Question

The chemical 5-amino-2,3-dihydro-1,4-phthalazinedione, better known as luminol, is used by forensic scientists in analyzing crime scenes for the presence of washed-away blood. Luminol is so sensitive that it can detect blood that has been diluted 10,000 times. A basic solution of luminol is often sprayed onto surfaces that are suspected of containing minute amounts of blood.

Luminol has a molecular weight of 177 g/mol.

Part A:

The forensic technician at a crime scene has just prepared a luminol stock solution by adding 11.0 g of luminol into a total volume of 75.0 mL of H2O.

What is the molarity of the stock solution of luminol?

Part B:

Before investigating the scene, the technician must dilute the luminol solution to a concentration of 6.00×10⁻²M . The diluted solution is then placed in a spray bottle for application on the desired surfaces.

How many moles of luminol are present in 2.00 L of the diluted spray?

Part C: What volume of the stock solution (Part A) would contain the number of moles present in the diluted solution (Part B)?

Answer 1

Part A:

Molecular weight of Luminol = 177 g/mol

Weight of Luminol taken = 11.0 g

Volume of the solution = 75.0 mL = 0.075 L

Moles of Luminol taken = 11.0 g / 177 gmol^-1 = 0.062 moles

Molarity of the solution = no. of moles / volume of the solution = 0.062 moles / 0.075 L = 0.827 M

so the molarity of the Luminol solution is 0.827 M

Part B:

Required / Diluted concentration of Luminol = 6.00×10⁻² M

Volume of the dilute solution = 2.00 L

No. of moles of Luminol present in 2.00 L of dilute solution = 6.00×10⁻² M x 2.00 L = 0.12 mol

Part C:

1: Stock

2: Dilute

M₁V₁ = M₂V₂

0.827 M x V₁ = 6.00×10⁻² M x 2 L

V1 = 0.12 / 0.827 = 0.145 L = 145 mL