A compound composed of only carbon and hydrogen is 7.743%
hydroge...

Question

A compound composed of only carbon and hydrogen is 7.743%
hydrogen by mass. Propose a Lewis structure for the compound.

Accepted Answer

Welcome back everyone in this example. We need to propose a lewis structure for a compound that is composed of only silicon and bromine. And we're told the percent by massive bombing being 91.9%. So what our first step is is because we know that this compound consists of both silicon S. I. And bromine. We want to find our percent by mass of our silicon. And so we're going to recognize that because they tell us our percent by mass of bromine. We will just have to take the difference by assuming we have 100 g total of our sample. And with this assumption we can say that our 100 g of our sample Subtracted from our percent by massive roaming. So we can just understand this as 91.9 g Since it's out of our 100 g sample. So we would say -1 or -91. So we'll circle this in purple minus 91.9 g of our bromine. And this is going to give us our percent by massive silicon equal to a value 8.1 g of our silicon. So that would be 8.1 by mass of our silicon. Now that we know this, we can use this information to get molds of each of our components silicon and roaming and that's going to help us determine our unknown molecular formula which will ultimately lead us into getting our lewis structure because our molecular formula will allow us to know how many atoms we have of silicon and bromine. So going into our calculation for the molds of each of our re agents, Let's begin with our moles of blooming. So we know our massive booming in our sample as as we stated 91.9 g of grooming. And We're going to do a bit of stock geometry here by recalling our molar mass from our periodic table of elements. We find bromine in group seven a on our periodic tables which corresponds to its smaller mass of 79.9 g per mole of bromine. And as you can see our units of grams are aligned accordingly so we can cancel our units grams of roaming. We're left with moles and this is going to give us our molds of romaine equal to a value of 1.15 moles of grooming. So following the same process for our silicon, we determined above that we have a percent by mass of silicon equal to 8.1 g of silicon. Multiplying by its molar mass and geometry step, we find silicon on our periodic tables and group four a corresponding to its molar mass of 28.1 g for one mole of silicon. So this allows us again to cancel our units of grams. We're left with units of moles and this gives us a mole value of silicon equal to 0.288 moles of silicon. So recall that to get our molecular formula, we want to divide by the smallest number of moles and as we can see 0.288 moles is less than 1.15. So we're gonna divide both of these quantities by our smallest number of moles. So by 0.288 moles. And in doing so we're going to get our quantities of each of our atoms here as the following values. So for browning we get a value of 3.99 and for silicon of course we get a value of one because it's just being divided by itself. So we can round our quantity for browning 3.99 to a single digit hole number as four moles. Or the ratio of four atoms of our booming where we have one atom of silicon. And so now this determines therefore our molecular formula. So we can say that our molecular formula now of our compound is going to be S I. B. R four. Because we recognize that these whole numbers that we came up with become our sub scripts. We don't really have to write our subscript of one for silicon and we should recognize this molecular formula as silicon petro bromide. So now that we know are molecular formula, it should be simple for us to get our lewis structure. So recall that the first step before drawing your lewis structure is to calculate total valence electrons and we can accurately do so now that we know the number of atoms of each of our re agents here. So let's scroll down for more room. So to calculate our total valence, we have just one atom of silicon. And as we stated earlier, silicon is found in Group Four. A recall that our group number corresponds to our valence electrons. So we have four valence electrons contributed from our silicon atom. And then added on to this, we have our bromine atom which as we stated earlier we recall is located in group seven a corresponding to seven valence electrons. However, we recognize that we because we have that subscript of four in our molecular formula, that tells us that we have four atoms of bromine. And so we're going to multiply our seven valence electrons by our four atoms of bromine. And this is going to when we get the sum here, We're going to yield a total of 32 vans electrons total. And we will use this information to draw our structure. So because our molecular formula tells us we have just one atom of silicon, we know that silicon is going to be in the center of our lewis structure and we're going to surround it by four bromine atoms. So 1234 according to our molecular formula. And what we can do is add in our valence electrons. So we know each atom of Wyoming because Wyoming is in Group seven A should have seven valence electrons. So we have 1234 pairing them up now. 56 and seven for the next roaming adam we have the same thing so 1234 pairing them up 56 and seven. Following the same steps 1234 pairing the valence electrons up 56 and seven. And for our last bromine atom 1234 pairing them up 56 and seven. And then Because we recognize that each blooming atom has seven valence electrons and we have four booming atoms. So that gives us seven times four which we recall equals the value 28. We have used up a total of 28 of our valence electrons out of our 32 total. So we would subtract 28 and that's going to leave us with four valence electrons left. Which we can use because we recall that Silicon as we stated as in Group four A. And has four valence electrons. So filling those valence electrons in around silicon, we have 123 and four. And we know that we need to form our molecules. So we need to use our valence electrons on bromine and silicon to make four covalin bonds. Because silicon and bromine we recall are both nonmetal atoms. So we recognize them as having a co violent relationship sharing these electrons in the bond. And so we would take these each of these two valence electrons from one from roman and one from silicon to form our first double or single bond here. We form a second single bond here by connecting these two valence electrons a third and then 1/4. And this gives us our bondage structure of silicon tetra bromide. According to our molecular formula, we have a net charge of zero. So all of the formal charges in our structure also have a net charge of zero for each of our atoms here. And that is true because we know that each of our atoms silicon and of women have their preference of valence electrons either being set as lone pairs or shared in a bond with each atom. And so what that means is that this is going to be our proposed lewis structure for silicon tetra bromide. And so this entire structure here is going to be our final answer, which is going to correspond to choice D in the multiple choice I hope everything I reviewed was clear. If you have any questions, leave them down below and I'll see everyone in the next practice video

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