BackMolarity and Solution Concentration: Practice and Concepts
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Molarity and Solution Concentration
Introduction to Molarity
Molarity (M) is a fundamental concept in chemistry used to express the concentration of a solution. It is defined as the number of moles of solute dissolved per liter of solution. Understanding molarity is essential for preparing solutions, performing stoichiometric calculations, and analyzing chemical reactions in aqueous solutions.
Solute: The substance being dissolved (e.g., NaCl, KNO3).
Solvent: The substance doing the dissolving (usually water in general chemistry).
Solution: A homogeneous mixture of solute and solvent.
The formula for molarity is:
Calculating Molarity: Step-by-Step Examples
Below are practice problems illustrating how to calculate the molarity of various solutions. Each example demonstrates the application of the molarity formula.
2.5 moles of sodium chloride is dissolved to make 0.500 liters of solution. Calculation:
3.0 moles of silver chloride is dissolved in enough water to make 2.0 liters of solution. Calculation:
0.60 moles of potassium nitrate is dissolved to make 0.80 liters of solution. Calculation:
4.2 moles of lithium fluoride are dissolved in 2.0 liters of solution. Calculation:
13 grams of hydrochloric acid (HCl) is dissolved to make 0.500 liters of solution. Step 1: Calculate moles of HCl. Molar mass of HCl = 36.5 g/mol Step 2: Calculate molarity.
33 grams of beryllium hydride (BeH2) is dissolved to make 0.60 liters of solution. Step 1: Calculate moles of BeH2. Molar mass of BeH2 = 11.0 g/mol Step 2: Calculate molarity.
734 grams of lithium sulfate (Li2SO4) is dissolved to make 2500 mL of solution. Step 1: Convert mL to L. Step 2: Calculate moles of Li2SO4. Molar mass of Li2SO4 = 110 g/mol Step 3: Calculate molarity.
8.0 × 1021 molecules of Cl2 dissolved to make 500 mL of solution. Step 1: Convert molecules to moles. Step 2: Convert mL to L. Step 3: Calculate molarity.
Conceptual Question: Comparing Solution Preparation Methods
Question: You have two solutions. In the first solution, 1.0 moles of sodium chloride is dissolved to make 1.0 liters of solution. In the second, 1.0 moles of sodium chloride is added to 1.0 liters of water. Is the molarity of each solution the same? Explain your answer.
Key Point: The molarity is defined as moles of solute per liter of solution, not per liter of solvent.
In the first case, the final volume is exactly 1.0 L (solution), so .
In the second case, the final volume will be slightly more than 1.0 L (since the solute adds volume), so the molarity will be slightly less than 1.0 M.
Conclusion: The solutions have slightly different molarities because the total volumes differ.
Summary Table: Molarity Calculations
Problem | Moles of Solute | Volume (L) | Molarity (M) |
|---|---|---|---|
1 | 2.5 | 0.500 | 5.0 |
2 | 3.0 | 2.0 | 1.5 |
3 | 0.60 | 0.80 | 0.75 |
4 | 4.2 | 2.0 | 2.1 |
5 | 0.356 | 0.500 | 0.712 |
6 | 3.0 | 0.60 | 5.0 |
7 | 6.67 | 2.5 | 2.67 |
8 | 0.0133 | 0.500 | 0.0266 |
Key Takeaways
Molarity is a measure of solution concentration, defined as moles of solute per liter of solution.
Always ensure volumes are in liters and amounts are in moles before calculating molarity.
When preparing solutions, the final volume (not just the volume of solvent) determines the molarity.
