HW1 F19

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Jeremiah Hutauruk
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Joined: Fri Sep 28, 2018 12:28 am

HW1 F19

Postby Jeremiah Hutauruk » Sun Oct 07, 2018 5:01 pm

The question is "F.19 Caffeine, a stimulant in coffee and tea, has a molar mass of 194.19 g mol 1 and a mass percentage composition of 49.48% C, 5.19% H, 28.85% N, and 16.48% O. What is the molecular formula of caffeine?"

What is the first step of this problem and why is the molar mass needed?

ellienelson4F
Posts: 7
Joined: Fri Sep 28, 2018 12:27 am

Re: HW1 F19

Postby ellienelson4F » Sun Oct 07, 2018 5:52 pm

The first step of this problem is to assume you have 100g, and then convert each element to moles by dividing whatever the percent (now, grams because you have 100g) was, for example 49.48g C/12.0107g/mol. Once you have converted each element to moles, divide by the smallest number of moles to get whole numbers. These whole numbers will serve as your ratios for the empirical formula. In order to get to the molecular formula, you must use the molar mass. The molecular formula is the actual number of each element you have, while the empirical formula is the the formula for the specific compound and is always the simplest ratio.

Lily Smith 4C
Posts: 16
Joined: Fri Sep 28, 2018 12:27 am

Re: HW1 F19

Postby Lily Smith 4C » Sun Oct 07, 2018 5:59 pm

Think of the total mass as 100 grams. Hydrogen would then be 5.19g of the 100g total, Nitrogen would be 28.85g of the total 100, Carbon would be 49.48g of the 100 and Oxygen would be 16.48g of the 100g.

Then divide each element by it's molar mass. The element with the smallest answer is what you will divide each by.
This is the empirical formula. It should be whole numbers. If it is within .10, I think, of a whole number round up or down to the closest whole number

From there you would use the molar mass stated in the question to find the molecular formula

Multiple the amount of each element in its empirical formula by its molar mass. Add these values of each element up and find the total mass.

Divide the mass stated in the problem by the one you find and see the ratio

That ratio is what you multiple the empirical form of the elements by.

This will be your answer for the molecular formula.


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