Molar Concentration (M)
Mass of Solute (grams)
Volume of Solution
Liters (L)
Milliliters (mL)
Understanding Molarity (Molar Concentration)
Molarity (M) is the most common unit of concentration used in chemistry. It represents the number of moles of a solute dissolved in exactly one liter of a solution.
The Core Formula
Molarity (M) = Moles of Solute (n) / Volume of Solution in Liters (V)
Because moles (n) is defined as Mass (m) / Molar Mass (MW), we can expand the formula to:
M = Mass (g) / [Molar Mass (g/mol) × Volume (L)]
Key Components Explained
Solute: The substance being dissolved (e.g., salt, sugar).
Solvent: The liquid doing the dissolving (usually water).
Solution: The combination of the solute and the solvent.
Molar Mass: The sum of the atomic weights of all atoms in a molecule (found on the periodic table).
Example Calculation
Suppose you want to prepare 500 mL of a 0.5 M Sodium Chloride (NaCl) solution. How much salt do you need?
Identify Molar Mass: NaCl is approx. 58.44 g/mol.
Convert Volume: 500 mL = 0.5 Liters.
Apply Formula: Mass = Molarity × Molar Mass × Volume
Calculate: 0.5 M × 58.44 g/mol × 0.5 L = 14.61 grams.
Important Tips
Always remember that the "Volume" in the molarity formula refers to the total volume of the final solution, not just the volume of the solvent added. In practice, you usually add the solute to a flask first, then fill it with solvent until the specific volume line is reached.
function updateMolarityFields() {
var mode = document.getElementById('calcMode').value;
var massCont = document.getElementById('input-mass-container');
var mwCont = document.getElementById('input-mw-container');
var molarityCont = document.getElementById('input-molarity-container');
var volCont = document.getElementById('input-volume-container');
// Reset visibility
massCont.style.display = "block";
mwCont.style.display = "block";
molarityCont.style.display = "block";
volCont.style.display = "block";
if (mode === 'molarity') {
molarityCont.style.display = "none";
} else if (mode === 'mass') {
massCont.style.display = "none";
} else if (mode === 'volume') {
volCont.style.display = "none";
}
document.getElementById('mol_result_box').style.display = "none";
}
function calculateMolarity() {
var mode = document.getElementById('calcMode').value;
var mass = parseFloat(document.getElementById('mol_mass').value);
var mw = parseFloat(document.getElementById('mol_mw').value);
var molarity = parseFloat(document.getElementById('mol_molarity').value);
var vol = parseFloat(document.getElementById('mol_vol').value);
var volUnit = document.getElementById('mol_vol_unit').value;
var resultText = "";
var resultValue = 0;
// Normalize volume to Liters for calculation
var volInL = vol;
if (volUnit === 'mL') {
volInL = vol / 1000;
}
if (mode === 'molarity') {
if (isNaN(mass) || isNaN(mw) || isNaN(vol) || mw <= 0 || vol <= 0) {
alert("Please enter valid positive numbers for mass, molar mass, and volume.");
return;
}
resultValue = mass / (mw * volInL);
resultText = "Concentration: " + resultValue.toFixed(4) + " M (mol/L)";
}
else if (mode === 'mass') {
if (isNaN(molarity) || isNaN(mw) || isNaN(vol) || mw <= 0 || vol <= 0) {
alert("Please enter valid positive numbers for molarity, molar mass, and volume.");
return;
}
resultValue = molarity * mw * volInL;
resultText = "Required Mass: " + resultValue.toFixed(4) + " grams";
}
else if (mode === 'volume') {
if (isNaN(mass) || isNaN(mw) || isNaN(molarity) || mw <= 0 || molarity <= 0) {
alert("Please enter valid positive numbers for mass, molar mass, and molarity.");
return;
}
resultValue = mass / (mw * molarity); // Result in Liters
if (volUnit === 'mL') {
resultText = "Required Volume: " + (resultValue * 1000).toFixed(2) + " mL";
} else {
resultText = "Required Volume: " + resultValue.toFixed(4) + " L";
}
}
var resBox = document.getElementById('mol_result_box');
var resTxtElem = document.getElementById('mol_result_text');
resTxtElem.innerHTML = resultText;
resBox.style.display = "block";
}