Notes

Solution Concentration & Titration

Sorry, we don't have 25% sugar here. How about 100 g/dm3 or 0.292 mol/dm3?

1. Sugar level? Sugar concentration!

At bubble tea shops, we describe the amount of sugar dissolved — or sugar level — as a percentage. But it’s so inconsistent. 30% at CHOI is as sweet as 2000% at EYTEA.

Concentration can be measured as the mass of solute in 1 dm3 of solvent

Your nerdy chemists propose a more consistent way of describing sugar level: by the concentration of dissolved sugar. We can express concentration as the mass of solute dissolved in 1 dm3 of solvent (whereby dm3 is a fancy unit for litre).

So if you add 100 g of sugar to 1 dm3 of tea, the concentration of sugar is simply 100 g/dm3.

concentration (g/dm3) = mass of solute ÷ volume of solvent


2. It’s getting crowded in here

A higher concentration means more solute in the same volume of solvent

Using the formula above, a higher concentration means a greater mass of sugar in the same volume of tea. But what does this mean at the molecular level? To answer this, we shall think in terms of moles.

Dilute Concentrated
Mass of sugar dissolved1 g100 g
Molar mass of sugar, C12H22O11342 g/mol342 g/mol
No. of moles of sugar 1 ÷ 342 = 0.00292 mol100 ÷ 342 = 0.292 mol

From the calculation, we see that a more concentrated tea with a greater mass of sugar has a larger number of moles of sugar dissolved in the same volume of solution. Simply put, the concentrated solution is more crowded, with more sucrose molecules packed in the same space.

This brings us to the other definition of concentration, which is expressed as the number of moles of solute in every dm3 of solvent. Concentration expressed this way is also called molarity. The unit is mol/dm3.

concentration (mol/dm3) = no. of moles ÷ volume of solvent

Alternatively, if you already know the concentration in g/dm3, you can divide it by the molar mass of solute to derive the corresponding concentration in mol/dm3.

concentration (mol/dm3) = concentration (g/dm3) ÷ molar mass


3. The other way round: converting volume to number of moles

The concentration of a solution is the link between its volume and the number of moles of solute. When we measure the volume of a solution in the laboratory, we can very quickly find out the number of moles by rearranging the formula for molarity:

no. of moles = concentration (mol/dm3) × volume of solvent


4. Titration, titration, this is a game of concentration

Oh no! The very last bottle of bubble tea in Singapore is accidentally contaminated with an unknown amount of ethanoic acid. But since we really crave for it, we shall find out if the amount of ethanoic acid is too much for the bubble tea to be drunk.

In a titration, an exact volume of titrant is added to an analyte of unknown concentration

We can uncover the amount of ethanoic acid by a technique called titration. When we titrate, we add just enough reactant to fully react with the unknown. This requires the use of an indicator that changes colour at the end point, at which the reaction is complete.

The contaminated bubble tea is titrated with 20.00 cm3 of sodium hydroxide with a concentration of 0.5 mol/dm3. To find out the amount of ethanoic acid:

  • STEP 1: Write the chemical equation and highlight the substances you are interested in

    CH3COOH (aq) + NaOH (aq) ⟶ CH3COONa (aq)+ H2O (l)

  • STEP 2: Calculate the amount of titrant needed to reach end point

    Volume of NaOH = 20 cm3 = (20/1000) dm3 = 0.02 dm3
    Number of moles of NaOH = cV = 0.5 × 0.02 = 0.01 mol

  • STEP 3: State the mole ratio of reactants

    Ratio of CH3COOH to NaOH = 1:1

  • STEP 4: Calculate the amount of analyte

    Number of moles of ethanoic acid = 1/1 × 0.01 = 0.01 mol