Explain how you would find the empirical and molecular formulas of an unknown compound if you already know the kinds of atoms in the compound. (You may use an example.)
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Assuming that you know the percentage of each atom in an unknown compound or the mass of each element in a given sample of that compound, you can follow this general procedure to know the empirical formula:
1.) If you are given percentages, consider the mass of a sample of the compound to be 100g (choose this value for simplicity). Then the percentages given will represent the mass of each element in that 100g sample.
2.) List the masses of each element and find the number of moles of each of these masses by dividing by the molar mass of the element.
3.) Of the values obtained for the number of moles, see which is smallest and then divide each number of moles by that value.
4.) The approximate answers after the division represent the number of each element in the empirical formula of the compound. If you have decimal values, multiply by an appropriate number to make the values whole since an empirical formula must contain whole numbers.
I will use the following example to demostrate the procedure:
Example: NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O. Calculate the empirical formula of NutraSweet.
1.) Let the mass of the substance be 100g. Then there are:
57.14 grams Carbon in this sample
6.16 grams Hydrogen in this sample
9.52 grams Nitrogen in this sample
27.18 grams Oxygen in this sample
2.) Find the number of moles of each element by dividing by the molar mass:
Number of moles of Carbon = (Mass of Carbon / Molar Mass of Carbon) = (57.14 / 12) = 4.762 moles
Number of moles of Hydrogen = (Mass of Hydrogen / Molar Mass of Hydrogen) = (6.16 / 1) = 6.16 moles
Number of moles of Nitrogen = (Mass of Nitrogen / Molar Mass of Nitrogen) = (9.52 / 14) = 0.68 moles
1.) If you are given percentages, consider the mass of a sample of the compound to be 100g (choose this value for simplicity). Then the percentages given will represent the mass of each element in that 100g sample.
2.) List the masses of each element and find the number of moles of each of these masses by dividing by the molar mass of the element.
3.) Of the values obtained for the number of moles, see which is smallest and then divide each number of moles by that value.
4.) The approximate answers after the division represent the number of each element in the empirical formula of the compound. If you have decimal values, multiply by an appropriate number to make the values whole since an empirical formula must contain whole numbers.
I will use the following example to demostrate the procedure:
Example: NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O. Calculate the empirical formula of NutraSweet.
1.) Let the mass of the substance be 100g. Then there are:
57.14 grams Carbon in this sample
6.16 grams Hydrogen in this sample
9.52 grams Nitrogen in this sample
27.18 grams Oxygen in this sample
2.) Find the number of moles of each element by dividing by the molar mass:
Number of moles of Carbon = (Mass of Carbon / Molar Mass of Carbon) = (57.14 / 12) = 4.762 moles
Number of moles of Hydrogen = (Mass of Hydrogen / Molar Mass of Hydrogen) = (6.16 / 1) = 6.16 moles
Number of moles of Nitrogen = (Mass of Nitrogen / Molar Mass of Nitrogen) = (9.52 / 14) = 0.68 moles
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