A chemist prepares a solution of vanadium
III bromide  VBr3  by measuring out 0.75g of VBr3 into a 250.mL volumetric flask and filling to the mark with distilled water.
ÂÂ
Calculate the molarity of
Br− anions in the chemist’s solution.
ÂÂ
Be sure your answer is rounded to the correct number of significant digits.
molL
The Correct Answer and Explanation is:To calculate the molarity of Br⁻ anions in the chemist’s solution, we follow these steps:
Step 1: Determine the molar mass of VBr₃
Vanadium (V): 50.94 g/mol
Bromine (Br): 79.90 g/mol × 3 = 239.70 g/mol
Total molar mass of VBr₃ = 50.94 + 239.70 = 290.64 g/mol
Step 2: Calculate the moles of VBr₃ used
Given mass of VBr₃ = 0.75 g
Moles of VBr₃ = 0.75 g290.64 g/mol=0.00258 mol\frac{0.75\ \text{g}}{290.64\ \text{g/mol}} = 0.00258\ \text{mol}
Step 3: Determine the moles of Br⁻ anions
Each formula unit of VBr₃ contains 3 Br⁻ ions, so: Moles of Br⁻=0.00258 mol VBr₃×3=0.00774 mol Br⁻\text{Moles of Br⁻} = 0.00258\ \text{mol VBr₃} × 3 = 0.00774\ \text{mol Br⁻}
Step 4: Calculate the molarity of Br⁻
Volume of solution = 250. mL = 0.250 L Molarity (M)=0.00774 mol Br⁻0.250 L=0.03096 mol/L\text{Molarity (M)} = \frac{0.00774\ \text{mol Br⁻}}{0.250\ \text{L}} = 0.03096\ \text{mol/L}
Rounded to 3 significant digits (since 0.75 g has 3 sig. figs):
✅ Final Answer: 0.03096 mol/L Br⁻
💡 Explanation
In this problem, we are asked to find the molarity of bromide ions (Br⁻) in a solution made by dissolving a specific mass of vanadium(III) bromide (VBr₃) in water. Molarity refers to the number of moles of a solute per liter of solution, expressed in mol/L. Since VBr₃ is an ionic compound, it dissociates completely in water into vanadium ions and bromide ions. Specifically, each mole of VBr₃ releases three moles of Br⁻ ions because its chemical formula contains three bromine atoms.
The process starts by determining the molar mass of VBr₃, which is the sum of the atomic masses of vanadium and three bromines. After that, we calculate the moles of VBr₃ by dividing the mass used (0.75 g) by the molar mass. From there, we multiply by 3 to find the moles of bromide ions, because each VBr₃ unit produces 3 Br⁻.
Finally, the moles of Br⁻ are divided by the total solution volume in liters (250 mL = 0.250 L) to obtain the molarity of Br⁻ ions. The answer is rounded to three significant digits, matching the precision of the given mass (0.75 g).
This calculation is important in chemistry, particularly in preparing solutions for reactions, titrations, or analytical procedures where knowing ion concentration is essential.
