Molarity Calculator

Find molarity, moles, volume, or the mass you need to weigh out. Pick what you want to solve for and fill in the rest.

What molarity actually measures

Molarity tells you how much dissolved substance sits in each liter of a solution. It is written with a capital M, so 2 M means two moles per liter. Chemists also write it as mol/L.

Molarity (M) = moles of solute ÷ liters of solution

One detail trips up almost everyone at first. The volume in that formula is the volume of the finished solution, not the volume of water you started with. When a solid dissolves it takes up space of its own, so 1 liter of water plus a scoop of salt gives you slightly more than 1 liter of salt water. That is why lab recipes say to add water up to the mark rather than adding a set amount of water.

The formula, rearranged three ways

All four quantities come from the same relationship, so you can solve for whichever one is missing:

In every row, V is in liters and n is in moles. If your volume is in milliliters, divide by 1000 first. The calculator above does that step for you.

Worked example: finding the molarity

Suppose you dissolve 58.44 g of table salt in enough water to make 500 mL of solution. What is the molarity?

Step 1. Turn grams into moles. The molar mass of NaCl is 58.44 g/mol, so:
n = 58.44 g ÷ 58.44 g/mol = 1.000 mol

Step 2. Turn milliliters into liters.
500 mL ÷ 1000 = 0.500 L

Step 3. Divide.
M = 1.000 mol ÷ 0.500 L = 2.00 M

Worked example: weighing out a solution

Now go the other way. You need 250 mL of 0.100 M glucose. How much solid do you weigh?

Step 1. Find the moles you need.
n = 0.100 mol/L × 0.250 L = 0.0250 mol

Step 2. Turn moles into grams. Glucose is C₆H₁₂O₆, molar mass 180.16 g/mol.
m = 0.0250 mol × 180.16 g/mol = 4.50 g

So you weigh 4.50 g, tip it into a 250 mL volumetric flask, swirl until it dissolves, then top up to the line.

How to prepare a solution in the lab

The order of these steps matters, and skipping one is the most common way students end up with the wrong concentration.

1. Work out the mass you need, then weigh it on a balance.
2. Tip the solid into a volumetric flask of the right size.
3. Add solvent until the flask is roughly two thirds full, then swirl until everything dissolves.
4. Add more solvent slowly until the bottom of the meniscus sits on the etched line.
5. Cap the flask and invert it several times so the contents mix evenly.

Never dissolve the solid directly in a full flask. If the solid has not dissolved by the time you reach the mark, you cannot add more liquid without ruining the concentration.

Molarity of some everyday solutions

Molarity, molality, and percent

These three sound alike and get mixed up constantly, so it is worth pinning down the difference.

Molarity shifts with temperature because liquids expand when warmed. Heat a 1.00 M solution and the same moles now spread through a slightly larger volume, so the molarity drops a little. Molality uses mass instead of volume, and mass does not change with temperature, which is why freezing point and boiling point calculations use molality rather than molarity.

Mistakes worth avoiding

• Using the volume of solvent. The formula wants the volume of the finished solution.
• Forgetting to convert milliliters. A 250 mL flask is 0.250 L. Leaving it as 250 makes your answer 1000 times too small.
• Ignoring ions. A 1 M solution of CaCl₂ is 1 M in the compound but 2 M in chloride ions, because each unit releases two of them.
• Reading the meniscus wrong. Read the bottom of the curve, at eye level.
• Rounding the molar mass too early. Carry the extra digits until the final step.

Frequently asked questions

Related terms

• Molarity
• Concentration
• Solute
• Molality
• Titration
• Aqueous

Next: use the dilution calculator to water a stock solution down, or the molar mass calculator to get the g/mol figure for any formula.