Calculating mass from molarity and volume is essential when preparing a specific chemical solution. This procedure determines the amount of solute (solid substance) that must be dissolved in a solvent (liquid) to achieve a target concentration. Molarity (M) measures this concentration, defining the number of moles of solute present in one liter of solution. Following a sequential process ensures accurate solution preparation.
Essential Definitions and Prerequisites
Three pieces of information must be established before calculation: the target Molarity (M), the required Volume (L), and the solute’s Molar Mass (g/mol). Molarity specifies the solution’s strength in moles of solute per liter of solution, usually determined by experimental requirements. The Volume is the total amount of solution needed and must be expressed in liters (L) to align with molarity units. If the volume is given in milliliters, it must be converted to liters first.
The Molar Mass (g/mol) is the mass in grams of one mole of the specific chemical compound. This value is derived from the periodic table by summing the atomic masses of all atoms in the compound’s chemical formula. For example, calculating NaCl requires adding the atomic mass of sodium and chlorine. This calculated value is necessary to convert the chemical quantity of moles into a measurable mass in grams.
The Sequential Conversion Formula
The conversion process is a two-step sequence moving from concentration to the amount of substance, and finally to its mass. The first step determines the number of moles (n) of solute required for the target concentration and volume. This is calculated by multiplying the Molarity (M) by the Volume (L) of the solution: Moles (n) = Molarity (M) x Volume (L).
The second step converts the calculated moles into a physical mass that can be weighed. This conversion uses the Molar Mass (g/mol) determined from the periodic table. The formula is Mass (g) = Moles (n) x Molar Mass (g/mol). Multiplying moles by the molar mass cancels the mole unit and yields the final answer in grams.
A Practical Walkthrough Example
Consider preparing a 0.5 M sodium chloride (NaCl) solution in a total volume of 0.25 L. First, determine the Molar Mass of NaCl by adding the atomic mass of sodium (22.99 g/mol) and chlorine (35.45 g/mol), resulting in 58.44 g/mol.
The initial step calculates the moles of NaCl needed by multiplying the target molarity by the volume. Moles (n) = 0.5 moles/L x 0.25 L, which results in 0.125 moles of NaCl.
The final step converts these 0.125 moles into a measurable mass using the calculated Molar Mass. The calculation is Mass (g) = 0.125 moles x 58.44 g/mol, resulting in a required mass of 7.305 grams of sodium chloride. To create the 0.5 M solution, 7.305 grams of sodium chloride must be weighed out and dissolved, bringing the total volume up to 0.25 L.