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Ch.3 - Molecules, Compounds & Chemical Equations
Tro - Chemistry: A Molecular Approach 4th Edition
Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831Not the one you use?Change textbook
All textbooksTro 4th EditionCh.3 - Molecules, Compounds & Chemical EquationsProblem 122
Chapter 3, Problem 122

A drop of water has a volume of approximately 0.05 mL. How many water molecules does it contain? The density of water is 1.0 g/cm3.

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First, convert the volume of the water drop from milliliters to cubic centimeters (cm3) since the density of water is given in g/cm3. Remember that 1 mL is equivalent to 1 cm3.
Next, use the density of water to convert the volume of the water drop to mass. Density is defined as mass per unit volume, so you can multiply the volume of the water drop by the density of water to find the mass of the water drop in grams.
Then, convert the mass of the water drop to moles using the molar mass of water. The molar mass of water (H2O) is approximately 18.015 g/mol. You can do this by dividing the mass of the water drop by the molar mass of water.
After that, use Avogadro's number to convert the number of moles of water to the number of water molecules. Avogadro's number (6.022 x 10^23) is the number of particles (atoms, molecules, ions, etc.) in one mole of a substance.
Finally, calculate the number of water molecules in the drop of water by multiplying the number of moles of water by Avogadro's number.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molar Mass of Water

The molar mass of water (H2O) is approximately 18.02 g/mol. This value is essential for converting grams of water to moles, which allows us to relate the mass of water to the number of molecules it contains. Since one mole of any substance contains Avogadro's number of entities (approximately 6.022 x 10^23), knowing the molar mass is crucial for calculating the total number of water molecules in a given volume.
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Density and Volume Relationship

Density is defined as mass per unit volume, and for water, it is approximately 1.0 g/cm³. This means that 1 mL of water has a mass of about 1 gram. Understanding this relationship allows us to convert the volume of water (0.05 mL) into mass, which is necessary for further calculations involving the number of molecules.
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Avogadro's Number

Avogadro's number (6.022 x 10^23) is the number of atoms, ions, or molecules in one mole of a substance. This constant is fundamental in chemistry for converting between moles and the number of individual particles. Once we determine the number of moles of water from its mass, we can use Avogadro's number to find the total number of water molecules present in the drop.
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