A reaction mixture is at equilibrium. What happens when the amount of one of the
reactants is lowered?

The equilirbium shifts to side where the reactant is lowered so that the reactant comes back to normal level for equilibrium.

Explanation:

This is the basic premise of Le Chatelier's principle which can be used to predict the effect of a change in conditions on some chemical equilibria.

If A--->B  is at equiblirum and if concentration of A goes down, then the equilirbium shifts left to make more A.

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