100 gram Method?

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Andrew Pfeiffer 2E
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Joined: Sat Sep 28, 2019 12:16 am

100 gram Method?

Postby Andrew Pfeiffer 2E » Thu Oct 03, 2019 6:10 pm

Looking around this forum, I've noticed that a lot of students have referred to and used the "100-gram" trick when calculating empirical formulas. I missed the first lecture because I enrolled recently and I was wondering if someone could explain this method in their own words. Thanks!

Abby Soriano 1J
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Joined: Sat Aug 24, 2019 12:16 am

Re: 100 gram Method?

Postby Abby Soriano 1J » Thu Oct 03, 2019 6:26 pm

The 100 gram trick is used to make calculations involving finding an empirical formula easier! If you're given the percent composition of a sample, you might assume that you would need to work backwards in order to find the original number of grams in order to convert them to moles. However, with the 100 gram trick, if you simply assume that your starting sample was already 100 grams, you can turn the percentages they give you into grams too. Instead of having to find out what 67.45% C, 12.13% H, and 20.42% O is in terms of grams, you can simply assume that there's 67.45g C, 12.13 g H, and 20.42g O, and go straight into converting them into moles.

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Re: 100 gram Method?

Postby Destiny_Ryales_3J » Thu Oct 03, 2019 7:17 pm

Be careful! Do not automatically make this assumption when the question gives you the amount of grams in the sample already, you would need to know the mass percentage composition already.

Trent Yamamoto 2J
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Joined: Sat Aug 17, 2019 12:18 am

Re: 100 gram Method?

Postby Trent Yamamoto 2J » Thu Oct 03, 2019 9:39 pm

The "100 gram" trick makes it really easy when using the mass composition to get to the empirical formula. If the mass composition were given to you in a problem (say, 25% C 50% H and 25% O), it would be very convenient to imagine these percentages out of 100 grams. There would be 25g C, 50g H, and 25g O. From here you would be able to move forward in converting to moles and work towards the empirical formula.

Simply put, imagine the percentages are out of 100g of your sample.

Rida Ismail 2E
Posts: 139
Joined: Sat Sep 07, 2019 12:16 am

Re: 100 gram Method?

Postby Rida Ismail 2E » Thu Oct 03, 2019 9:49 pm

The only reason 100g is used is that it is simple to think of a percentage as a number out of 100. Therefore, you can assume that 1% is equivalent to 1g. If given mass composition, you are able to easily use the percentages given and assume that that percentage was taken from a 100g sample

Nare Nazaryan 1F
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Joined: Fri Aug 09, 2019 12:17 am

Re: 100 gram Method?

Postby Nare Nazaryan 1F » Thu Oct 03, 2019 10:08 pm

If you are given the mass percent composition, it's simple to assume that your sample is 100 grams in order to easily convert the mass percent composition, for that certain element, into the mass of that element in grams. That way you can easily calculate the moles of that element using its mass (g) and molar mass (g/mol). Furthermore, this would help you find the ratios of each element in the compound once you divide your the moles of each element by the least amount of moles you found. You'll finally get your molar ratios (empirical formula). For example, if the question states that your sample has 29.34% of Carbon, 14.12% of Hydrogen, and 56.54% Oxygen, you can assume that if the sample is 100 grams, each percent would be equal to 1 gram of that sample (because it's 100%). Then you would easily go about solving the problem.

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Joined: Sat Sep 07, 2019 12:15 am

Re: 100 gram Method?

Postby 005321227 » Thu Oct 03, 2019 11:12 pm

The 100-gram trick essentially simplifies calculations when given percent compositions of elements in a molecule. What you do, is convert the mass percentages to grams (as if out of 100) and divide by the molar masses in order to find the moles of each element and be able to determine your ratios from there.

Nohemi Garcia 1L
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Joined: Fri Aug 02, 2019 12:15 am

Re: 100 gram Method?

Postby Nohemi Garcia 1L » Thu Oct 03, 2019 11:21 pm

The 100 gram method is especially useful for when the problems don't give a specific mass for a molecule, but it gives the mass compositions. Just be careful when there is a specific mass given for a molecule; the 100 gram method probably won't work.

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Re: 100 gram Method?

Postby Tooba_1A » Fri Oct 04, 2019 12:24 am

Simply put, if the problem doesn't already give you a given mass, then you assume that the percent given is out of 100 grams. This makes the work much easier! Instead of having to go about solving this, you can just assume it's out of 100 and turn the percent into a whole number with less of a hassle. This helps with empirical formulas, etc.

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Joined: Wed Sep 18, 2019 12:19 am

Re: 100 gram Method?

Postby sbottomley3a » Fri Oct 04, 2019 9:12 am

Just be careful and read the problem thoroughly! This trick only works if you are given mass percentage composition but aren't given any specific mass of your sample.

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