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Here's a quick overview of how to prepare a solution when the final concentration is expressed as M or molarity.
You prepare a solution by dissolving a known mass of solute (often a solid) into a specific amount of a solvent. One of the most common ways to express the concentration of the solution is M or molarity, which is moles of solute per liter of solution.
Example of How to Prepare a Solution
Prepare 1 liter of 1.00 M NaCl solution.
First, calculate the molar mass of NaCl which is the mass of a mole of Na plus the mass of a mole of Cl or 22.99 + 35.45 = 58.44 g/mol
- Weigh out 58.44 g NaCl.
- Place the NaCl in a 1-liter volumetric flask.
- Add a small volume of distilled, deionized water to dissolve the salt.
- Fill the flask to the 1 L line.
If a different molarity is required, then multiply that number times the molar mass of NaCl. For example, if you wanted a 0.5 M solution, you would use 0.5 x 58.44 g/mol of NaCl in 1 L of solution or 29.22 g of NaCl.
Important Points to Remember
- Molarity is expressed in terms of liter of solution, not liters of solvent. To prepare a solution, the flask is filled to the mark. In other words, it is incorrect to a 1 liter of water to a mass of sample to prepare a molar solution.
- Sometimes it's necessary to adjust the pH of a solution. To do this, add enough water to dissolve the solute. Then add an acid or base solution dropwise (usually a hydrochloric acid or HCl solution for acid or sodium hydroxide or NaOH solution for a base) to reach the desired pH. Then add more water to reach the mark on the glassware. Adding more water won't change the pH value.