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The density of a certain gaseous fluoride of phosphorus is 3.93 g/L at STP. Calculate the molar mass of this fluoride and determine its molecular formula.

$\mathrm{PF}_{3}$

01:45

Aadit S.

02:02

Charotte M.

Chemistry 101

Chapter 9

Gases

Rice University

Drexel University

University of Maryland - University College

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Yeah. All right. So, let's try to find um the formula. And the molar mass of the substance that we know contains phosphorus and floor een. And what we do know is that the density of this substance is 3.93 grams per leader at STP. So STP is standard temperature and pressure 0°C, one atmosphere. Now, this is an important um uh thing to know because there's a shortcut for us to be able to get to our molar mass. Because at STP one mole of any gas doesn't matter what it is. Any gas has a volume Of 22.4 L. So, how are we gonna use that? Well, we know what the definition is of molar mass. It's grams per mole. So, let's use our density. And the fact that 22.4 leaders is one mole at STP to convert two g per mole. So, if we take our 3.93 grams per leaders, that's per one leader. And we multiply that by the fact that we know that there's 22 .4 leaders in every one mole, then that means to our three significant figures that we have because our one leader and our one moller infinite That we have a molar mass of 88.0 g per mole. Mhm. All right. So, we've got molar mass. How are we going to use that to figure out the formula? Well, um what do we know about phosphorous and flooring? Well, the Mass of phosphorus. If you look at your periodic table is 30.97 grams per mole. Floor een Is 19.0 g per mole. So how can we get these two equal 88. Well, um, Let's make this number 23 significant. We know that we're going to have to use only three significant figures with addition and subtraction. So we can say that this is essentially equal to 31.0. And so what we're gonna do with that? Well, if we had one phosphorus at 31, but the total was 88, 88.0 -31.0 is 57 zero. So by using this 31, I've used a phosphorus now Florian is 19 and it turns out that if I use 3 19 that will use all of my mass. So I used a floor een and a floor green and a floor een. So now I've used all 88 g per mole. I've used a phosphorus and three floor Eanes. So we write that is P. F three. So phosphorus try fluoride is the formula of our substance

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