A salt crystal has a mass of 0.12 mg. How many NaCl formula units does it contain?

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Step 1: Convert the mass of the salt crystal from milligrams to grams. This is because the molar mass of NaCl, which we will use in the next step, is typically given in grams per mole. The conversion factor is 1 gram = 1000 milligrams.

Step 2: Calculate the number of moles of NaCl in the salt crystal. The molar mass of NaCl is approximately 58.44 g/mol. Use the formula: number of moles = mass (in grams) / molar mass (in g/mol).

Step 3: Calculate the number of formula units of NaCl in the salt crystal. Use Avogadro's number, which states that one mole of any substance contains approximately 6.022 x 10^23 entities (atoms, molecules, ions, or formula units). Use the formula: number of formula units = number of moles x Avogadro's number.

Step 4: The result from step 3 is the number of NaCl formula units in the salt crystal.

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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). For sodium chloride (NaCl), the molar mass is approximately 58.44 g/mol, calculated by adding the atomic masses of sodium (Na) and chlorine (Cl). Understanding molar mass is essential for converting between mass and the number of moles in chemical calculations.

Avogadro's Number

Avogadro's number, approximately 6.022 x 10²³, is the number of formula units, atoms, or molecules in one mole of a substance. This constant allows chemists to relate the macroscopic scale of substances (grams) to the microscopic scale (individual particles). It is crucial for determining how many formula units of NaCl are present in a given mass.

Unit Conversion

Unit conversion is the process of converting a quantity expressed in one set of units to another. In this context, converting the mass of the NaCl crystal from milligrams to grams is necessary to use the molar mass effectively. This step is vital for accurately calculating the number of formula units present in the salt crystal.

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