken (15.0 mL = 0.015 L) to find the moles using: \( \text{moles} = \text{concentration} \times \text{volume} \).
5. Finally, calculate the concentration of the final solution. Use the formula: \( \text{concentration} = \frac{\text{moles}}{\text{volume in liters}} \). Convert the final solution volume from mL to L (65.0 mL = 0.065 L) and substitute the moles calculated in the previous step.