This week in Chem I, we have added to our chemistry knowlege by using our understanding of moles and molar mass to find percent composition, empirical formulas, and molecular formulas. A brief review is below:
% Composition = (Mass of element / Mass of Compound) X 100
If given % composition, you can find the empirical formula by:
Step 1: Assume you have 100g. This way, your percentages will be the number of grams. Example: 40% of a compound is carbon. 40% of 100 g is 40 grams. It’s a miracle!
Step 2: Change grams to moles. Since empirical formulas are mole comparisons, we need to put our numbers in moles first.
Step 3: Compare the number of moles of each to the smallest number of moles by dividing by the smallest number.
Step 4: Round your ratio to the nearest whole number as long as it is “close.” For example, 1.99987 can be rounded to 2, but 1.3333 cannot be rounded to 1. It is four-thirds, so we must multiply all ratios by 3 to rid ourselves of the fraction.
Step 5: Write the formula using your ratio numbers as subscripts.
To find the molecular formula, you must be given the actual molar mass of the compound. Find the ratio of the actual mass to the empirical mass. For example, if it is one, the empirical formula is equal to the molecular formula. If it is 2, the subscripts must be multiplied by two.