SCH 3U - Solution Concentration

Solutions

A solution can either be dilute or concentrated, depending on how much solute is dissolved in the solvent – this is a qualitative measure of the concentration of a solution. 

 

We can also get a quantitative measure of concentration:

• percentage concentration – m/V, m/m or V/V
• parts per million (ppm) or parts per billion (ppb)
• molar concentration (usually referred to as simply “concentration’)

Percentage Concentration

Sometimes concentrations are expressed as percentages.  For instance, the label of a vinegar bottle shows 5% V/V acetic acid (meaning there are 5 mL of acetic acid for every 100 mL of vinegar solution).

Percentage concentration is expressed as m/V (mass by volume), m/m (mass by mass) or V/V (volume by volume):

m/V  ↝ c_solution = m_solute/V_solution × 100, where mass is in g and volume is in mL

m/m ↝ c_solution = m_solute/m_solution × 100, where mass is in g

V/V  ↝ c_solution = V_solute/V_solution × 100, volume is in mL           

ex. A salt solution is formed by mixing 1.50 g of sodium chloride in enough water to make 250 mL of solution.  What is the m/V percentage concentration of salt in the solution?

m_solute = 1.50 g                   c_solution = m_solute/V_solution × 100

V_solution = 250 mL                          = (1.50 g/250 mL) × 100

c = ?                                               = 0.60%

                                         ∴ the salt solution is 0.60% m/V

Parts per Million (Parts per Billion)

This is used when we want to express the concentration of a very dilute solution (ppm – one part solute for every million parts of solution)

ppm ↝ c = m_solute/m_solution × 10⁶, where m is in g

ppb  ↝ c = m_solute/m_solution × 10⁹, where m is in g

ex. In a chemical analysis, 2.2 mg of oxygen was measured in 250 mg of pond water.  What is the concentration of oxygen in ppm?

m_O2 = 0.0022 g            c_O2 = m_O2/ m_H2O × 10⁶

m_H2O = 0.25 g                     = (0.0022 g/0.25 g) × 10⁶

c_O2 = ?                               = 8 800 ppm                     

                                    ∴ the concentration is 8 800 ppm.

Molar Concentration

If we wish to make up a solution of known concentration, we need to know how much solute to measure.

n = cV, where n is the number of moles (mol), c is concentration (mol/L) and V is volume (L)

ex. What amount of lithium bromide is required to make 250 mL of 1.25 mol/L solution?

V = 250 mL                       n = cV

c = 1.25 mol/L                      = (1.25 mol/L)(0.25 L)

n = ?                                   = 0.313 mol                        

                                         ∴ 0.31 mol of lithium bromide is required 

Homework:

m/V, m/m, V/V → Practice, p. 373 # 1, 7 ; p. 375 # 11, 19; p. 376 # 21, 27

ppm → Practice, p. 378 # 31, 33, 38

molar conc → Practice, p. 381 # 41, 42, 47

 

 

Student Questions:
1.  Hey miss, you wonderful woman, can you post answers for p. 381 # 42 & 47?

Sure I can, since you asked so nicely.

                                                

2. I’m struggling with question 21 on page 376.

3.  Please do p. 378 # 33.

 

 

4.  We demand more answers...