The chemical 5-amino-2,3-dihydro-1,4-phthatazenedeone, better known as luminol,

Question

The chemical 5-amino-2,3-dihydro-1,4-phthatazenedeone, 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 x 10^-2 M. 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?

Explanation / Answer

PART A)

Molar mass of luminol: 177 g/mol

Then, 20g were dissolved in 75ml of water.

20g of luminol are 20g/177g/mol = 0.112 moles of luminol in 75ml of water.

molarity is number of moles dissolved in 1000 ml = (1000ml * 0.112moles ) / 75ml = 1.51 M

PART B) The concentration of the dilluted spray is 5.00 x 10-2 M; which means 5.00 x 10-2? moles of luminol dissolved in 1000 ml of water or in 1L of water. Thus , in 2L of solution you have 2*5.00 x 10-2? = 0.1 moles of luminol dissolved

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