Concentration

 This is part of Year 11 HSC Chemistry course under the topic of Molarity

HSC Chemistry Syllabus

  • Manipulate variables and solve problems to calculate concentration, mass or volume using:
    • C = `\frac{n}{v}` (molarity formula (ACSCH063)

    Concentration

     

    Concentration describes the amount of solute present in a specified volume of solvent.

     

    A solution is a homogeneous mixture wherein a solute is dissolved in a solvent. For instance, salt water consists of the salt solute dissolved in water as the solvent.

     

    The descriptors 'dilute' and 'concentrated' indicate the relative concentrations of solutions.

     

    A solution is termed 'unsaturated' if it can dissolve more solute at a given temperature.

     

    Conversely, a 'saturated' solution cannot dissolve any more solute at a specified temperature.

     

    Concentration: amount of solute present in solution

    Concentration term

    Amount of solute

    Amount of solution

    Concentration units

     

    Mass/volume

    Mass

    Volume

    gL-1

    Percent by mass

    % w/w or %m/m

    Mass

    Mass

    g/100g

    Mass/volume percentage

    w/v% or m/v%

    Mass

    Volume

    g/100mL

    Volumes/volume percentage

    v/v%

    Volume

    Volume

    mL/100g

    Parts per million

    Ppm

    Mas

    Mass

    Mg kg-1, mg g-1

    Mass

    Volume

    Mg L-1, mg mL-1

    Mole percent 

    %

    Moles

    Moles

    %

    Molarity

    M

    Moles

    Volume

    Mol L-1

    Molar, M

     

    Concentration Units

    The concentration of a solution in grams per litre (`gL^-1`) indicates the mass of the solute in grams dissolved in one litre of the solution. Here's a practical example:

     

    • Example: if the concentration of sodium chloride in seawater is 20 `gL^-1`, this means that in 1 L of seawater, there are 20g of sodium chloride

     

    Formula to calculate concentration in `gL^-1` :

     

    Concentration = `\frac{\text{mass of solute (g)}}{\text{volume of solution (L)}}`

    Understanding the formula is straightforward when considering that "g" represents mass and "L" represents volume in litres. 

     

    Concentration in Parts Per Million (ppm)

    PPM is often used for very dilute solutions, and it can be expressed as weight/weight (w/w) or weight/volume (w/v):

     

    • Weight/volume:  1 ppm = 1mg `L^-1` = 1 µg `mL^-1`
    • Weight/weight: 1 ppm = 1mg `kg^-1` =  1 µg `g^-1` 

     

    Weight/volume ppm conversion Examples 

     

    1. 2 g `L^-1` to ppm:

    Convert grams to milligrams: 2 g = 2000 mg

    The conconcentration is 2000 ppm

    2. 1.5 g `L^-1` to ppm:

    Convert grams to milligrams: 1.5 g = 1500 mg

    The concentration is 1500 ppm

     

    Weight/weight ppm conversion

     

    1. 0.33 g `Kg^-1` to ppm:

    Convert grams to milligrams: 0.33 g = 330 mg

    The conconcentration is 330 ppm

    2. 2000 µg `L^-1` to ppm:

    Convert micrograms to milligrams: 2200µg `\divide` 1000 = 2.2 mg

    The concentration is 2.2 ppm

     

    Concentration in Moles per Litre, Molarity (mol `L^-1`)

    Molarity (M) describes the concentration of a solution given by the number of moles in a litre. It can also be denoted as mol `L^-1` or mol/L. The symbol "c" can represent molarity. 

     Formula:

    C = `\frac{n}{v}`

    where

    • C = concentration of solution in mol `L^-1`
    • n = moles of substance being dissolved
    • v = volume of solution in litres (L)

     

    Examples:

    1. Calculate the concentration of a 0.5L solution with 0.125 moles of NaCl

    c(NaCl) = `\frac{0.125}{0.5}` = 0.25 M

     

    2. Calculate the concentration of a 250 mL solution with 0.02 moles of copper nitrate

    c(NaCl) = `\frac{0.02}{0.25}` = 0.08 M

     

    3. Calculate moles of HCl in 300mL of a 2 M solution

    n = 2 `\times` 0.3 moles = 0.6 moles

     

    4. Calculate the volume of a solution with a concentration of 0.3 M and 0.03 moles

    v = `\frac{0.03}{0.3} = 0.1 L = 100 mL`

     

    Summary

    • Concentrations can be represented in different ways, such as `gL^-1`, ppm, or mol `L^-1`, depending on the context
    • Understanding the units and what they represent helps in deriving and using formulas
    • The conversion between different units of concentration is a fundamental skill in chemistry which often requires only simple multiplication or division. 

     

    BACK TO MODULE 2: INTRODUCTION TO QUANTITATIVE CHEMISTRY