Reagent tables provide an easy way to view the quantities of different reagents and solvents used in a reaction, including stoichiometries and reaction scale. For scripted practicals a reagent table is usually provided for you (albeit with gaps for some values you will need to calculate). For unscripted practicals, drawing up a reagent table is often a key part of the reaction planning.

Looking at a reaction in a table format is often helpful to a chemist's understanding of the reaction, considering what components are present in excess or catalytic quantities, and the scale at which the reaction is being undertaken. These are also crucial pieces of safety information which feed into risk assessments.

This page covers the reagent tables which are used within our taught practical scripts, however different chemists have varying approaches to reagent tables, so you may encounter different styles at times.

Reagent tables contain a series of chemical parameters against each component used in a reaction. The lab scripts provide a column for each component and a row for each parameter, but it is not uncommon for these to be reversed. The typical parameters which might be included in a reagent table are given below:

This section is often ommitted within taught practicals due to the nature of chemical provision. However, it is often very useful to include notes as to the source of a chemical, as they do vary in purity and form. This entry might be used to indicate a lab book reference where a compound had been prepared, a supplier, catalogue number, or other identifier for a commercial compound, or to indicate pre-purification (eg. dried, redistilled) etc.

The formula weight (fw) for the chemical, usually given in units of g mol⁻¹. This is often referred to as the relative molecular mass (RMM), although these aren't strictly the same, as RMM is a dimensionless unit.

A common consideration needed with formula weight is the presence of any solvents (typically water) of crystallisation present in the compound. If a compound such as copper(II) sulfate is used, this is normally a pentahydrate compound, so the formula weight needs to take into account the five water molecules contributing to the mass. If the anhydrous fw is used, the calculated mass would be lower than intended.

The quantity required to be used in the lab expressed as a mass, usually in grams (g) or milligrams (mg). This table is calculated from the amount (mol) and formula weight (g mol⁻¹) values.

The molar amount of each substance required. For the scale of reactions typically undertaken in Teaching & Research labs these are typically expressed in units of mmol (or sometimes µmol). The amount for the limiting reagent is also used to refer to the scale the reaction is being carried out at, e.g. 5 mmol scale, or 0.1 mol scale etc).

Procedures will typically provide the amount of the limiting reagent, and using the equivalents information (see below), the amount of other reagents can easily be calculated by multiplying the equivalents value with the amount for the limiting reagent.

E.g. If a reaction is being carried out with 5 mmol of the limiting reagent, and you require 1.1 equivalents for another reagent, this would be 5.0 mmol × 1.1 = 5.5 mmol.

The equivalents provides information on the stoichiometry of the reaction.

In most reactions, it is common for there to be a single limiting reagent (i.e. only one substance with 1 equivalent), with all other reagents present either in excess or in catalytic amounts. The reason for this is that it is highly unlikely that exactly the same equivalents of two or more compounds could be reliably measured in the lab, so that there is going to be one compound which is slightly in excess. Rather than leaving this to chance, chemists usually decide which compound should be the limiting reagent, therefore controlling which compounds will be fully consumed (assuming a reaction is able to reach completion), and for which compounds slight excess amounts will remain at the end of the reaction. These become important considerations for chemists in the lab, particularly with work up procedures.

For catalysts, equivalents are often expressed as mol% values, eg 5mol%. This means that you only require 5% of the quantity for this substance compared the the limiting reagent, i.e. 1.0 equivalent would correspond to 100mol%, so 5mol% is the same as 0.05 equivalents.

Many compounds may be provided as solutions of a compound in a solvent at a known concentration, rather than being provided as a neat compound. There are many reasons that this might be desirable, for instance:

• Neat compounds might have significant hazards which can be reduced by using a solution

• Quantities required of the neat compound may be unrealistic to measure, whereas the use of a solution enables using larger volumes which may be much easier to measure.

• Substances which are gases are difficult to handle, but solutions are much easier to work with. A common example would be hydrochloric acid (HCl), which is regularly used as a solution, and is only occasionally used in its gaseous form.

• Compounds with melting (or boiling) points which make handling awkward. Generally chemists want to either weigh solids, or measure volumes of liquids. Substances with melting points around room temperature can be difficult to measure, especially for small quantities, and it is quite common to use solutions to avoid either compounds melting during weighing or solidifying whilst measuring volumes

Concentrations are most commonly encountered in units of mol dm⁻³, as these are the most meaningful units for chemists. However, there are many other concentration units which might be encountered, for instance:

• various mass per volume units, for example g dm⁻³ or mg mL⁻¹ are both regularly encountered (and are equivalent units).

• parts per quantity e.g. parts per million (ppm), parts per billion (ppb).

• volume/volume units, eg 5% v/v ethanol/water implies a solution which is 5% by volume ethanol and 95% by volume water.

Care needs to be taken with units in all calculations, but calculations involving concentration manipulations are particularly prone to errors arising. Including units alongside values in calculations helps to reduce the prevalence of calculation errors.

Density is a measurement of the mass per volume of a substance, most commonly given in units of grams per millilitre, g mL⁻¹ (which is the same as g cm⁻³). The density is used to calculate the volume required for a compound in a liquid state. For commercial chemicals, the density can be found listed on chemical bottles or on the supplier websites for the specific chemical.

Volumes are used for measuring out quantities of liquids, whether a neat chemical or as a solution. Whilst in principle it is possible to weigh liquids, in practice it tends to be easier to use volumes. There are a number of reasons this might be the case:

• Liquids handling may be easier on safety grounds. eg volatile liquids are challenging to weigh.

• Liquids may have greater reactivity. eg compounds could react in the presence of air.

Volumes are routinely encountered in reagents tables in the following forms:

• Volumes of solvents

• Volumes of reagents which have been calculated from a concentration

• Volumes of reagents which have been calculated from a mass and density

For any calculations involving volumes, the correct arithmatic can usually be determined by considering the units:

• Using a mass (in g) and a volume (in g mL⁻¹) the volume (in mL) can be calculated by: mass ÷ density = volume

• Using an amount (in moles) and a concentration (mol dm⁻³) the volume (in dm⁻³ (= L)) can be calculated by amount ÷ concentration = volume.

The volume of solvent(s) in reactions has a important effect. Reactions are often described with their solvent concentration, calculated from the amount of the limiting reagent and the solvent volume to give a concentation for the reaction.

Eg. 5 mmol of the limiting reagent in 20 mL of reaction solvent:

5 × 10⁻³ mol ÷ (20 / 1000) dm³ = 0.25 mol dm⁻³ reaction concentration.

The reaction concentration can effect many aspects of reactions including:

• Heat dissipation from reactions.

• What reacts with what. Eg dilute systems may favour intramolecular reactions whereas concentrated systems may favour intermolecular reactions.

With a completed reagent table, it is common practice to then append an 'Amount taken' row to the table, and record the actual masses used in the lab. These are likely to differ, especially with masses, as it is unlikely you will weigh out the precise amount calculated. It is also good practice to record the quantities directly in the units used to measure in the lab, even if your calculations are in a different unit. Mistakes do happen with converting between units, and recording what you actually did (and at the time you did this) may prove crucial in uncovering your mistakes. For instance if you wanted to weigh 50 mg in the lab, and your balance is in grams you should record the exact mass and units you weighed out on the balance (with all decimals), e.g. 0.0494 g. For volumes it is common to record the mode (eg 25 mL measuring cylinder or 2 mL syringe) which provides an idea of the accuracy used. Some people may additionally add a 'Source' row to their table, and record the manufacturer or other details of the chemicals used.