What is the formula for molarity from density and purity?

M = (density × purity% × 10) ÷ molar mass. The factor of 10 converts g/mL and percent (÷100) into consistent units: (g/mL × % × 10) = (g/mL × fraction × 1000 mL/L).

Why do I need molar mass?

Molarity is moles per liter. To go from mass concentration (g/L) to molar concentration (mol/L) you divide by molar mass (g/mol), which converts grams into moles.

Where do I find density and purity for my acid?

Both values are printed on the reagent bottle label and in the product's Safety Data Sheet (SDS). For example, concentrated H2SO4 is typically 1.84 g/mL at 98% purity.

What does the factor 10 represent?

It combines two unit-conversion steps: multiplying by 1000 to convert mL to L, and dividing by 100 to convert percent to a decimal fraction. 1000 ÷ 100 = 10.

Can I use this for bases or other solutions?

Yes. The formula applies to any solution where you know the density, mass percent of the solute, and the molar mass of the solute. It is not limited to acids.

What if purity is listed as a decimal (e.g. 0.98 instead of 98%)?

Enter it as a percentage: 98. The calculator expects a value between 0 and 100. Entering 0.98 would give a result 100 times too small.

Molarity of Acid from Density

Name: Molarity of Acid from Density
Availability: InStock
Author: Nham Vu

Enter the density, percent purity, and molar mass of a concentrated acid to instantly calculate its molar concentration (mol/L).

Molarity Formula

M = (density × purity% × 10) ÷ molar mass

Common Acids (auto-fill)

Density (g/mL)

Mass Percent Purity (%)

Molar Mass (g/mol)

Result

Fill in the values and click Calculate.

Common Concentrated Acids

Acid	Density	Purity	~Molarity
H₂SO₄	1.84 g/mL	98%	18.4 M
HCl	1.19 g/mL	37%	12.1 M
HNO₃	1.41 g/mL	68%	15.2 M
CH₃COOH	1.05 g/mL	100%	17.5 M

Formula: M = (ρ × P% × 10) ÷ MW

Summary

Enter the density, percent purity, and molar mass of a concentrated acid to instantly calculate its molar concentration (mol/L).

How it works

Enter the density of the acid solution in grams per milliliter (g/mL).
Enter the mass percent purity (the percentage shown on the reagent bottle label).
Enter the molar mass of the pure acid in grams per mole (g/mol).
Click Calculate to apply the formula M = (density × purity × 10) ÷ molar mass.
The result shows the molarity in mol/L with a detailed step-by-step derivation.
Use the preset buttons for common lab acids to auto-fill known values.

Use cases

Preparing dilute acid solutions from concentrated stock reagents.
Verifying the molar concentration of commercially supplied acids.
Calculating how many moles of acid are contained in a given volume.
Setting up stoichiometric calculations for neutralization reactions.
Teaching solution chemistry and unit-conversion concepts.
Cross-checking SDS (Safety Data Sheet) concentration data.
Converting between density/purity specs and molarity for lab protocols.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu