Lesson 7: Concentrations & Consumer Products
Percentage Concentration
Percentage Volume/Volume (% V/V)
cV/V = volume of solute/volume of solvent x 100%
Percentage Weight/Volume (% W/V)
cW/V = weight of solute/volume of solvent x 100%
Percentage Weight/Weight (% W/W)
cW/W = weight of solute/weight of solvent x 100%
Sample Problem #1 (% V/V)
A photographic “stopbath” contains 140 mL of pure acetic acid in a 500 mL bottle of solution. What is the percentage by volume concentration of acetic acid?
Given
VHC2H3O2(l) = 140 mL
VHC2H3O2(aq) = 500 mL
cHC2H3O2 = ?
cHC2H3O2 = VHC2H3O2(l) / VHC2H3O2(aq) x 100%
cHC2H3O2 = 140 mL / 500 mL x 100%
cHC2H3O2 = 28.0% V/V
Sample Problem #2 (% W/V)
What is the weight/volume concentration of hydrogen peroxide when 4.5 g of hydrogen peroxide is within a 150 mL solution
Given
WH2O2(l) = 4.5 g
VH2O2(aq) = 150 mL
cH2O2 = ?
cH2O2 = WH2O2(l) / VH2O2(aq) x 100%
cH2O2 = 4.5 g /150 mL x 100%
cH2O2 = 3.0% W/V
Sample Problem #3 (% W/W)
A sterling silver ring has a mass of 12.0 g and contains 11.1 g of pure silver. What is the percentage weight by weight concentration of silver in the ring?
Given
WAg = 11.1 g
Walloy = 12.0 mL
cAg = ?
cAg = WAg / Walloy x 100%
cAg = 11.1 g / 12.0 g x 100%
cAg = 92.5 % W/V
Extremely Low Concentrations
Very dilute concentrations are often expressed in parts per million (ppm, 1:106), parts per billion (ppb, 1:109) and even parts per trillion (ppt, 1:1012)
“Parts per” concentrations are actually a special case of % W/W concentrations. Since a very dilute aqueous solution is very similar to pure water, it density is assumed to be 1 g/mL at 20 ℃. Therefore, a solute concentration of 1 ppm means that there is 1 g of solute in every 1 000 000 g or 1 000 000 mL of solution. \n
cppm = msolute / msolution x 10^6 \n
cppb = msolute / msolution x 10^9
cppt = msolute / msolution x 10^12
Parts Per Equivalences
Sample Problem #4 (ppm)
Dissolved oxygen in natural water is an important measure of the health of hte ecosystem. In a chemical analysis of 250 mL of water at SATP, 2.2 mg of oxygen was measured. What is the concentration of oxygen in parts per million?
\n
Given
msolute = 2.2 mg = 0.0022g
msolvent = 250 g
cppm = ?
\n
cppm = msolute / msolution x 10^6
cppm = 0.0022 g / 250 g x 10^6
cppm = 8.8 ppm
