Concentration of Solutions

Knowns

• solution concentration 0.90 g NaCl/100 mL • molar mass NaCl 58.5 g/mol

Unknown

• solution concentration =? M

What to do

Convert the concentration from g/100 mL to mol/L. The sequence is g/100 mL → mol/100 mL → mol/L.

What effect does dilution have on the total moles of solute in solution?

Diluting a solution reduces the number of moles of solute per unit volume, but the total number of moles of solute in solution does not change.

Example Problem

Intravenous (IV) saline solutions are often administered to patients in the hospital. One saline solution contains 0.90 g NaCl in exactly 100 mL of solution. What is the molarity of the solution?

Equation for Molarity

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

Moles of solute = molarity (M) × liters of solution (V)

Moles of solute = M₁ × V₁ = M₂ × V₂

Moles of solute before dilution = moles of solute after dilution

Moles of solute before dilution = moles of solute after dilution

What are two ways to express the percent concentration of a solution?

The concentration of a solution in percent can be expressed in two ways: as the ratio of the volume of the solute to the volume of the solution or as the ratio of the mass of the solute to the mass of the solution.

molarity (M)

the concentration of solute in a solution expressed as the number of moles of solute dissolved in 1 liter of solution

=

0.15 mol/L = 0.15M

solution concentration =

(0.90g NaCl/100 mL) x (1mol NaCl/58.5 g NaCl) x (1000 mL/1 L)

Percent by Mass

(mass of solute/mass of solution) x 100%

Percent by Volume

(volume of solute/solution volume) x 100%

How do you calculate the molarity of a solution?

To calculate the molarity of a solution, divide the moles of solute by the volume of the solution.

Solve for the Unknown

Use the molar mass to convert g NaCl/100 mL to mol NaCl/100 mL. Then use the conversion factor between milliliters and liters to convert to mol/L, which is molarity.

concentration

a measurement of the amount of solute that is dissolved in a given quantity of solvent; usually expressed as mol/ L

concentrated solution

a solution containing a large amount of solute

dilute solution

a solution that contains a small amount of solute