Nobody asks you to "make aspirin". You get a real problem and work backwards to the science that solves it. Multiple answers are valid.
A rural hospital has run out of painkillers and can only source willow bark, acetic acid derivatives, and basic lab glassware. Work backwards from 'reduces pain and fever without dissolving the stomach'.
Yields are dropping and the plants are pale yellow. The atmosphere is 78% of the missing nutrient, but plants cannot touch it. Work backwards to a compound roots can actually absorb.
A coastal town wants food packaging strong enough to hold a litre of liquid that fully breaks down in seawater within a year.
Solar panels are useless overnight. Design storage chemistry that survives 55°C days and does not catch fire.
A sea-spray bridge is corroding. Work backwards from the electrochemistry of rusting to a metal system that protects itself.
Warming water expelled the algae from a coral reef. Work backwards from the symbiosis to what the corals actually lost.