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.
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
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.
|Mass of sugar dissolved||1 g||100 g|
|Molar mass of sugar, C12H22O11||342 g/mol||342 g/mol|
|No. of moles of sugar||1 ÷ 342 = 0.00292 mol||100 ÷ 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.
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