Aspirin preparation records and purification evidence

What you’ll learn

• Distinguish reaction yield from purification recovery.
• Evaluate recrystallisation, drying and melting-range evidence.

Understand the idea

Core practical 16 uses aspirin to connect preparation and purification. A supplied reaction record can establish a theoretical product amount from the limiting reactant without becoming a synthesis recipe. The percentage yield compares dry isolated product with the stoichiometric maximum. Percentage recovery during purification instead compares the recovered mass with the crude material taken into that purification stage. These percentages answer different questions and need different denominators.

Recrystallisation relies on a suitable solvent in which the desired solid is substantially more soluble hot than cold. Some impurities remain in solution, while insoluble impurities can be separated at an appropriate stage. Product that stays dissolved or is lost during transfers lowers recovery. A suitable small amount of cold washing solvent removes surface solution while limiting dissolution of the crystals; the choice depends on the supplied solubility data and approved laboratory method.

Drying matters both for mass and melting measurements. A sharp melting range close to the reference supports purity, whereas many impurities depress and broaden the range. This is supporting evidence rather than absolute proof of identity. In an aspirin context, salicylic-acid contamination may be investigated using supplied comparative chemical or spectral data. Corrosive acylating reagents and hot solvents require the teacher's specified containment and heating controls; no preparation quantities or operating sequence are needed to interpret the record.

Key formulae

$$n_{\mathrm{aspirin,max}}=n_{\mathrm{salicylic\ acid}}\quad\text{for a }1:1\text{ reaction}$$

$$\mathrm{yield}\ (\%)=\frac{m_{\mathrm{dry\ product}}}{m_{\mathrm{theoretical}}}\times100$$

$$\mathrm{recovery}\ (\%)=\frac{m_{\mathrm{purified\ material}}}{m_{\mathrm{crude\ material}}}\times100$$

$$\mathrm{atom\ economy}\ (\%)=\frac{M_{\mathrm{desired\ product}}}{\sum M_{\mathrm{reactants}}}\times100\quad\text{for a }1:1\text{ equation}$$

Worked example

A supplied aspirin preparation record identifies $4.14\ \mathrm{g}$ of salicylic acid, molar mass $138\ \mathrm{g\,mol^{-1}}$, as the limiting reactant. It forms aspirin in a $1:1$ amount ratio; aspirin has molar mass $180\ \mathrm{g\,mol^{-1}}$. The dry isolated aspirin mass is $4.05\ \mathrm{g}$. Find the percentage yield.

1. The salicylic-acid amount is $4.14/138=0.0300\ \mathrm{mol}$.
2. The theoretical aspirin amount is $0.0300\ \mathrm{mol}$ and its theoretical mass is $0.0300\times180=5.40\ \mathrm{g}$.
3. The percentage yield is $100(4.05/5.40)=75.0\%$.

Answer: $75.0\%$.