A Comprehensive Guide to Calculating Molarity
Molarity is a crucial concept in chemistry that represents the concentration of a solution. It is defined as the number of moles of solute per liter of solution. Understanding how to calculate molarity is essential for various chemical applications, including preparing solutions with specific concentrations and performing quantitative analyses.
Understanding Molarity
A mole is the SI unit of amount, representing a specific number of entities, such as atoms, molecules, or ions. One mole is defined as 6.022 × 10^23 entities. Molarity, denoted by the symbol M, is expressed as:
Molarity (M) = Moles of solute / Volume of solution (in liters)Step-by-Step Calculation of Molarity
1. Determine the Moles of Solute:
- Convert the given mass of solute to moles using its molar mass.
- Molar mass is the mass of one mole of a substance, found in its periodic table entry or from chemical calculations.
2. Determine the Volume of Solution:
- Convert the given volume of solution from milliliters (mL) to liters (L).
- 1 liter (L) = 1000 milliliters (mL)
3. Calculate Molarity:
- Divide the number of moles of solute by the volume of solution in liters.
Example Calculation
Problem: Calculate the molarity of a solution prepared by dissolving 12.5 g of sodium chloride (NaCl) in 500 mL of water.
Solution:
1. Moles of Solute:
- Molar mass of NaCl = 22.99 g/mol + 35.45 g/mol = 58.44 g/mol
- Moles of NaCl = 12.5 g / 58.44 g/mol = 0.214 mol
2. Volume of Solution:
- Volume = 500 mL = 0.5 L
3. Molarity:
- Molarity = 0.214 mol / 0.5 L = 0.428 M
Applications of Molarity
Calculating molarity is essential in various chemical applications:
- Preparing Solutions of Specific Concentration: By controlling the molarity, solutions with the desired concentrations can be prepared for specific purposes, such as experiments or analytical procedures.
- Quantitative Analysis: Molarity is used in titrations and other quantitative analyses to determine the concentration of unknown solutions by reacting them with solutions of known concentrations.
- Stoichiometric Calculations: Molarity helps determine the stoichiometric ratios of reactants and products in chemical reactions, allowing for the prediction of reaction outcomes.
- Understanding Solution Properties: Molarity provides insights into the behavior and properties of solutions, such as their freezing point depression, boiling point elevation, and osmotic pressure.
Frequently Asked Questions (FAQs)
Q: What is the difference between molarity and normality?
A: Normality (N) is another unit of concentration that considers the number of equivalents per liter of solution. While molarity focuses on the number of moles, normality considers the number of moles that can react with or be replaced by a specific reference substance, usually H+ or OH-.
Q: How do I convert between moles and millimoles?
A: 1 mole = 1000 millimoles (mmol). To convert moles to millimoles, multiply by 1000. To convert millimoles to moles, divide by 1000.
Q: What is the molarity of a solution with 0.05 g of NaOH in 200 mL of water?
A:
- Moles of NaOH = 0.05 g / 40.00 g/mol = 0.00125 mol
- Volume = 200 mL = 0.2 L
- Molarity = 0.00125 mol / 0.2 L = 0.00625 M
Q: How do I prepare a 2.5 M solution of sulfuric acid (H2SO4) using a 98% concentrated solution (d = 1.84 g/mL)?
A:
- 2.5 M = 2.5 mol / L
- Use density to calculate the mass of 98% H2SO4 needed:
- Mass = Volume × Density
- Let’s assume we need 1 L of 2.5 M solution.
- Mass = 1 L × 1.84 g/mL = 1840 g
- To calculate the volume of 98% H2SO4 needed:
- Volume = Mass / (Density × Concentration)
- Volume = 1840 g / (1.84 g/mL × 98%)
- Volume = 100.5 mL
- Add 100.5 mL of 98% H2SO4 to a 1 L volumetric flask and fill with water to the mark.
Q: What is the molarity of a solution that contains 1.5 × 10^22 molecules of glucose (C6H12O6) in 250 mL of water?
A:
- Moles of glucose = (1.5 × 10^22 molecules) / (6.022 × 10^23 molecules/mol) = 0.0249 mol
- Volume = 250 mL = 0.25 L
- Molarity = 0.0249 mol / 0.25 L = 0.1 M
