This article provides an overview of the units of measurement commonly used in the preparation of solutions, including mass, relative mass of elements and substances, volume, density, quantity of substance, and molar mass, and explains the conversion relationship of these units. The article also details the different ways to represent the concentration of a solution, such as the amount concentration, mass concentration, mass molar concentration of a substance, etc., and how to calculate the concentration. This knowledge helps to accurately prepare and analyze solutions.
Section 1 Units of measurement commonly used in the preparation of solutions
quality
Unit: kilogram (kg), gram (g), milligram (mg), microgram (μg)
Relationship: 1 kg = 1000 g, 1 g = 1000 mg, 1 mg = 1000 μg
The relative atomic mass of an element
The ratio of the average atomic mass of an element to 1/12 of the atomic mass of carbon-12
Symbol – A (dimensionless)
The relative molecular mass of a substance
The ratio of the average mass of a molecule or unit of matter to 1/12 of the atomic mass of carbon-12
Symbol – M (dimensionless)
volume
Units: cubic meters (m³), liters (L), milliliters (mL or ml), microliters (μL or μl)
Relationship: 1 m³ = 1000 L, 1 L = 1000 mL, 1 mL = 1000 μL
density
The ratio of the density of a substance to the density of a reference material
Units – kilograms per cubic meter (kg/m³), grams per cubic centimeter (g/cm³), grams per milliliter (g/mL)
The amount of matter
The ratio of the number of units of matter to the number of carbon-12 atoms of 0.012 kg
Unit – Mole (mol)
Avogadro's constant: 6.022 × 10²³
Molar mass
Mass (m) divided by the amount of substance (n)
Units – kilograms per mole (kg/mol), grams per mole (g/mol)
Calculation formula: M = m/n
A comparison of the old and new units of measurement that is common in the laboratory
Compare the names and concepts of the old and new units of measurement, introduce the evolution of the units of measurement, and help understand their practical application and importance in the laboratory.
Section 2 Representation of solution concentration and its calculation
Representation of the concentration of the solution
Quantity concentration of a substance (c) – indicates the amount of substance in the solute per liter of solvent in the solution. Commonly used units are mol/L (moles per liter) or mmol/L (millimoles per liter).
Mass concentration (p) – indicates the mass of the solute per liter of solvent in the solution. Commonly used units are g/L (grams per liter) or mg/L (milligrams per liter).
Mass molar concentration (b) – indicates the amount of solute per kilogram of solvent. The unit is mol/kg (mol/kg).
Mass fraction (w) – indicates the mass of the solute as a percentage of the total mass of the solution. The unit is in percentage (%).
Volume fraction – indicates the volume of solute as a percentage of the total volume of the solution. The unit is in percentage (%).
Volumetric specific concentration – Indicates the ratio of solute volume to solvent volume, commonly used for concentration representation of gas and liquid mixtures.
Titer (TBA) – Indicates the relationship between a unit volume of a standard titrant and the mass of the substance being measured, and is commonly used as a standard solution in analytical chemistry.
The amount of concentration of the substance
The quantity concentration of a substance refers to the amount of substance contained in a unit volume of solution.
c: Quantity and concentration of the substance (mol/L)
n: Quantity of substance (mol)
V: Solution volume (L)
Symbols –It is usually denoted by c, but it can also be written as [B] for chemical reaction equilibrium.
Commonly used units are mol/L or mmol/L.
Calculation of solution concentration
The concentration is calculated by the relationship between the quantity (n), mass (m), molar mass (M), volume (V) and other quantities of the substance.
This includes the conversion between mass concentration, quantity concentration of the substance, and mass molar concentration.
Molar mass (M): The amount of substance contained in a unit mass of substance.
Calculate the solute mass or the amount of a substance using molar mass: m = c⋅M⋅V
The mass of the solute in the solution is calculated according to the mass fraction (w): m=w⋅M⋅V
Example of concentration calculation
Calculation of the quantity concentration of the substance
For sulfuric acid (H₂SO₄), the concentration is 0.1003 mol/L, which means that there is 0.1003 moles of H₂SO₄ per liter of solution.
Mass molar concentration calculation
For sodium chloride (NaCl), a mass molar concentration of 0.020 mol/kg indicates 0.020 moles of NaCl per kilogram of water.
summary
This article details the basics of solution preparation and concentration calculation, covering commonly used units of measurement such as mass, volume, density, etc., as well as the representation of solution concentration, such as the quantity concentration of a substance, mass concentration, mass molar concentration, etc. The basic formulas and conversion relationships for concentration calculations are also explained, and examples are given to show how this knowledge can be applied to practical calculations. The overall content is designed to help readers master the methods of measurement units and concentration calculations in solution preparation, and to provide scientific basis for experimental operations.
