Molecular and Empirical formulas
How to find molecular and empirical formulas.
[Note: I do not 100% guarantee that any of my work is correct- it's just done to the best of my abilities- so please double check your answers if following my method to solve a similar problem]
*Remember significant figures! I'm not the best at sig. fig., but hopefully all my work is accurate.*
Formaldehyde has an empirical formula that is 39.99% C, 53.28% O and 6.73% H. What is the empirical formula? If the actual molar mass for formaldehyde is 30.03 g/mol, what is the molecular formula?
Needed information-
The molar mass of C = 12.01
The molar mass of O = 16.00
The molar mass of H = 1.01
Since percentages are given, assume that there are 100 grams. Percent of each element given = mass of that element. Thus, there are 39.99 g of C, 53.28 g of O and 6.73 g of H.
Convert grams to moles for each element.
39.99 g C x (1 mol C) / (12.01 g C) = 3.33 mol C
53.28 g O x (1 mol O) / (16.00 g O) = 3.33 mol O
6.73 g H x (1 mol H) / (1.01 H) = 6.66 mol H
Find the smallest whole number ratio by dividing the # of mol of each element by the # mol of the element that is in the smallest amount. As we can see above, C and O are in a 1:1 ratio, since their numbers are the same. However, we need to know the ratio of H.
6.66 mol H/ 3.33 mol C = a ratio of 2 (as you can also see by looking: 6.66 is 2 x 3.33)
Thus the ratio of C : H : O is as follows- 1 : 2 : 1 (note: this ratio is true of sugars)
The empirical formula is CH2O
Save this answer. We also need to know the molecular formula. To do this, first calculate the mass of the empirical formula. We go back to the molar mass information we used earlier, and using the ratios in the empirical formula CH2O, we can make the following equation:
12.01 + 16.00 + (2) (1.01) = 30.03
The formula mass of CH2O is 30.03. Remember the following from our original problem: “If the actual molar mass for formaldehyde is 30.03 g/mol”- this is the same as the equation that we did above, so we don‘t have to do any changes here.
The molecular mass (or mw, or molecular weight) is also 12.01 + 16.00 + (2) (1.01) = 30.03.
As we can see, the molecular formula of Formaldehyde is the same as its empirical formula. Thus we go back to the saved answer, and give this as being both the empirical and molecular formula.
Helpful Reference: http://chemistry.about.com/library/weekly/bl032803a.htm
[Note: I do not 100% guarantee that any of my work is correct- it's just done to the best of my abilities- so please double check your answers if following my method to solve a similar problem]
*Remember significant figures! I'm not the best at sig. fig., but hopefully all my work is accurate.*
Formaldehyde has an empirical formula that is 39.99% C, 53.28% O and 6.73% H. What is the empirical formula? If the actual molar mass for formaldehyde is 30.03 g/mol, what is the molecular formula?
Needed information-
The molar mass of C = 12.01
The molar mass of O = 16.00
The molar mass of H = 1.01
Since percentages are given, assume that there are 100 grams. Percent of each element given = mass of that element. Thus, there are 39.99 g of C, 53.28 g of O and 6.73 g of H.
Convert grams to moles for each element.
39.99 g C x (1 mol C) / (12.01 g C) = 3.33 mol C
53.28 g O x (1 mol O) / (16.00 g O) = 3.33 mol O
6.73 g H x (1 mol H) / (1.01 H) = 6.66 mol H
Find the smallest whole number ratio by dividing the # of mol of each element by the # mol of the element that is in the smallest amount. As we can see above, C and O are in a 1:1 ratio, since their numbers are the same. However, we need to know the ratio of H.
6.66 mol H/ 3.33 mol C = a ratio of 2 (as you can also see by looking: 6.66 is 2 x 3.33)
Thus the ratio of C : H : O is as follows- 1 : 2 : 1 (note: this ratio is true of sugars)
The empirical formula is CH2O
Save this answer. We also need to know the molecular formula. To do this, first calculate the mass of the empirical formula. We go back to the molar mass information we used earlier, and using the ratios in the empirical formula CH2O, we can make the following equation:
12.01 + 16.00 + (2) (1.01) = 30.03
The formula mass of CH2O is 30.03. Remember the following from our original problem: “If the actual molar mass for formaldehyde is 30.03 g/mol”- this is the same as the equation that we did above, so we don‘t have to do any changes here.
The molecular mass (or mw, or molecular weight) is also 12.01 + 16.00 + (2) (1.01) = 30.03.
As we can see, the molecular formula of Formaldehyde is the same as its empirical formula. Thus we go back to the saved answer, and give this as being both the empirical and molecular formula.
Helpful Reference: http://chemistry.about.com/library/weekly/bl032803a.htm