Molarity Calculator
Please provide any three values in the fields below to calculate the fourth value in the molarity equation for solutions:
Molarity = MassMolecular Weight × Volume
Molarity (M) is one of the most commonly used units for measuring the concentration of a solution in chemistry. It helps express how much solute (the substance being dissolved) is present in a specific volume of solvent (the substance doing the dissolving), usually water. Molarity plays a critical role in laboratory experiments, pharmaceutical preparations, and industrial chemical processes.
Molarity is expressed in moles per liter (mol/L). The mole is a standard unit in chemistry used to count particles such as atoms, molecules, or ions. One mole of a substance contains approximately 6.022 × 1023 particles, a number known as Avogadro's number.
The unit of molarity, M or mol/L, simply tells you how many moles of solute are dissolved in one liter of solution. For example, a 1 M (1 mol/L) solution contains 1 mole of solute in every liter of solution. For very low concentrations, mM (millimoles per liter), μM (micromoles per liter), or other smaller units are also used as units of molarity.
By its definition, the basic formula for molarity is:
M = nV
Where:
- M is the molarity of the solution (mol/L),
- n is the number of moles of solute (mol),
- V is the volume of the solution (L).
If you dissolve 2 moles of sodium chloride (NaCl) in 1 liter of water, what is the Molarity of the solution?
M = nV = 2 mol1 L = 2 M
This means the sodium chloride solution has a concentration of 2 M (2 moles per liter).
If you have a 1 M solution of hydrochloric acid (HCl) and need 0.5 moles of HCl, what volume of solution is required?
V = nM = 0.5 mol1 M = 0.5 L
So, 0.5 liters (or 500 mL) of the 1 M HCl solution is required to obtain 0.5 moles of HCl.
Molarity with Molecular Weight
Often, you don't have the number of moles directly but instead know the mass of the solute. To calculate molarity in such cases, you need to convert the mass of the solute to moles using its molecular weight (also called molar mass). The molecular weight is the mass of one mole of a substance, typically expressed in grams per mole (g/mol).
Based on this definition, the formula to calculate molarity when molecular weight is involved is:
M = mMW V = mMW×V
Where:
- M is the molarity (mol/L),
- m is the mass of the solute (g),
- MW is the molecular weight of the solute (g/mol),
- V is the volume of the solution (L).
Suppose you have 10 grams of sodium chloride (NaCl) and dissolve it in water to form a solution of 500 mL (0.5 L). The molecular weight of NaCl is 58.44 g/mol. To find the molarity of the solution:
M = mMW×V = 10 g58.44 g/mol × 0.5 L = 0.342 M
Thus, the molarity of the sodium chloride solution is 0.342 M.
Suppose you need to prepare 2 liter of a 1 M solution of potassium nitrate (KNO3). The molecular weight of KNO3 is 101.1 g/mol. How much potassium nitrate should you weigh out? First, rearrange the molarity equation to solve for the mass of solute m:
m = M×MW×V = 1 M × 101.1 g/mol × 2 L = 202.2 g
So, you need to weigh out 202.2 grams of potassium nitrate to make 2 liter of a 1 M solution.