Concentration and Standard Solution – HSC Chemistry
This is part of Year 11 HSC Chemistry course under the topic of Molarity.
HSC Chemistry Syllabus
 Conduct practical investigations to determine the concentrations of solutions and investigate the different ways in which concentrations are measured
 Manipulate variables and solve problems to calculate concentration, mass or volume using:
Concentration and Standard Solution Explained
What is 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 as Molarity (mol L^{–1})
Molarity (mol/L or M) is a common way to express the concentration of a solution, given by the number of moles in a litre.
Molarity is given by:
$$c = \frac{n}{V}$$
where:
 `c` = concentration of solution in mol L^{–1}
 `n` = moles of substance being dissolved
 `V` = volume of solution in litres (L)
Molarity Calculation Examples
Concentration Units
Since there are numerous ways to describe the quantity of solutes and the amount of solvent, the unit for concentration can also be expressed in various ways.
Concentration: amount of solute present in solution 

Concentration term 
Amount of solute 
Amount of solution 
Concentration units

Mass/volume 
Mass 
Volume 
g L^{1} 
Percent by mass % w/w or %m/m 
Mass 
Mass 
g /100 g 
Mass/volume percentage w/v% or m/v% 
Mass 
Volume 
g /100 mL 
Volumes/volume percentage v/v% 
Volume 
Volume 
mL /100 g 
Parts per million ppm 
Mass 
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 in Grams Per Litre (g L^{–1})
The concentration of a solution in grams per litre (g L^{–1}) indicates the mass of the solute in grams dissolved in one litre of the solution.
Here's a practical example:
If the concentration of sodium chloride in seawater is 20 g L^{–1}, this means that there are 20 g of sodium chloride in 1 L of seawater
To calculate concentration in g L^{–1} we simply divide the mass of solute by the volume in litres.
$$c= \frac{\text{mass of solute (g)}}{\text{volume of solution (L)}}$$
Converting Between Molarity and g L^{–1}
Recall that the number of moles of a substance is given by:
$$n = \frac{m}{MM}$$
where `M` is the molar mass of the substance.
To convert from molarity (mol L^{–1}) to g L^{–1}, we simply need to divide the concentration by the molar mass.
To convert from g L^{–1} to mol L^{–1}, we multiply the concentration by the molar mass.
Examples
1. 10.0 g of NaOH is added to 250.0 mL of water. Calculate the molarity of the solution.
$$n = \frac{m}{MM}$$
$$n = \frac{10.0}{22.99 + 16.00 +1.008} = 0.25 \text{ mol}$$
$$c = \frac{n}{V}$$
$$c = \frac{0.25}{0.25} = 1.00 \text{ mol/L (3 s.f.)}$$
2. Convert 2.00 mol L^{–1 }of HCl solution to g L^{–1}.
$$MM = 1.008 + 35.45 = 36.458 \text{ g/mol}$$
$$c = 2.00 \times 36.458 = 72.9 \text{ g/L (3 s.f.)}$$
Concentration in Parts Per Million (ppm)
Parts per million (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 = 1 mg L^{–1} = 1 µg L^{–1}
 Weight/weight: 1 ppm = 1 mg kg^{–1} = 1 µg kg^{–1}
Examples
 2 g L^{–1 }to ppm:
Convert grams to milligrams: 2 g = 2000 mg
The concentration is 2000 ppm
 0.33 g kg^{–1} to ppm:
Convert grams to milligrams: 0.33 g = 330 mg
The concentration is 330 ppm
What is a Standard Solution?
A standard solution is a solution whose concentration is known accurately. It's prepared by dissolving a primary standard, a specific solute, in an appropriate solvent
Primary Standard
A primary standard is a solute used to prepare a standard solution. It must fulfil the following criteria:
 High level of purity
 Accurately known composition
 Stable and unaffected by air when weighing
 Readily soluble (in water for high schoollevel studies)
 High molar weight
 Quickly and completely reacting
Certain substances, such as hydrochloric acid, are not suitable as primary standards due to their tendencies to fume, absorb moisture in the air, or effervesce when in contact with water.