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10-3: Empirical and Molecular Formulas. Percentage Composition. The mass of each element in a compound, compared to the mass of the entire compound (multiplied by 100) Tells you the percent of the mass made up by each element in the compound. How to calculate % comp:.
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Percentage Composition • The mass of each element in a compound, compared to the mass of the entire compound (multiplied by 100) • Tells you the percent of the mass made up by each element in the compound.
How to calculate % comp: • (grams of X)/(total grams) x 100% • Complete sample problem #7 on pg 334 • Complete practice problem #14 on pg 334
Empirical Formula • The smallest whole number ratio of the atoms of the elements in a compound • Remember glucose? • What is its molecular formula? • What is its empirical formula?
Calculating Empirical Formula • 1) If given as percents, read them as grams (accurate if a 100 g sample) • 2) Convert each gram amount to moles • 3) Find the smallest whole number ratio by dividing each mole amount by the smallest number. Round as appropriate. • 4) Use that ratio to write the empirical formula
Molecular Formula • Gives the actual number of atoms of each element in a molecular compound. • Is a whole number multiple of the empirical formula.
Finding the Molecular Formula • You will be given the molar mass of the molecular formula in the question. • Find the empirical formula. • Find the molar mass of the empirical formula. • Divide molecular formula mass by empirical formula mass. • This is the whole number multiple that you should multiply the empirical formula by.
Practice problem #18 (pg 339) • Sample problem 9 (pg 338)