Molecular Formulas

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duverge_maciel
Posts: 21
Joined: Fri Sep 28, 2018 12:16 am

Molecular Formulas

Postby duverge_maciel » Thu Oct 04, 2018 4:29 pm

On problem F9, I understand how to find the atom ratios and the limiting reactants, but what I don't understand is how to get the molecular formula of vanillin based on the calculations. How do I figure out that the molecular formula is C8H8O3 based on my calculations?

Kate_Santoso_4F
Posts: 72
Joined: Fri Sep 28, 2018 12:29 am

Re: Molecular Formulas

Postby Kate_Santoso_4F » Thu Oct 04, 2018 4:40 pm

For problem F9, they are specifically asking you "in what atom ratios are the atoms present in vanillin." They are simply asking for the empirical formula of vanillin. Since the molecular weight was not given, you cannot determine what the molecular formula is. Using the mass percentage compositions given, convert the percentages to grams by imagining a sample mass of 100g. Convert the grams to moles and divide by the smallest number of moles, which will give you a ratio of 2.67:2.67:1 for C:H:O. To get a whole number of atoms, multiply the entire ratio by 3 to give you a ratio of 8:8:3, which will give you an empirical formula of C8H8O3.

aaronharouni
Posts: 72
Joined: Fri Sep 28, 2018 12:15 am

Re: Molecular Formulas

Postby aaronharouni » Thu Oct 04, 2018 4:46 pm

The molecular formula cannot be found in this question. Usually, the molecular mass is needed and the problem provides you with this information. In this question, C8H8O3 is the empirical formula found through the mass percentages. To find the molecular formula, you would need the MM.

Rebecca Altshuler 1D
Posts: 61
Joined: Fri Sep 28, 2018 12:15 am

Re: Molecular Formulas

Postby Rebecca Altshuler 1D » Thu Oct 04, 2018 4:47 pm

For this problem, at least in my 6th edition textbook, you only need to provide the ratio of the atoms, which essentially means the empirical formula. In order to calculate the molecular formula, we would need the molar mass of the compound, which is not given.

Germar G 4F
Posts: 38
Joined: Fri Sep 28, 2018 12:28 am

Re: Molecular Formulas

Postby Germar G 4F » Thu Oct 04, 2018 4:53 pm

This problem isn't asking you for the molecular formula, it's asking for the empirical formula. Note that they ask "In what atom ratios are the atoms present in vanillin?" implying that they are not looking for the exact amount of atoms in vanillin, but just the ratios. There is also no given molar mass, so it would be impossible to calculate/verify the empirical formula. Follow the steps for finding the empirical formula, and you should get the answer. You should have to multiply by 3 at the end to obtain whole numbers and you should end up with C8H8O3.
Last edited by Germar G 4F on Thu Oct 04, 2018 10:04 pm, edited 2 times in total.

Rhea Churi 4K
Posts: 62
Joined: Fri Sep 28, 2018 12:28 am

Re: Molecular Formulas

Postby Rhea Churi 4K » Thu Oct 04, 2018 5:19 pm

Because it's asking you to find the atom ratios, you're basically trying to find the empirical formula. You would assume that you have 100 g of Vanillin so you can convert each percent composition to grams, giving you the mass. So, 63.15% C would be 63.15 g C, and so on and you would convert it to moles. Once you do that to all of them, divide each by the smallest number of moles, and multiply all by a number to reach that smallest whole number ratio between the atoms in your molecule. From that, you should be able to find the formula as C8H8O3.
This isn't in the problem but if you wanted to find the molecular formula, you would need to know the molar mass (g/mol) for the molecular mass and divide by the molar mass of the empirical formula which usually will give you a whole number. For example, if you were to do that equation and get 2 as your answer, you would multiply the whole formula by 2 and get C16H16O6 as your answer.
Hope this helps!


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