Question

What is the final concentration of methyl red if:

A 0.05% solution of methyl red is prepared by dissolving 0.025 g in 20 mL of 95% ethanol in a 50- mL volumetric flask. Water is then added to within a few mL of the mark. ~0.1 M NaOH is added dropwise until all the solid dissolves and then diluted to the mark. 20 mL of this solution is then transfered into 50 mL of 95% ethanol in a 200 mL volumetric flask. This is then diluted to the mark with water.

Answer 1

We start with a 0.05% methyl red solution and dilute to prepare a final solution having volume 200 mL. 20 mL of 0.05% methyl red solution was used to prepare the dilute solution. We are required to find the concentration of methyl red in the final solution.

Use the dilution equation:

C₁*V₁ = C₂*V₂ where C₁ = concentration of stock methyl red = 0.05%; V₁= volume of stock methyl red solution taken = 20 mL and V₂ = final volume of the dilute solution = 200 mL. Therefore,

(0.05%)*(20 mL) = C₂*(200 mL)

====> C₂ = (0.05%*20)/(200) = 0.005%

The final concentration of the dilute methyl red solution is 0.005% (ans).