Understanding Volume and Amount of Substance
Milliliters (mL) are a standard unit for measuring liquid volume. This unit relates directly to the liter, with 1000 milliliters equaling one liter. Volume measurements indicate the three-dimensional space a substance occupies.
Moles represent a specific quantity of a substance, defined as Avogadro’s number of particles (approximately 6.022 x 10^23 atoms, molecules, or ions). This unit bridges the macroscopic mass of a substance and the microscopic number of particles it contains. Moles allow chemists to work with precise ratios in chemical reactions.
Molarity (M) quantifies solution concentration, expressing moles of solute dissolved per liter of solution. This measurement helps understand how much of a specific substance is present within a given volume of a mixture. Molarity directly converts solution volume to moles of solute.
Density describes the relationship between a substance’s mass and its volume, typically in grams per milliliter (g/mL) or grams per cubic centimeter (g/cm³). For pure liquids, density converts a measured volume into a corresponding mass. This conversion is necessary when molarity cannot determine the amount of substance.
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). This value is unique for each chemical compound, calculated by summing the atomic masses of all atoms in its chemical formula. Molar mass converts a substance’s mass into its equivalent number of moles.
Converting Volume to Moles Using Molarity
When working with chemical solutions, converting volume to moles often uses the solution’s molarity. This method applies when a substance is dissolved in a solvent to form a homogeneous mixture with a known concentration.
The first step converts volume from milliliters (mL) to liters (L). Since molarity is moles per liter, this conversion aligns the units. Divide the volume in milliliters by 1000. For example, 250 mL becomes 0.250 L.
Once the volume is in liters, calculate the moles of solute by multiplying the solution’s molarity by the volume in liters. The formula is: Moles = Molarity (mol/L) × Volume (L).
For instance, 150 mL of a 0.50 M sodium chloride (NaCl) solution first converts to 0.150 L. Then, multiply 0.50 mol/L by 0.150 L, resulting in 0.075 moles of NaCl.
Converting Volume to Moles Using Density and Molar Mass
For pure liquid substances where molarity is not relevant, converting volume to moles requires two steps: density and molar mass. This approach suits compounds existing as pure liquids, not dissolved in a solvent. The process first converts volume into mass, then mass into moles.
The initial step converts the given volume in milliliters to mass using the substance’s density. Density is typically provided in grams per milliliter (g/mL). The calculation is: Mass (g) = Density (g/mL) × Volume (mL).
After determining the mass, convert it into moles using the substance’s molar mass. Molar mass is expressed in grams per mole (g/mol) and found by summing the atomic masses of all atoms in the compound’s chemical formula. The formula is: Moles = Mass (g) / Molar Mass (g/mol).
For example, to find the moles in 20 mL of pure ethanol (C₂H₅OH) with a density of 0.789 g/mL and a molar mass of 46.07 g/mol: first calculate the mass (20 mL × 0.789 g/mL = 15.78 g). Then, convert the mass to moles (15.78 g / 46.07 g/mol ≈ 0.342 moles of ethanol).