Molarity

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Core Concept

Molarity (M) is a unit of concentration used to describe how much solute is dissolved in a given volume of solution. It is defined as the number of moles of solute per liter of solution.

Molarity is calculated using the formula:

M=moles of soluteliters of solution

Practice Tips

  • Confusing molarity (M) with molality (m), which is moles of solute per kilogram of solvent.

  • Using the mass of the solution instead of the volume in liters.

  • Forgetting that diluting a solution decreases molarity but does not change the number of moles of solute.

  • Molarity is temperature dependent! Since molarity depends on the volume of the solution, it is temperature-dependent (volume changes with temperature).

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Molarity Formula

Molarity is calculated using the formula:

M=moles of soluteliters of solution

Where:

  • M = Molarity of the solution (in moles per liter, or mol/L)

  • Moles of solute = The amount of solute in moles

  • Liters of solution = The total volume of the solution in liters

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Key Concepts

  1. Solute and Solvent:

    • Solute: The substance that is dissolved (e.g., salt in a saltwater solution).

    • Solvent: The substance in which the solute is dissolved (e.g., water in a saltwater solution).

    • Solution: The homogeneous mixture of solute and solvent.

  2. Units of Molarity:

    • Molarity is measured in moles per liter (mol/L), often abbreviated as M.

    • For example, a 1.0 M solution of NaCl contains 1 mole of NaCl dissolved in 1 liter of solution.

  3. Dilution:

    • Diluting a solution reduces its molarity by adding more solvent.

    • Dilution formula: M1V1=M2V2​

    • Where:

      • M1​ and V1​: Initial molarity and volume

      • M2​ and V2: Final molarity and volume

  4. Preparing Solutions:

    • To prepare a solution of a given molarity, calculate the moles of solute needed, then dissolve it in enough solvent to reach the desired final volume.

  5. Temperature Dependence:

    • Since molarity depends on the volume of the solution, it is temperature-dependent (volume changes with temperature).

Steps for Calculating Molarity

  1. Calculate Moles of Solute:

    • If the amount of solute is given in grams, convert it to moles using the molar mass of the solute. moles=mass (g)molar mass (g/mol)

  2. Determine Volume of Solution in Liters:

    • Convert the solution volume to liters if necessary (1 L = 1000 mL).

  3. Apply the Molarity Formula:

    • Substitute the moles of solute and the liters of solution into the formula: M=moles of soluteliters of solution

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