Molarity

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:

$M = \frac{\text{moles of solute}}{\text{liters of solution}}$

  • 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).

Test Yourself

Assorted Multiple Choice
A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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Molarity

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Direct Calculation of Molarity

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Calculating Moles or Volume from Molarity

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Molarity from Mass (Single-Step Conversion)

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Molarity Formula

Molarity is calculated using the formula:

$M = \frac{\text{moles of solute}}{\text{liters of solution}}$

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

Brain Hack Tip

🧠 Here is a tip of how to remember how to remember molarity 🧠

Two cute mice holding onto the edge of a glass container.

Key Concepts

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.

Dilution:

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

  • Dilution formula: $M_1 V_1 = M_2 V_2$​

  • Where:

    • $M_1$​ and $V_1$​: Initial molarity and volume

    • $M_2$​ and $V_2$: Final molarity and volume

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. $\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

  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: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

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