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A limiting reactant is the reagent that is completely consumed during a chemical reaction. Once this
reagent is consumed the reaction stops. An excess reagent is the reactant that is left over once the
limiting reagent is consumed. The maximum theoretical yield of a chemical reaction is dependent upon
the limiting reagent thus the one that produces the least amount of product is the limiting reagent. For
example, if a 2.00 g sample of ammonia is mixed with 4.00 g of oxygen in the following reaction, use
stoichiometry to determine the limiting reagent.
4NH3 + 5O2 4NO + 6H2O
Since the 4.00 g of O2 produced the least amount of product, O2 is the limiting reagent.
In this experiment you will be given a mixture of two ionic solids, AgNO3 and K2CrO4, that are both
soluble in water. When the mixture is dissolved into water they will react to form an insoluble
compound, Ag2CrO4, which can be collected and weighed. The reaction is the following:
2 AgNO3(aq) + K2CrO4(aq) Ag2CrO4(s) + 2 KNO3(aq)
Colorless Yellow Red colorless
Precipitate
Precipitate is the term used for an insoluble solid which is produced by mixing two solutions. Often the
solid particles are so fine that they will pass right through the pores of a filter. Heating the solution for a
while causes the particles to clump together, a process called digesting. The precipitate coagulates upon
cooling and eventually settles to the bottom of a solution container. The clear liquid above the
precipitate is called the supernatant. When the mixture is poured into a filter, solid is trapped by the
filter and the clear liquid passes through as the filtrate. You will then test the filtrate to find out the
limiting reagent and then weigh the precipitate collected to determine the mass of each reagent
originally present in the solid mixture.
Equipment and Reagents
Unknown solid mixture hot plate or burner 2 watch glasses 0.2 M K2CrO4
2 Shell vials beaker tongs 150 mL beaker 0.1 M AgNO3
Analytical balance filter paper wash bottle
Limiting Reactant
Experiment
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