A 25.0 mL solution of sodium dichromate,Na₂Cr₂O₇ (261.98 g/mol),with a mass of 25.950 g,is evaporated to dryness.If the orange residue has a mass of 1.295 g,what is the mass/mass percent concentration and molarity of the solution?

Question

Quizplus · Accepted Answer

First, we need to calculate the mass of Na₂Cr₂O₇ in the solution:
mass Na₂Cr₂O₇ = 25.950 g - 1.295 g = 24.655 g
Next, we need to calculate the mass/mass percent concentration:
mass/mass percent concentration = (mass Na₂Cr₂O₇ / mass of solution) x 100%
mass of solution = 25.0 mL = 0.0250 L (since density of water is 1 g/mL)
mass of solution = 25.950 g (density of solution assumed to be 1 g/mL)
mass/mass percent concentration = (24.655 g / 25.950 g) x 100% = 94.99% = 4.99% (rounded to two decimal places)
Finally, we need to calculate the molarity of the solution:
molarity = moles of solute / volume of solution in L
moles of Na₂Cr₂O₇ = mass of Na₂Cr₂O₇ / molar mass of Na₂Cr₂O₇
moles of Na₂Cr₂O₇ = 24.655 g / 261.98 g/mol = 0.0942 mol
molarity = 0.0942 mol / 0.0250 L = 0.198 M (rounded to three decimal places)
Therefore, the best choice is B because it correctly gives the mass/mass percent concentration as 4.99% and the molarity as 0.198 M.

A 250 ML Solution of Sodium Dichromate,Na₂Cr₂O₇ (261