How to Convert g/mL to Molarity (with Density and Examples)

You have a bottle of concentrated acid labeled with a density in g/mL and a mass percent and you need to know its molarity to make a working solution. The conversion is one formula, but the step people miss is that density alone only gives you molarity if the reagent is 100% pure. For real reagents you also need the mass percent on the label.

What is the formula to convert g/mL to molarity?

Molarity (M) = (density in g/mL × 1000 × mass percent) ÷ molar mass in g/mol. The 1000 converts mL to L, the mass percent (as a decimal) accounts for the fact that concentrated reagents are not pure and dividing by molar mass converts grams to moles. If the substance is 100% pure (like pure water or a pure liquid reagent), drop the mass percent term.

Worked example for concentrated sulfuric acid: density 1.84 g/mL, 98% w/w, molar mass 98.08 g/mol. Molarity = (1.84 × 1000 × 0.98) ÷ 98.08 = 18.4 M. For concentrated HCl at 1.18 g/mL and 37% w/w with molar mass 36.46 g/mol: (1.18 × 1000 × 0.37) ÷ 36.46 = 12.0 M.

For other reagents, plug your numbers into the molarity calculator to skip the arithmetic.

How do I convert g/L to molarity?

Divide concentration in g/L by molar mass in g/mol. The g unit cancels with g/mol and you are left with mol/L, which is molarity. If you're wondering how this differs from the g/mL case, it's just a factor of 1000 - 1 g/mL is 1000 g/L, so the formula is the same minus the unit conversion step.

Examples for common stock solutions:

• 40 g/L NaOH (molar mass 40 g/mol) = 1.0 M
• 58.44 g/L NaCl (molar mass 58.44 g/mol) = 1.0 M
• 180.16 g/L glucose (molar mass 180.16 g/mol) = 1.0 M
• 9 g/L NaCl (physiological saline) = 0.154 M

How do I convert molarity to g/mL or g/L?

To go the other way, multiply molarity by molar mass to get g/L, then divide by 1000 to get g/mL. Example: a 0.5 M glucose solution is 0.5 × 180.16 = 90.08 g/L, or 0.09008 g/mL. To find the volume needed to deliver a target number of moles, use volume = moles ÷ molarity.

g/mL to molarity conversion chart for common reagents

Densities and percent values vary slightly by manufacturer - always check the certificate of analysis on your specific bottle. Ambient temperature also shifts density by a small amount, but at room temperature the difference is under 0.5%.

Why is my molarity calculation wrong?

Three mistakes account for almost every wrong answer:

• Forgetting the mass percent. Concentrated HCl is not pure HCl - it is 37% HCl in water. If you compute molarity as 1.18 × 1000 ÷ 36.46 you get 32.4 M, which is nearly triple the real value of 12.0 M. Always multiply by the mass percent expressed as a decimal.
• Mixing g/L and g/mL. A factor of 1000 separates them. If your answer is off by exactly 1000, you confused the two.
• Using molecular weight instead of molar mass for hydrates. Copper sulfate pentahydrate (CuSO4·5H2O) has a molar mass of 249.69 g/mol, not 159.61 g/mol. If your reagent is a hydrate, use the hydrated molar mass.

What is the difference between molarity, molality and normality?

Molarity (M) is moles of solute per liter of solution - the standard for most lab work. Molality (m) is moles per kilogram of solvent and is temperature-independent, because it uses mass instead of volume - it's the right unit for boiling point and freezing point calculations. Normality (N) is equivalents per liter and depends on the reaction context: 1 M H2SO4 is 2 N for acid-base titrations, because it donates two protons, but 1 N for redox reactions where only one electron transfers.

Once you have your stock molarity, the next step is usually dilution. Use the dilution calculator (C1V1 = C2V2) to figure out how much stock to take for a target working concentration, or the molarity calculator for any g/mL or g/L conversion not in the table above.