Question 4.11: Molecules per Gram An aspirin tablet, C9H8O4, contains 0.360...

Introduction to General, Organic, and Biochemistry

Molecules per Gram
An aspirin tablet, $\text{C}_{9}\text{H}_{8}\text{O}_{4}$ , contains 0.360 g of aspirin. (The rest of the tablet is starch or other fillers.) How many molecules of aspirin are present in this tablet?

Strategy
The formula weight of aspirin is 9(12.0) + 8(1.0) + 4(16.0) = 180.0 amu, which gives us the conversion factor 1 mol aspirin = 180.0 g aspirin. To convert moles of aspirin to molecules of aspirin, we use the conversion factor 1 mole aspirin $= 6.02 \times 10^{23}$ molecules aspirin.

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First, we need to find out how many moles of aspirin are in 0.360 g:

0.360\ \cancel{\text{g aspirin}}\times \frac{1\ \text{mol aspirin}}{180.0\ \cancel{\text{g aspirin}}}= 0.00200 \ \text{mol aspirin}

The number of molecules of aspirin in a tablet is:

0.00200\ \cancel{\text{mol}}\times 6.02 \times 10^{23}  \frac{\text{molecules}}{\cancel{\text{mol}}}= 1.20 \times 10^{21}\ \text{molecules}

■ Quick Check 4.11
How many molecules of water, $\text{H}_{2}\text{O}$ , are in a glass of water (235 g)?

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