Amount of Substance

Amount of Substance

Define relative isotopic mass.

The mass of an isotope relative to 1/12 of the mass of an atom of carbon-12.

Define relative atomic mass.

The relative atomic mass is the weighted mean mass of an atom of an element relative to 1/12 of the mass of an atom of carbon-12.

Describe how to determine the relative atomic mass of an element from the relative abundances of its isotopes. (Always check significant figures!!!)

(Atomic Mass X Percentage) + (Atomic Mass X Percentage) + (Atomic Mass X Percentage)


1) Multiply each isotopic mass by its relative abundance

2) Add them all together

3) Divide by 100

Explain the difference between relative formula mass and relative molecular masses and their calculation from relative atomic masses.

The relative molecular mass is used for simple molecules and relative formula mass for giant structures. It is found by adding up the mass of each individual element.

Define the amount of substance. Define the Avogadro constant.

The amount of substance (n) is the number of particles in a substance. This is measured in moles. One mole is the amount of substance is called the Avogadro constant, the amount of substance that contains 6.02 X 1023 particles.

Define a mole

1 mole is the amount of substance which contains as many particles as there are atoms in exactly 12g of carbon-12. One mole of an atom is equal to the atomic mass of that element.

How do you find the molar mass?

How do you find the amount of atoms in a compound?

Moles X Avogadro’s constant

Define empirical formula.

The empirical formula is the simplest whole number ratio of atoms of each element present in a compound.

Define molecular formula.

The molecular formula is the number and type of atoms of each element in a molecule.

How to calculate the empirical formula?

Step 1: Divide the mass by the molecular mass. (Find the moles)

Step 2: Find the simplest whole number ratio by dividing each side by the smallest number.

Step 3: Write out the empirical formula.

How would you work out the molecular formula from the empirical formula?

Step 1: Write the relative mass of the empirical formula.

Step 2: Divide the molecular mass by the relative mass.

Step 3: Write out the molecular formula.

Describe stoichiometry.

The relationship between the amounts of reactants and products is called the STOICHIOMETRY of a reaction. We use the stoichiometry of a reaction to calculate the amount of product will be formed, or how much of a reactant will be needed. For example, using the equation above; if we started with 20 moles of NaHCO3, we would obtain 10 moles of CO2 as equation shows them in 2:1 ratio.

Step 1: Work out the moles of the products and reactants mentioned in the question.

Step 2: Display this as a ratio

Define the terms anhydrous, hydrated and water of crystallisation.

Anhydrous: A structure without water.

Hydrated: Water molecules are part of the structure.

Water of Crystallisation: The water molecules part of the (crystalline structure)

Give the basic gas equation.

Moles = volume of gas (dm3)


Give the ideal gas equation.

1 atm = 101325 Pa                           Temp = Celsius + 273


* P = pressure in Pa. One atmosphere, which is normal atmospheric pressure, is 101325 Pa.

* V = volume in m3. Remember; 1 m3 = 1000 dm3.

* n = number of moles of gas

* R = Universal Gas constant = 8.31 J K-1 mol-1

* T = the absolute temperature in Kelvin. Remember; 0 oC = 273 K.

Give the equation for concentration.

Moles of solute (mol) = concentration (moldm-3) x volume of solution (dm3)

Explain the difference between a concentrated and dilute solution.

Solutions containing a higher number of moles of solute in a given volume of solution are described as concentrated.

Solutions containing a fewer number of moles of solute in a given volume of solution are described as dilute.

Give the three molar equations.

Give the formula for percentage yields.

Give four reasons why the theoretical yield may not be achieved.

  • The reaction may have not gone to completion
  • Purification of the product led to the loss of some products
  • Other side reactions may have taken place along the main reaction.
  • It is a reversible reaction

What is the limiting reagent?

The reactant that is not in excess and thus limits the reaction. To find the limiting reagent compare the moles of the reactants by balancing the equation. The reactant with more moles is the limiting reagents as more of this product is needed.

Give the formula for atom economy.


Why is a high atom economy important?

Reactions with high atom economies produce a large proportion of the desired products and few unwanted products. This is important for sustainability as they make the best use of natural resources; improving atom economy makes industrial processes more efficient, preserves raw materials, and reduces waste. However, percentage yields is also important for efficiency.