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 20.0 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 5.00×10⁻²M . 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)?

Express your answer with the appropriate units.

Answer 1

Part A:

Mass of luminol in stock solution is : 20.0 grams

Moles of Luminol in stock: Mass/ Molar mass = 20.0 g/ (177 g/mole) = 0.113 mole

Volume of stock solution = 75 mL = 0.075 L

Molarity of the stock solution = 0.113 mole/ 0.075 L = 1.5 M

Part B:

Moles of Luminol present = Volume in Liters * Moalrity = 2 L * 5.00×10⁻² M = 0.1 mole

Part C:

The volume = moles/ concentration = 0.1 mole/ 1.5 M = 0.067 Liters