4 Ways to Find Molarity

2024 Author: Samantha Chapman | [email protected]. Last modified: 2024-01-15 13:47

Molarity describes the ratio of moles of a solute to the volume of a solution. For a more detailed explanation on how to find molarity by having moles, liters, grams, and / or milliliters, read on.

Steps

Method 1 of 4: Calculate the Molarity with Moles and Volume

Step 1. Learn the basic formula for calculating molarity

Molarity is equal to the number of moles of the solute divided by the volume of the solution in liters. For this reason, it is written as: molarity = moles of solute / liters of solution

Example problem: What is the molarity of a solution containing 0.75 mol of NaCl in 4.2 liters?

Step 2. Examine the problem

Identifying molarity requires having the number of moles and the number of liters. If the problem provides any of these quantities, no preliminary calculations are needed.

• Example problem:

  • Moles = 0.75 mol of NaCl
  • Volume = 4, 2 L.

  

  Step 3. Divide the number of moles by the number of liters

  The resulting quotient will give you the number of moles per liter of solution, otherwise known as molarity.

  Example problem: molarity = moles of solute / liters of solution = 0.75 mol / L 4.2 = 0.17857142

  

  Step 4. Write your answer

  Round off the number of digits after the decimal point to two or three, according to your teacher's preference. When you write the answer, abbreviate "molarity" with "M" and add the chemical abbreviation of the solute involved.

  Example problem: 0.19 M NaCl

  Method 2 of 4: Calculate the Molarity with Mass and Volume

  

  Step 1. Learn the basic formula for calculating molarity:

  expresses the ratio between the number of moles of solute and the liters of solution or the volume of this solution. In the form of a formula, molarity is expressed as follows: molarity = moles of solute / liters of solution

  Example problem: What is the molarity of a solution obtained by dissolving 3.4 g of KMnO₄ in 5, 2 liters of water?

  

  Step 2. Examine the problem

  Finding molarity requires you to have the number of moles and the number of liters. If you have the volume and mass of the solution, but the number of moles is not given, you need to use these two digits to calculate the number of moles before continuing.

  • Example problem:

    • Mass = 3.4 g of KMnO₄
    • Volume = 5.2 L.

    

    Step 3. Find the molar mass of the solute

    In order to calculate the number of moles from the mass or grams of solute used, you must first determine the molar mass of the solute. This can be done by adding the separate molar masses of each element found in the solution. Find the molar mass of each element using the periodic table of elements.

    • Example problem:

      • 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 = 158.0 g

      

      Step 4. Convert grams to moles

      Now that you have the molar mass of the solute, you need to multiply the number of grams of solute in the solution by a conversion factor of 1 mole to the formula weight (molar mass) of the solute. This will give you the number of moles of solute for this equation.

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

      

      Step 5. Divide the number of moles by the number of liters

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

      Example problem: molarity = moles of solute / liters of solution = 0.0215 mol / 5.2 L = 0.004134615

      

      Step 6. Write your answer

      You must round the number of digits after the decimal point in the place requested by your teacher. Usually, this will be two or three places after the decimal point. Also, when you write your answer, abbreviate "molarity" with "M" and state which solute it is.

      Example problem: 0, 004 M of KMnO₄

      Method 3 of 4: Calculate the Molarity with Moles and Milliliters

      

      Step 1. Learn the basic formula for calculating molarity

      To find molarity, you need to calculate the number of moles of solute in a solution per liter of solution. Milliliters cannot be used. The general formula used to express molarity is written as: molarity = moles of solute / liters of solution

      Example problem: What is the molarity of a solution containing 1.2 moles of CaCl₂ in 2.905 milliliters?

      

      Step 2. Examine the problem

      Calculating molarity requires knowing the number of moles and the number of liters. If the volume is given in milliliters instead of liters, you need to convert the volume to liters before continuing with your calculations.

      • Example problem:

        • Moles = 1.2 mol of CaCl₂
        • Volume = 2.905 ml

        

        Step 3. Convert milliliters to liters

        Find the number of liters by dividing the number of milliliters by 1,000, as there are 1,000 milliliters in 1 liter. Note that you could also just move the decimal point three places to the left.

        Example problem: 2,905ml * (1L / 1,000ml) = 2,905L

        

        Step 4. Divide the number of moles by the number of liters

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

        Example problem: molarity = moles of solute / liters of solution = 1.2 moles of CaCl₂ / 2, 905 L = 0, 413080895

        

        Step 5. Write your answer

        Complete the number of digits after the decimal point to the amount requested by your teacher (usually two or three places). When writing the answer, you should also abbreviate "molarity" to "M" and give the name of the solute.

        Example problem: 0.413 M of CaCl₂

        Method 4 of 4: Another Practical Problem

        

        Step 1. Find the molarity of a solution made by dissolving 5.2g of NaCl in 800ml of water

        Identify the values given by the problem: mass in grams and volume in milliliters.

        • • Mass = 5.2 g of NaCl
          • Volume = 800 ml of water

          

          Step 2. Find the molar mass of NaCl

          To do this, add the molar mass of sodium, Na, and that of chlorine, Cl.

          • The molar mass of Na = 22.99 g
          • The molar mass of Cl = 35.45 g
          • The molar mass of NaCl = 22.99 + 35.45 = 58.44 g

          

          Step 3. Multiply the solute mass by the molar mass conversion factor

          In this case, the molar mass of NaCl is 58.44 g, so it's conversion factor is 1 mol / 58.44 g.

          Moles of NaCl = 5.2 g NaCl * (1 mol / 58.44 g) = 0.08898 mol = 0.09 mol

          

          Step 4. Divide the 8,000ml of water by 1,000

          Since there are 1,000 milliliters per liter, you need to divide the number of milliliters of this problem by 1,000 to find the number of liters.

          • You could also interpret the operation as if it were a question of multiplying 8,000 ml by a conversion factor of 1 L / 1,000 ml.
          • To speed up the process, you could just move the decimal three places to the left, rather than multiplying or dividing by something.
          • Volume = 800ml * (1L / 1,000ml) = 800ml / 1,000ml = 0.8L

          

          Step 5. Divide the number of moles of solute by the number of liters of solution

          To find molarity, you need to divide 0.09 mol, the number of moles of NaCl solute, by 0.8 L, the volume of the solution in liters.

          molarity = moles of solute / liters of solution = 0.09 mol / 0.8 L = 0.1125 mol / L

          

          Step 6. Reorder your answer

          Round off your answer to two or three decimal places and shorten the molarity with "M".