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Calculating Molar Mass and Amount of Molecules of Aspirin When The Formula and Weight is known

February 16th, 2008

Chemistry Tutor Site Exercise

The molecular formula of acetylsalicylic acid (aspirin), one of the most commonly used pain relievers, is C₉H₈O₄.

1. calculate the molar mass of aspirin
2. a typical aspirin tablet contains 500 mg of C₉H₈O₄. How many moles of C₉H₈O₄ molecules and how many molecules of acetylsalicylic acid are in a 500 mg tablet?

Chemistry Tutor Site Solution

The molar mass is the mass in grams of one mole of the compound. The molar mass is obtained by summing the masses of the component atoms. In one mole of aspirin there are 9 moles of carbon atoms, 8 moles of hydrogen atoms, and 4 moles of oxygen atoms.

9 C: 9x 12.01 = 108.09 g

8 H: 8 x 1.008 = 8.064 g

4 O: 4 x 16.00 = 64 g

Mass of 1 mole C₉H₈O₄ = 132.154 g

So the molar mass of aspirin is 132.154 g

Moles of aspirin are determined by calculate the ratio between its mass and the molar mass. The mass of 1 mole of aspirin is 132.154 g, thus 5 x 10^-2 g ( 500 mg) is much less than 1 mole, you can calculate using this fraction:

5 x 10^-1 g x ( 1 mol / 132.154 g ) = 3.78 x 10^-3 mol of aspirin

Since 1 mole is 6.022 x 10²³ units, we can determine the number of molecules:

3.78 x 10^-3 x 6.022 x 10²³ molecules = 22.76×10²⁰ molecules

and can be rounded to be 23×10²⁰ molecules

 

 

Entry Filed under: Molar Mass