How to Calculate Concentration Molarity

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Molarity Calculator

Calculate Concentration (mol/L) from Mass, Molar Mass, and Volume

Milliliters (mL) Liters (L)

Calculation Result:

How to Calculate Molarity: A Comprehensive Guide

Molarity (M) is the most common unit used to express the concentration of a solution in chemistry. It represents the number of moles of a solute per liter of solution.

The Molarity Formula

The standard formula for molarity is:

Molarity (M) = Moles of Solute (n) / Volume of Solution in Liters (V)

If you are starting with the mass of a substance (grams), the calculation involves two steps:

  1. Find the Moles: Divide the mass of the solute by its molar mass (g/mol).
  2. Divide by Volume: Divide the moles by the total volume of the solution in liters.

Step-by-Step Example: NaCl Solution

Suppose you dissolve 5.84 grams of Sodium Chloride (NaCl) in enough water to make 500 mL of solution. Here is how to find the molarity:

  • Step 1: Identify Mass. Mass = 5.84 g.
  • Step 2: Identify Molar Mass. The molar mass of NaCl is approximately 58.44 g/mol.
  • Step 3: Calculate Moles. 5.84 g / 58.44 g/mol = 0.1 moles.
  • Step 4: Convert Volume to Liters. 500 mL = 0.5 Liters.
  • Step 5: Calculate Molarity. 0.1 moles / 0.5 L = 0.2 M.

Why Molarity Matters

In lab settings, molarity allows scientists to accurately measure out specific amounts of molecules. Since chemical reactions happen based on mole ratios (stoichiometry), knowing the molar concentration is essential for predicting reaction outcomes and ensuring safety.

Tips for Accuracy

  • Total Volume: Always remember that the volume in the formula is the total volume of the final solution, not just the volume of the solvent added.
  • Temperature: Molarity can change slightly with temperature because liquids expand and contract, affecting the volume.
  • Units: Ensure your volume is in Liters. If you have milliliters (mL), divide by 1,000.
function calculateMolarity() { var mass = parseFloat(document.getElementById('soluteMass').value); var molarMass = parseFloat(document.getElementById('molarMass').value); var volume = parseFloat(document.getElementById('solVolume').value); var unit = document.getElementById('volUnit').value; var resultDiv = document.getElementById('molarityResult'); var output = document.getElementById('resultOutput'); var steps = document.getElementById('calculationSteps'); if (isNaN(mass) || isNaN(molarMass) || isNaN(volume) || mass <= 0 || molarMass <= 0 || volume <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Convert volume to liters if necessary var volumeInLiters = volume; if (unit === 'ml') { volumeInLiters = volume / 1000; } // Calculate moles var moles = mass / molarMass; // Calculate molarity var molarity = moles / volumeInLiters; // Display results resultDiv.style.display = 'block'; output.innerHTML = molarity.toFixed(4) + " mol/L (M)"; steps.innerHTML = "Process: " + mass + "g / " + molarMass + " g/mol = " + moles.toFixed(4) + " moles. " + "Then: " + moles.toFixed(4) + " moles / " + volumeInLiters + " L = " + molarity.toFixed(4) + " M."; }

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