How many molecules (or formula units) are in each sample? a. 67.9 g H2O b. 112 mg NaPO4 c. 32.1 kg C6H12O d. 56.1 g N2

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Step 1: Determine the molar mass of each compound.

Step 2: Convert the given mass of each sample to moles using the formula: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \).

Step 3: Use Avogadro's number \( 6.022 \times 10^{23} \) to convert moles to molecules or formula units using the formula: \( \text{molecules} = \text{moles} \times 6.022 \times 10^{23} \).

Step 4: For each sample, perform the calculations using the steps above.

Step 5: Ensure units are consistent and check calculations for accuracy.

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Key Concepts

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

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in a molecule. For example, the molar mass of water (H2O) is approximately 18.02 g/mol, derived from 2 hydrogen atoms (1.01 g each) and 1 oxygen atom (16.00 g). Understanding molar mass is essential for converting between grams and moles in stoichiometric calculations.

Avogadro's Number

Avogadro's number, approximately 6.022 x 10^23, is the number of particles (atoms, molecules, or formula units) in one mole of a substance. This constant allows chemists to relate the macroscopic scale of substances (grams) to the microscopic scale (number of molecules). When calculating the number of molecules in a sample, the number of moles is multiplied by Avogadro's number to find the total count of molecules present.

Unit Conversion

Unit conversion is the process of converting a quantity expressed in one set of units to another. In chemistry, this often involves converting grams to moles using molar mass, and then to molecules using Avogadro's number. For instance, to find the number of molecules in 67.9 g of H2O, one would first convert grams to moles using the molar mass of water, and then multiply the result by Avogadro's number to obtain the total number of molecules.

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