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Understanding Percent Composition in Chemical Formulas

Learn how to calculate the percentage of elements N, P, and K in fertilizers using chemical formulas and mass calculations. Practice examples provided.

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Understanding Percent Composition in Chemical Formulas

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  1. The label on a bag of fertilizer states the percentages of N, P, and K How do they know that? Chemical Quantities Percent Composition

  2. Percent Composition Is the percent by mass of each element in a formula • First… Figure out the total mass of each element to get the Formula Mass • Second… Divide the Mass of the particular element by the total Formula Mass • Third… Multiply that number by 100 to get percent!

  3. Example: Calculate the percent composition of Carbon in CO2 First… Figure out the total mass of each element to get the Formula Mass: CO2 = 1C (12.01g) + 2 O(16.00 g) = 44.01 g/mol Second…Divide the Mass of the particular element by the total Formula Mass 12.01 g C = 27.289 44.01 g CO2 Third…Multiply the amount by 100 to get a percent! 27.289 x 100 = 27.29% Carbon in CO2

  4. Example: Calculate the percent composition of Oxygen in CO2 First… Figure out the total mass of each element to get the Formula Mass: CO2 = 1C (12.01g) + 2 O(16.00 g) = 44.01 g/mol Second…Divide the Mass of the particular element by the total Formula Mass 32.00 g O = 72.71 44.01 g CO2 Third…Multiply the amount by 100 to get a percent! 72.71 x 100 = 72.71% Oxygen in CO2

  5. Check your answers… • Add up your percentages • They MUST add up to 100! Carbon = 27.29 + Oxygen = 72.71 Total = 100

  6. Learning Check What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?

  7. Learning Check The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent of carbon in isoamyl acetate? 1) 7.102 %C 2) 35.51 %C 3) 64.58 %C

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