Blog

Bases are proton acceptors by definition. They produce OH– ions in aqueous solution, either by accepting a H+ from water, or by the base itself containing OH– ions. All the examples above are strong bases because the reactions go to completion. This means we can work [OH–] simply by knowing the initial amount of base…

The strong acid assumption is that the acid (HA) in solution dissociates completely into ions (i.e., H+ and A–). This assumption is useful because we can work out the end [H+] and pH simply from the concentration of acid that we start with and we don’t have to worry about any equilibrium constants. An ICE…

One of the first facts that Year 7 students learn in Chemistry is that water has a pH of 7. However, this isn’t some fundamental law of the universe, but simply reflects the value of an equilibrium constant at 25 oC. Water contains H+ and OH– ions, formed by partial dissociation of water molecules. This…

A very short post on a very useful formula: The concentration of H+ ions in solutions vary by many orders of magnitude but the pH equation allows us to express this very quickly using convenient numbers (typically between -1 and 15 ish). This equation means that if the [H+] increases by a factor of 10, the…

The ideal gas law is pV = nRT where: p = pressure (Pa) V = volume (m3) n = amount (mol) R = 8.31 J mol-1 K-1 T = Temperature (K) Question: 0.168 g of a hydrocarbon is held under conditions of 227 oC and 100 kPa and occupies a volume of 50 cm3. A…

If you are given the initial amounts of substances, the balanced symbol equation and the final amount of one substance, you can work out the final molar amounts of ALL substances using an ICE table. Question: 6.00 g of ethanoic acid (CH3COOH) and 6.00 g of propan-1-ol (CH3CH2CH2OH) are heated under reflux with H2SO4. The…

A very common mistake is to confuse the relative atomic mass with the mass number. The relative atomic mass is the mean mass of 1 atom of an element on a scale in which one atom of 12C has a mass of exactly 12. It is a weighted average mass that takes into account the…

We know that terrestrial Cl is 75% 35Cl and 25% 37Cl.  A molecule containing one Cl would show the same distribution, i.e., 75% H35Cl and 25% H37Cl. This would appear in mass spectrometry as two molecular ion peaks with an intensity ratio of 3:1. An interesting question is therefore to ask what pattern would we…

Another interesting variation of an isotopes question is where you need to determine the mass number of a missing isotope. This is another type of question where algebra is needed. Question: A sample of boron contains 2 isotopes and has an Ar of 10.8. The lighter isotope has a mass number of 10 and abundance…

A common GCSE question is to calculate the relative atomic mass of an element, given the mass and abundance of a series of isotopes. e.g., Terrestrial (i.e., as found on Earth) Mg is: Mass 24 25 26 Abundance 80% 10% 10% The Ar is calculated by [(24 x 80%) + (25 x 10%) + (26 x…