Quantitative analysis: the mole and concentration in mol/dm3, converting between g/dm3 and mol/dm3, the acid-alkali titration core practical, and calculating an unknown concentration from titration results.

What this dot point is asking

Edexcel wants you (in separate chemistry) to use the mole and the relationship between moles, concentration and volume, convert between g/dm$^3$ and mol/dm$^3$, carry out the acid-alkali titration core practical, and calculate an unknown concentration from titration results. The titration calculation is one of the highest-value question types on the paper.

The mole and concentration

The mole is the unit for amount of substance; one mole contains $6.02 \times 10^{23}$ particles and has a mass in grams equal to the relative formula mass, so $\text{moles} = \dfrac{\text{mass}}{M_r}$.

Concentration in mol/dm$^3$ relates moles to volume:

Rearranged, $\text{moles} = \text{concentration} \times \text{volume (dm}^3)$. Always convert volumes from cm$^3$ to dm$^3$ by dividing by $1000$.

Converting between g/dm3 and mol/dm3

The two concentration units are linked by the relative formula mass:

The titration core practical

The Edexcel core practical is an acid-alkali titration to find an unknown concentration.

1. Use a pipette to measure exactly $25.0$ cm$^3$ of the alkali into a conical flask, and add a few drops of indicator (such as phenolphthalein or methyl orange).
2. Fill a burette with the acid and record the start reading.
3. Add the acid slowly, swirling, until the indicator just changes colour (the end point), then record the final reading. The volume added is the titre.
4. Repeat until concordant titres (within $0.10$ cm$^3$) are obtained, and use their mean.

Using a pipette and burette gives accurate, repeatable volumes; swirling and adding dropwise near the end point avoids overshooting.

Calculating an unknown concentration

The titration calculation follows three steps:

1. Calculate the moles of the solution whose concentration you know ($\text{moles} = \text{concentration} \times \text{volume in dm}^3$).
2. Use the mole ratio from the balanced equation to find the moles of the unknown.
3. Calculate the concentration of the unknown ($\text{concentration} = \text{moles} \div \text{volume in dm}^3$).

Try this

Q1. Calculate the moles in $50.0$ cm$^3$ of $0.200$ mol/dm$^3$ acid. [2 marks]

• Cue. $0.200 \times (50.0/1000) = 0.200 \times 0.0500 = 0.0100$ mol.

Q2. Convert $0.250$ mol/dm$^3$ of $HCl$ to g/dm$^3$ ($M_r$ HCl = 36.5). [2 marks]

• Cue. $0.250 \times 36.5 = 9.13$ g/dm$^3$.

Q3. Why is the titration repeated until concordant titres are obtained? [1 mark]

• Cue. To improve accuracy and reliability by taking a mean of close results.