Basic chemistry

# Molarity how to calculate ?

## Molarity

### Molarity :

Molarity is the relationship between the number of moles of a solute and the volume of a solution. Here are some details on how to find the molarity when you have the number of moles, liters, grams or millimeters.

1

### Calculate molarity from moles and volume

1. 1

You must know the basic formula for calculating the molarity. The molarity is equal to the number of moles of a solute divided by the volume of the solution in liters. The formula is therefore the following: molarity = moles of a solute / liters of a solution .

• Example: What is the molarity of a 4.2 liter solution containing 0.75 mol NaCl?
2. 2

Examine the statement more closely. To calculate the molarity, you must have the number of moles and the number of liters. If these data are provided in the statement, you do not need to make any preliminary calculations.

• Example of a problem:
• moles = 0.75 mol NaCl
• volume = 4.2 l
3. 3

Divide the number of moles by the number of liters. This quotient will give you the number of moles per liter of solution, that is, the molarity.

• Example: molarity = moles of solute / liters of solution = 0.75 / 4.2 l = 0.1785742
4. 4

Write down your answer. Round off the number of decimals after the decimal point to the tenth or hundredth, as directed by your teacher. When writing your answer, use the abbreviation “M” for “molarity” and use the chemical abbreviation of the solute in question.

• Example: 0.179 M NaCl

2

### Calculate molarity from mass and volume

1. 1

You must know the basic formula for calculating the molarity. The molarity is equal to the number of moles of a solute divided by the volume of the solution in liters. The formula is therefore the following: molarity = moles of a solute / liters of a solution .

• Example: what is the molarity of a solution resulting from the dissolution of 3.4 g of KMnO 4 in 5.2 liters of water?
2. 2

Examine the statement more closely. To calculate the molarity, you must have the number of moles and the number of liters. If these data are not provided in the statement, but you have the volume and mass of the solution, you will need to use both of these data to calculate the number of moles before continuing.

• Example:
• mass = 3.4 g KMnO 4
• volume = 5.2 l
3. 3

Find the molar mass of the solute. In order to calculate the number of moles from the mass in grams of the solute used, you first need to determine the molar mass of the solute. For this you have to add the molar masses of each element found in the solution. Find the molar masses of each element in the periodic table of the elements.

• Example:
• molar mass of K = 39.1 g
• molar mass of Mn = 54.9 g
• molar mass of O = 16.0 g
• total molar mass = K + Mn + O + O + O + O = 39.1 + 54.9 + 16 + 16 + 16 + 16 = 158 g
4. 4

Convert the grams to moles. Now that you have the molar masses of the solute, you need to multiply the number of grams of solute present in the solution by a conversion factor of 1 mol divided by the formula of the mass (molar mass) of the solute. You will then obtain the number of moles of solute for this equation.

• Example: grams of solute * (1 / molar mass of solute) = 3.4 g * (1 mol / 158 g) = 0.0215 mol.
5. 5

Divide the number of moles by the number of liters. Now that you have the number of moles, you can divide that value by the number of liters of solution to find the molarity.

• Example: molarity = moles of solute / liters of solution = 0.0215 mol / 5.2 l = 0.004134615
6. 6

Write down your answer. You should round the number of decimals after the decimal point as instructed by your teacher. Generally, you leave two or three digits after the decimal point. In addition, when writing your answer, use the abbreviation “M” for “molarity” and name the compound in question.

• Example: 0.004 M KMnO 4