## Audio Visual Topic Video Question

Eileen Quach Dis 2A
Posts: 109
Joined: Wed Sep 30, 2020 9:49 pm

### Audio Visual Topic Video Question

Hi!
For the audio visual topic video on empirical and molecular formulas, Professor Lavelle said in his example after he calculated the mass percent composition that the easiest procedure when determining the mass of each element is to imagine that the sample mass is 100 grams. I was wondering why does this method always work no matter how much the sample mass is? Thanks!

Arielle Sass 2A
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Joined: Wed Sep 30, 2020 9:43 pm
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### Re: Audio Visual Topic Video Question

It works because no matter how large or small of a sample of something you take, the percent composition of its components will always be the same. And what matters when finding empirical formula is the ratio of each component, not the actual amount of each element in the sample. So if you have the percent composition of each element, which is already out of 100, then you can just “pretend” to have a sample of 100 grams out of convenience. Even if you pretended to have a sample of 38.52 grams the percent composition would be the same so when you simplified all the values down to ratios, you’d get the same answer. 100 grams is just easier and quicker :)

Jaclyn Dang 3B
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Joined: Wed Sep 30, 2020 10:02 pm

### Re: Audio Visual Topic Video Question

I think its because he does it to every single one? but I'm not too sure. I know that when you change something for one part of a question you have to do the same changes across the whole question so that it isnt off. For example in math if you have an equation and you multiply one side by 2 you have to do the same to the other side. I think this concept may apply here as well

Yuelai Feng 3E
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### Re: Audio Visual Topic Video Question

Hi! That's because, after you convert the mass of each element into the number of moles, you still need to divide each mole number by the smallest one to get the empirical formula. Only the molar ratio matters.

For example, if you imagine there are 100 grams of sample, and you get 1mol C + 4mol H. The empirical formula is CH4.
Alternatively, if you imagine the sample mass is 200 grams, you'll get 2mol C + 8mol H. Then you divide both by 2 and still get CH4.

Tiao Tan 3C
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Joined: Wed Sep 30, 2020 9:59 pm

### Re: Audio Visual Topic Video Question

Hi! we only care about molar ration when dealing with empirical formulas. The mass percent composition can be seen as the mass ratio of different elements in a sample. The ratio won't be affected by sample mass.
And I want to add that when we want to find the molecular formula when given an empirical formula, mass would matter.

Yuelai Feng 3E
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### Re: Audio Visual Topic Video Question

Tiao Tan 3K wrote:Hi! we only care about molar ration when dealing with empirical formulas. The mass percent composition can be seen as the mass ratio of different elements in a sample. The ratio won't be affected by sample mass.
And I want to add that when we want to find the molecular formula when given an empirical formula, mass would matter.

Good point! Actually when finding the molecular formula, what matters is the molar mass of the molecule, not the mass of the sample.

Morgan Gee 3B
Posts: 60
Joined: Wed Sep 30, 2020 9:48 pm

### Re: Audio Visual Topic Video Question

Dr. Lavelle uses 100 grams because the questions very rarely gives the mass of the unknown compound provided. That means we are at liberty to come up with any arbitrary mass of compound to work with. Because the values are given as percentages, it's a lot easier to just use 100 grams. This is because you do the percentage of element times 100 to get the grams of a certain element. This allows us to forego this thought process and change the percent sign to grams. It just makes everything easier to calculate!