The chemical 5 amino 23 dihydro 14 phthalazinedione better...

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. 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 solution. What is the concentration of the luminol stock solution? Show all work.

Transcript

00:01 Okay, we've got a solution of luminal, and we've been told that luminal has molecular weight of 177 grams per mole.

00:15 We've also been told that we've added 20 grams of luminal into 0 .75 .75 millimeters of water.

00:39 Okay.

00:39 And the question is, what's the concentration? okay.

00:47 Well, what are the units of concentration? what do we need when we're doing a problem that has to do with concentration? we typically want moles per liter.

00:56 That's what we're looking for.

00:58 We're looking for moles of luminal and liters of solution.

01:03 Right now, we've got grams of luminal and milliliters of solution.

01:07 So really what we're going to have to do is unit analysis, right? we're going to have to do some multiplications of conversion factors that will convert grams.

01:22 We need grams to turn into moles, and we need milliliters to turn into liters.

01:27 That's what we're trying to do.

01:31 So how can we turn grams into moles? well, we used to molecular weight.

01:36 Molecular weight relates grams to molds, right? so if we have grams right there in our numerator of our fraction, if we want to turn that into, the moles, we need to multiply it by the molecular weight in such a way that grams is on the bottom and moles is on the top.

01:55 That way the grams will cancel.

01:57 Okay.

01:58 That's just a little bit of algebra.

02:00 And then we need to make sure, okay, what's the relationship between grams and moles of luminal? there's 177 grams in one mole of luminal.

02:12 Okay.

02:12 So this multiplication will convert grams of luminal to moles of luminal...