Master the Art of Formula Finding: Learn Easy Steps to Remember the Process!"
Dr. Zahid H Shar
Analytical Chemistry Expert with PhD and Hands-on Experience in Chromatography and Spectroscopy
While teaching analytical chemistry, I found that some students were struggling to remember the steps involved in determining the empirical and molecular formula of a compound. To help them, I searched for mnemonic devices and found that Joel S. Thompson's rhyme is the best way to memorize the steps for empirical formula determination.
The rhyme goes as follows:
"Percentage to mass, mass to mole, divide by small, multiply 'til whole."
Each line of the rhyme represents one of the steps involved in determining the empirical formula:
"Percentage to mass": Convert the percentages of each element to masses.
"Mass to mole": Convert the masses of each element to moles using their respective atomic masses or molar masses.
"Divide by small": Divide each number of moles by the smallest number of moles to get the mole ratios.
"Multiply 'til whole": Multiply the mole ratios by a whole number to obtain the simplest whole-number ratio of atoms.
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Example
A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What is the empirical formula?
(1) Percent to mass: Assume 100 g of the substance, then 72.2 g magnesium and 27.8 g nitrogen.
(2) Mass to moles: far Mg: 72.2 g Mg X (1 mol Mg124.3 g Mg) = 2.97 mol Mg far N: 27.8 g N X (1 mol Nl14.0 g N) = 1.99 mol N
(3) Divide by small: for Mg: 2.97 moV1.99 ma1 = 1.49 for N: 1.99 mo111.99 mol = 1.00
(4) Multiply till whole: for Mg: 2 X 1.49 = 2.98 (i.e., 3) for N: 2 X 1.00 and the formula of the compound is Mg3N2.
By reciting this rhyme, students can remember the steps involved in determining the empirical formula, making it easier for them to apply this concept to their coursework and exams.
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