Lesson 4: Concentrations of Solutions, Dissociation Equations
Measuring Concentration of Solutions
1. Concentration as Mass/Mass Percent (solid solutions)
Mass Percent (% w/w) = Mass of solute (g) x 100%
Mass of solution (g)
Example 1: Calcium chloride is used to melt ice on roads. A sample of slush had a mass
of 23.47 g. When the solution was evaporated the residue had a mass of 4.58 g. What
was the mass/mass percent of calcium chloride in the slush?
Mass Percent (% w/w) = Mass of solute (g) x 100%
Mass of solution (g)
= 4.58 g x 100%
23.47 g
= 19.5 %
2. Concentration as Volume/Volume Percent (mixing two liquids)
Volume/volume (% v/v) = Volume of solute (mL) x 100%
Volume of solution (mL)
Example 2: Rubbing alcohol is 70% (v/v), 70 mL alcohol in 100 mL water
What volume of isopropyl alcohol is used to make 500 mL of rubbing alcohol?
Volume/volume (% v/v) = Volume of solute (mL) x 100%
Volume of solution (mL)
Volume of solute = 500 mL x 0.70
= 350 mL
3. Concentration as Mass/volume Percent (mixing a solid in a liquid)
Example 3: A pharmacist dissolves 1.50 g of a powdered drug to a final volume of 2.00
mL. What is the concentration of the drug in g/100 mL?
Mass/volume (% w/v) = Mass of solute (g)
x 100%
Volume of solution (mL)
= 1.50 g x 100%
2.00 mL
= 75.0 %
4. Molar Concentration
Molarity: the number of moles of solute that can dissolve in 1 Liter of solution.
Molarity = c= n
v
where c= molarity (mol/L or M), n = mol, v = volume (in L)
Example 4: What is the molarity of 0.750 mol of HCl in 335 mL of solution?
c=n
v
c = 0.750 mol
0.335 L
= 2.24 mol/L or M
Example 5: What mass of hydrogen chloride is present in 250.0 mL of a 20.0% solution
of hydrochloric acid whose density is 1.10 g/mL?
Step 1
Volume/volume (% v/v) = Volume of solute (mL) x 100%
Volume of solution (mL)
Volume of solute = 250 mL x 0.20
= 50 mL
Step 2
d=m
v
So m = v x d
m = 50 mL x 1.10 g
mL
= 55.0 g
Dissociation Equations
KCl(s) → K+(aq) + Cl- (aq)
CuSO4(s)
→ Cu+2(aq) + SO4-2(aq)
CuCl2(s)
→
Fe2(SO4)3(s)
Cu+2(aq) + 2Cl-(aq)
→
2Fe+3(aq) + 3SO4-2(aq)
Concentration of ions in solution
Example 6: A 0.5 M solution of calcium chloride(CaCl2) was prepared. What is the
concentration of calcium ions and chloride ions in the solution?
CaCl2(s)
0.5 M
→ Ca+2(aq) + 2Cl-1(aq)
0.5 M
2 x 0.5 M
=1M
Example 7: Calculate the concentration (in mol/L) of chloride ions in the solution
a) 19.8 g of CaCl2 in 150 mL of solution
a) CaCl2(s) → Ca+2(aq) + 2Cl- (aq)
Given g of CaCl2 and 100 mL (volume of solution)
We can solve for moles, then for M, then ion concentration
n CaCl2= 19.8 g
111.1 g/mol
= 0.1782 mol
c CaCl2 = n
v
= 0.1782 mol
0.150 L
= 1.89 M
CaCl2(s)
1.89 M
→ Ca+2(aq) + 2Cl-1(aq)
1.89 M
2 x 1.89 M
= 3.78 M
Problems
1. Determine the dissociation equations for the following ionic solutes in water.
a. KCl
b. CaCl2
c. Ca(NO3) 2
d. MgCl2
2. What is the concentration of each ion in solution in #1 if the solution molarity is
0.20 M?
3. Calculate the amount of each ion (in mol.L) in solution #1 c if 5.00 g of ionic
compound is dissolved in 1.0 L.
4. A solution contains 25.0 g of solute dissolved in 100.0 g of water. What is its
mass/mass percent? How much solute is contained in 20.0 g of the solution?
5. If I make a solution by adding water to 75 mL of ethanol until the total volume of
the solution is 375 mL, what’s the percent by volume of ethanol in the solution?
6. What is the molarity of the following solutions?
a. 0.8 moles of magnesium acetate in 5 liters of solution.
b. 0.75 moles of lithium fluoride in 65 mL of solution.
c. 120 grams of calcium nitrite in 240 mL of solution.
7. If 65 g of sugar is dissolved in 750ml of water what is the concentration of the
solution?
8. Which is more concentrated 34 g of salt dissolved in 100 ml of water or 100 g of
salt in 1500 ml of water?
9. What mass, in grams, of calcium nitrate are there in 867 mL of a 2.00 M calcium
nitrate solution?
10. A solution was prepared by dissolving 70.0 g of silver nitrate in water in a 1.0 L
volumetric flask. What is the molarity of the solution?
11. How many grams of solute are present in 25.0 mL of a 2.00 M solution of
potassium chloride?
12. The density of a 60.00 % (m/v) ethanol, C2H5OH, solution is 0.8937 g/mL.
Assuming a volume of 1000 L
a. What mass of solute is dissolved in 1000 mL (536.2 g)
b. What is the molarity of the solution? (molar mass = 46 g/mole) (11.66 M)