Molarity Calculator (Mass, Volume & Concentration)
Calculate solute mass, molar concentration, or final solution volume instantly. Features pre-loaded common laboratory biochemicals, hydration states, and step-by-step buffer preparation protocols.
Molarity & Solution Preparation Calculator
Calculate mass, molarity, or volume from the reagent molecular weight
1. Select Reagent or Input Molecular Weight
2. Input Solution Target Parameters
- Weigh out 2.9220 g of Sodium Chloride (NaCl).
- Dissolve in approximately 80.0 mL of deionized water (ddH₂O) or buffer.
- Stir thoroughly until completely dissolved. Adjust pH if required.
- Bring final solution volume up to 100 mL using a volumetric flask or graduated cylinder.
Mass (g) = Molarity (mol/L) × Volume (L) × Molecular Weight (g/mol)
Tip: Always adjust pH before bringing the solution to its final volume, as adding concentrated acid (HCl) or base (NaOH) adds extra liquid volume.
Molarity Calculator Formulas & Equation Derivations
1. Primary Molarity Equation
Molarity (M) defines the molar concentration of a chemical solution expressed as moles of solute per liter of solution:
2. Calculating Mass of Solute to Weigh Out
When preparing a reagent at a target concentration (C) and final volume (V):
3. Molarity (M) vs. Molality (m) vs. Normality (N)
Molarity Solution Preparation Bench Protocol
1. Pre-dissolve in 70–80% Final Volume
Never add solute to the final volume directly. Dissolve the weighed reagent in approximately 70–80% of distilled or deionized water (Milli-Q) using a magnetic stir bar.
2. Adjust pH Before Bringing to Final Volume
If pH adjustment is required (such as 1 M Tris-HCl pH 8.0 or 0.5 M EDTA pH 8.0), calibrate the pH meter and titrate with concentrated HCl or NaOH before bringing to the final volume mark.
3. Bring to Final Volume (Q.S.) in a Volumetric Flask
Transfer the solution to a calibrated volumetric flask or graduated cylinder and add ultrapure water until the bottom of the meniscus touches the calibration line.
Molarity Calculator Worked Lab Examples
Example 1: 500 mL of 5 M Sodium Chloride (NaCl) Stock
Mass = 5.0 mol/L × 0.500 L × 58.44 g/mol = 146.10 g
Weigh 146.1 g NaCl, add ~350 mL dH₂O, stir until completely dissolved, then bring to 500 mL and autoclave at 121°C for 20 minutes.
Example 2: 100 mL of 0.5 M EDTA Solution (pH 8.0)
Mass = 0.5 mol/L × 0.100 L × 372.24 g/mol (EDTA·2Na·2H₂O) = 18.61 g
Note: Disodium EDTA dissolves more readily near pH 8.0. Add NaOH gradually while stirring and measuring pH, then bring to final volume after dissolution. The required NaOH amount depends on the reagent and starting conditions.
Frequently Asked Questions
What is the difference between Molarity (M) and Molality (m)?
Molarity (M) is moles of solute per liter of total solution, which is volume-based and slightly varies with temperature due to thermal expansion. Molality (m) is moles of solute per kilogram of pure solvent, which is mass-based and temperature-independent.
Why should I not add the full volume of water to the solute at once?
Dissolving a solid solute increases the total liquid volume (displacement volume). Adding the final volume of solvent directly causes the final concentration to be lower than desired. Always dissolve solute in ~70–80% of final volume, adjust pH, then bring to target volume (Q.S.).
How do hydration states (e.g., anhydrous vs. dihydrate) affect molar mass calculations?
Hydrated chemical salts include bound water molecules in their crystal lattice (such as CaCl2·2H2O or EDTA·2H2O). You must use the formula weight of the hydrated form on your specific bottle; using anhydrous molecular weight will result in under-dosing the solute.
Why does EDTA not dissolve easily when preparing a 0.5 M solution?
Disodium EDTA is insoluble at acidic or neutral pH. It requires a pH of approximately 8.0 for the four carboxyl groups to ionize into their soluble sodium salt form. Add solid NaOH pellets or 10N NaOH while stirring vigorously until the solution clears.
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