Calculating Molarity
3
Calculat ing Molarity Home The properties and behavior of many solutions depend not only on the nature of the solute and solvent but also on the concentration of the solute in the solution. Chemists use many different units when expressing concentration; however, one of the most common units is molarity. Molarity (M) is the concentration of a solution expressed as the number of moles of solute per liter of solution: Molarity (M) = moles solute liters solution For example, a 0.25 M NaOH solution (this is read as 0.25 molar) contains 0.25 moles of sodium hydroxide in every liter of solution. Anytime you see the abbreviation M you should immediately think of it as mol/L. In order to calculate the molarity of a solution, you need to know the number of moles of solute and the total volume of the solution. To calculate molarity: 1. Calculate the number of moles of solute present. 2. Calculate the number of liters of solution present. 3. Divide the number of moles of solute by the number of liters of solution. The following example will illustrate both methods. Example: What is the molarity of a solution prepared by dissolving 15.0 g of sodium hydroxide in enough water to make a total of 225 mL of solution?
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Transcript of Calculating Molarity
Calculating
Molarity Home The properties and behavior of many solutions depend not
only on the nature of the solute and solvent but also on the concentration of the solute in the solution. Chemists use many different units when expressing concentration; however, one of the most common units is molarity. Molarity (M) is the concentration of a solution expressed as the number of moles of solute per liter of solution:
Molarity (M) = moles soluteliters solution
For example, a 0.25 M NaOH solution (this is read as 0.25 molar) contains 0.25 moles of sodium hydroxide in every liter of solution. Anytime you see the abbreviation M you should immediately think of it as mol/L.
In order to calculate the molarity of a solution, you need to know the number of moles of solute and the total volume of the solution.
To calculate molarity:
1. Calculate the number of moles of solute present.2. Calculate the number of liters of solution present.3. Divide the number of moles of solute by the number of
liters of solution.
The following example will illustrate both methods.
Example: What is the molarity of a solution prepared by dissolving 15.0 g of sodium hydroxide in enough water to make a total of 225 mL of solution?
Method 1:
Calculate the number of moles of solute present.
mol NaOH = 15.0g NaOH x 1 mol NaOH40.0 g NaOH
mol NaOH = 0.375 mol NaOH
Calculate the number of liters of solution present.
L soln = 225 mL x 1 L = 0.225 L soln1000 mL
Divide the number of moles of solute by the number of liters of solution.
M = 0.375 mol NaOH = 1.67 M NaOH
0.225 L solnPractice
Problems1. What is the molarity of a solution that contains 1.724
moles of H2SO4 in 2.50 L of solution?2. What is the molarity of a solution prepared by dissolving
25.0 g of HCl (g) in enough water to make 150.0 mL of solution
MolaritySolutions to
Practice Problems
Home 1. What is the molarity of a solution that contains 1.724 moles of H2SO4 in 2.50 L of solution?
o Calculate the number of moles of solute present.mol H2SO4 = 1.724 mol (given)
o Calculate the number of liters of solution present.L soln = 2.50 L (given)
o Divide the number of moles of solute by the number of liters of solution.
M =1.724 mol H2SO4 = 0.690 M H2SO42.50 L soln
o
2. What is the molarity of a solution prepared by dissolving 25.0 g of HCl (g) in enough water to make 150.0 mL of solution?
