Chemistry 110 Concentration

Question

A student placed 12.5 g of glucose (C6H12O6) in a volumetric flask, added enough water to dissolve the glucose by swirling, then carefully added additional water until the 100. mL mark on the neck of the flask was reached. The flask was then shaken until the solution was uniform. A 25.0 mL sample of this glucose solution was diluted to 0.500 L. How many grams of glucose are in 100. mL of the final solution?

Answer 1

In this problem, an initial solution is made and then from that a more dilute solution is made, so you have 2 concentrations to calculate

You need to determine the molar mass for glucose [(12g x 6) + (1g x 12) + (16g x6)]

Use the molar mass to convert 12.5 g glucose to moles by dimensional analysis:

12.5 g glucose x ^{1 mol glucose} / _{??g glucose} = mol glucose (??g represents the molar mass)

Use the mol glucose and the solution volume (in L) to determine the molarity (concentration) of your initial solution: Molarity = ^{moles glucose} / _{L solution}

To calculate the concentration of the dilute solution, use M₁V₁ = M₂V₂ where M₁ is the molar concentration of the original solution, V₁ is the volume of that original solution you withdraw, 0.25 L (must be in L rather than mL), and V₂ is the final volume of the dilute solution, 0.5 L. Solve for M₂, the concentration of the dilute solution.

For the final dilute solution you are asked to determine the mass of glucose in 100 mL, so you will again use the relationship between molar concentration and volume of solution (L) to determine moles of glucose:

Molarity = ^{moles glucose} / _{L solution} or moles glucose = Molarity, M₂ x 0.100 L.

Once you have this last calculation, convert moles glucose back to mass using the molar mass of glucose that you determined at the beginning of this solution.

moles glucose x ^{molar mass glucose} / _{1 mol glucose} = mass of glucose in the 100 mL of the dilute solution.