One of the most common misconception for SPM Form 4 Chemistry students is the perception that strength of acids/bases is actually the same as concentration of acids/bases. This cannot be more wrong a perception. The strength of acids and bases is linked to the ionisation and dissociation in solutions. While concentration is defined by parameters such as molarity or amount present. So this post, Part 7 from Berry Berry Easy study notes on the topic of Acids and Bases for Form 4 SPM Chemistry students is on the subtopics of Concentration of Acids and Alkalis, Relationship between Number of Moles with Molarity and Volume of a Solution, Preparation of Standard Solutions, Dilution Method and Relationship between pH Values, and Molarities of Acids or Alkalis.
Although this subtopic do not appear per se in examination as frequently as it should but it is important to understand later concepts especially for experiments. So try your best to understand this post.
[Tips: The easiest way to distinguish between the strength and concentration of acid/base is to use a set of strong and weak acids examples, then imagine both of them in diluted and concentrated form, allowing 4 different samples.
Some Berry Readers also asked what defines a concentrated acid/base? By concensus, usually concentrated acids/bases have high molarities exceeding 6 mol/L. Likewise, a diluted acid/base has molarities below 6 mol/L.]
SPM Chemistry Form 4 Notes – Acids and Bases (Part 7)
Concentration of Acids and Alkalis
- Quantity of solute can be measured in grams or moles: g dm-3 or mol dm-3.
- Concentration (g dm-3) = mass of solute (g) / volume of solution (dm3)
- Concentration (mol dm-3) = number of moles of solute (mol) / volume of solution (dm3)
Relationship between Number of Moles with Molarity and Volume of a Solution
- Molarity (mol dm-3) = number of moles of solution (mol) / volume of solution (dm3)
- M = molarity of solution (mol dm-3), V = Volume of solution (dm3), n = Number of moles of solute (mol)
- M = n/V
Preparation of Standard Solutions
- Standard solution = a solution in which its concentration is accurately known.
- Standard solution is prepared by using volumetric flask with a fixed volume (100 cm3, 200 cm3, 250 cm3, 500 cm3 and 1000 cm3)
Preparation of a Solution by Dilution Method
- Dilution = a process of diluting a concentrated solution by adding a solvent (water) to obtain a more diluted solution.
- The concentration of the solution decreases after dilution.
- The number of moles of solute in the solution remains unchanged after dilution.
- (MaVa) / 1000 = (MbVb) / 1000
- Ma = Initial molarity of solution, Mb = Final molarity of solution, Va = Initial volume of solution and Vb = Final volume of solution
Relationship between pH Values and Molarities of Acids or Alkalis
- pH values depends on degree of dissociation and molarity / concentration of hydrogen ions, H+/ hydroxide ions, OH- in the solution.
- The higher the molarity of a strong acid, the lower is its pH value.
- The higher the molarity of a strong alkali, the higher is its pH value.
- The molarity of an acid can be changed when (i) water is added, (ii) an acid of different concentration is added and (iii) an alkali is added.
This marks the end of Part 7 of the study notes from Berry Berry Easy on the Acids and Bases chapter for SPM Chemistry Form 4 students. In the next post, we’ll be discussing about the “difficult” subtopic of Neutralisation.