# 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.