Molarity describes the concentration of a solution, quantifying how much of a solute is dissolved within a given volume of solvent. Understanding how to determine this concentration is important for various scientific applications. This article explains how to calculate molarity when starting with the mass of a solute in grams and the volume of the solution in milliliters.
Understanding Molarity
Molarity, symbolized by an uppercase “M,” represents the concentration of a solution as moles of solute dissolved per liter (mol/L). This measurement accounts for the number of particles of a substance, unlike expressions based solely on mass or volume.
The concept of moles provides a standardized way to count atoms or molecules. Molarity allows chemists to understand the reactive proportions of substances in a solution, making it a widely used measure for preparing solutions and carrying out chemical reactions.
Key Unit Conversions
Calculating molarity from grams and milliliters requires two unit conversions. The first involves changing the mass of the solute from grams to moles, and the second involves changing the volume of the solution from milliliters to liters.
Converting grams to moles relies on the molar mass of the substance, typically expressed in grams per mole (g/mol). To find the molar mass, sum the atomic masses of all the atoms in the compound’s chemical formula, found on the periodic table.
The second conversion involves changing the volume of the solution from milliliters (mL) to liters (L). There are 1000 milliliters in every 1 liter. To convert, simply divide the milliliter value by 1000.
Step-by-Step Molarity Calculation
The fundamental formula for molarity (M) is defined as the moles of solute divided by the liters of solution.
The first step involves identifying the initial given values, which include the mass of the solute in grams and the volume of the solution in milliliters. The mass of the solute must be converted from grams into moles using the substance’s molar mass.
Next, the volume of the solution, initially given in milliliters, needs to be converted into liters. This is achieved by dividing the milliliter value by 1000. Finally, with the moles of solute and liters of solution determined, the molarity is calculated by dividing the moles of solute by the liters of solution.
Applying the Formula: A Worked Example
Consider a scenario where you need to determine the molarity of a solution prepared by dissolving 29.22 grams of sodium chloride (NaCl) in enough water to make a total solution volume of 500 milliliters.
First, identify the molar mass of sodium chloride. Sodium (Na) has an atomic mass of approximately 22.99 g/mol, and chlorine (Cl) has an atomic mass of approximately 35.45 g/mol. Summing these values gives a molar mass for NaCl of 58.44 g/mol. Convert the 29.22 grams of NaCl to moles: 29.22 g / 58.44 g/mol = 0.500 moles of NaCl.
Next, convert the solution volume from milliliters to liters. The given volume is 500 mL. Dividing this by 1000 yields 0.500 liters.
Finally, calculate the molarity: 0.500 moles / 0.500 L = 1.00 M. The molarity of the sodium chloride solution is 1.00 M.