Fundamentals Section M Homework M19

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Victoria Zheng--2F
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Fundamentals Section M Homework M19

Postby Victoria Zheng--2F » Wed Oct 02, 2019 8:20 pm

M19: A stimulant in coffee and tea is caffeine, a substance of molar mass 194g.mol-1. When 0.376g of caffeine was burned, 0.682g of carbon dioxide, 0.174g of water, and 0.110g of nitrogen were formed. Determine the empirical and molecular formulas of caffeine, and write the equation for its combustion.

My question for this problem is since the problem gave the masses of compounds, do I still follow the normal way to find the empirical and molecular formula?

Michelle Le 1J
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Joined: Fri Aug 09, 2019 12:16 am

Re: Fundamentals Section M Homework M19

Postby Michelle Le 1J » Wed Oct 02, 2019 8:52 pm

I believe you would have to first do the mass % of each of them and then convert the grams of them into moles in order to find the empirical and molecular formula of caffeine.

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Re: Fundamentals Section M Homework M19

Postby san_2F » Wed Oct 02, 2019 9:08 pm

First you would find the mass percent composition of each element. Imagine that the sample is 100g and then assume that the percents are just grams. Find how many moles of each element there are and divide all of them by the smallest value. If these values are not whole numbers, multiply them by a number to make sure all the numbers are integers. This is your empirical formula. Calculate the molar mass of this formula and see if it matches the molar mass of the substance and if it doesn’t, divide the actual moral mass by the molar mass to find the multiple to multiply each of the elements by.

Hannah Lee 2F
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Re: Fundamentals Section M Homework M19

Postby Hannah Lee 2F » Wed Oct 02, 2019 9:34 pm

In this problem, they don't give you the mass of each element in a compound, but rather the masses of the compounds that were produced as a result of the burning of caffeine. Because of this, you have to take an extra step and "break down" the compounds into their component elements in order to determine the empirical and molecular formula.

First, convert the masses of each compound (CO2, H2O, N2) into moles. For instance, H2O would be 0.174 g H2O divided by 18.016 g/mol = 0.009658 mol H2O.

Second, use mole ratios to convert the moles of the compounds into moles of C, H, and N only, which make up caffeine. For instance, to convert 0.009658 mol H2O into mol H, you would do: 0.009658 mol H2O (2 mol H / 1 mol H2O) = 0.01932 mol H. You can't find the moles of O yet since it's found in both CO2 and H2O.

Then convert the moles of C, H, and N into grams. To find O, you would need to subtract the total masses of C, H, and N from 0.376 which is the mass of caffeine burned; the amount left should be the mass of O. After finding the mass of O, convert it into moles.

After finding the moles of C, H, N, and O, use the standard process for determining the empirical formula by dividing the moles of C, H, N, and O by the smallest number of moles, which in this case is mol O.

Lastly, compare the mass given to you in the problem to the molar mass of the empirical formula you produce. If it's not the same, divide the given molar mass by the empirical molar mass or vice versa and adjust the empirical formula accordingly to get the molecular formula.

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