The forensic technician at a crime scene has just prepared a luminol stock solution by adding 20.0 g of luminol to H2O creating a solution with a total volume of 75.0 mL.
1. What is the molarity of the stock solution of luminol?

The molarity of the stock solution of luminol is 1.51

Explanation:

Molarity (M) is the number of moles of solute that are dissolved in a given volume. This is expressed as follows.:

Molarity is expressed in units .

So, to calculate the molarity of luminol, you must know the amount of moles that are 20 grams of the compound. For that you must know the molar mass of luminol.

Luminol is a compound whose formula is C₈H₇N₃O₂. In the Periodic Table you can see that the atomic mass of the elements that make up the compound is:

Then the molar mass of luminol is calculated taking into account the relative abundance of each element as follows:

C₈H₇N₃O₂= 8*12 g/mol + 7*1 g/mol +3*14 g/ol + 2*16 g/mol= 177 g/mol

Then, to calculate the moles that represent 20 g of luminol and taking into account the calculated molar mass, a rule of three is applied as follows: if 177 grams is 1 mole of the compound, 20 grams how many moles are?

moles=0.11299

So, being 75 ml = 0.075 L (1 L = 1000 mL or 1 mL = 0.001 L), the molarity is:

molarity≅ 1.51

The molarity of the stock solution of luminol is 1.51