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# What information do we need to determine the molecular formula of a compound if we know only the empirical formula?

## If we know the composition of a compound in terms of the masses (or mass percentages) of theelements present, we can calculate the empirical formula but not the molecular formula. Toobtain the molecular formula we must know the molar mass.

Composition

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##### Top Chemistry 101 Educators ##### Stephanie C.

University of Central Florida  ##### Jake R.

University of Toronto

### Video Transcript

all right. In today's video, we're gonna examine how we go from empirical formula to molecular formula. So, like, what do we need when were given empirical formula to go all the way too much for you? Okay, let's first start off by defining each quantity each quantity. But each term imperial formula is the smallest ratio of elements in a compound. All right, so it's always the smallest. Entered your value of ratios, off elements, tours one each, one another. Okay, so it's like we can't divide them. But anything further for it to remain home numbers, it's OK, while molecular formula is the actual Russia off elements to one another in a compound. So the one that if you were you zoom in under like an election microscope would weep, we would see would be the Maliki form formula. How do you spell so the actual ratio of elements, you know, and it can be like multiples. But that's still all right, because it's the actual pre shell. Okay, So given impure formula, what do we need then to find molecular formula? Right. So given this formula, we know the ratio in which each element is to one another. So let's say that the arbitrary compound organic compound see carbon hydrogen oxygen and there were just say, 12 three. What would we need to have been taking to the molecular form? Will see. This is the smallest ratio. We can't let this by anything else. So what would give us? What wanted you do we know would give us what parameter would give us information about the elements themselves. That's what we need. We need to know information about each element individually. So let's think about, um, Mats. Maybe it was always, you know, something that always pops up in people's heads. What would. But there we had mass and empirical formula with that, Then give this molecular formula Well, what would what could we do with that? So with this, we have mass percent mass percent with another mass value that can't give this molecular formula. And that's not possible? No. Well, if they gave us Smalls, would moles work? Well, this, um, empirical formula over here sore kind of already giving away the molar ratio of each compound is to each other because the mass percent for an empirical formula is the same for that of a molecular form humor. They have the same as percent. So if we had one mole of this of this compound, we would have one mole of carpet to multiply gin, three moles of oxygen. Just, you know, by working out it, knowing the mass percent. So this cannot give us information about the molecular formula. So what else do you know? What about molecular formula or nothing with me, But about Moeller, Mass that this primary will give us information about the compound it would give us. It would tell us elemental value. So we know that carbon always be 12. Argenis, where's from? Old Harder is one, and oxygen is around 16. Graham formal. So then giving it us more mouths. You would know then how many? We could work with the division and know how many Hodgins we have. How many, um, multiples of 12. That we have harmony, Multiple of 16. We have. So we can work with this Knowing master sent them more mass. That could then give us the molar issue. The actual mole ratio of each compound is each other in the compound. So Moeller mass would work. Yeah, McMaster University

#### Topics

Composition

##### Top Chemistry 101 Educators ##### Stephanie C.

University of Central Florida  ##### Jake R.

University of Toronto