Limiting and Excess Reactants - Practice Problems

What is the purpose of identifying a limiting reactant in a chemical reaction?

• To measure the speed of the reaction

• To identify the reactant with the highest molar mass

• To calculate the total mass of the reactants

• To determine which reactant will be completely used up first

How many moles of oxygen are required to completely react with 7.8 moles of magnesium?

• 3.9 moles

• 7.8 moles

• 15.6 moles

• 1.95 moles

Which reactant is the limiting reactant when reacting 7.8 moles of magnesium with 4.7 moles of oxygen?

• None, they are in perfect stoichiometric amounts

• Magnesium Oxide (MgO)

• Magnesium (Mg)

• Oxygen (O2)

Why do we use stoichiometry in limiting reactant problems?

• To calculate the energy released

• To determine the reactants' molar ratios

• To balance the chemical equation

• To identify the physical states of reactants

What role does the conversion factor play in these calculations?

• It measures the reaction rate

• It shows the molar ratio between reactants and products

• It converts grams to moles

• It determines the reaction's temperature

What is the maximum amount of magnesium oxide (MgO) that can be produced from 7.8 moles of magnesium?

• 9.4 moles

• 15.6 moles

• 3.9 moles

• 7.8 moles

How many moles of oxygen will be left over as the excess reactant?

• 0.8 moles

• 0 moles

• 1.6 moles

• 3.9 moles

What happens to the excess reactant in a chemical reaction?

• It disappears without a trace

• It is completely used up

• It the limiting reagent remains unchanged

• A portion of it remains unreacted

After identifying the limiting reactant, what is the next step?

• Calculate the amount of product formed

• Calculate the total mass of reactants

• Determine the reaction rate

• Measure the temperature change

Identify the limiting reagent when 6.00 g HCl combines with 5.00 g Mg to form MgCl₂.
a.  Mg +  2HCl --> MgCl₂  +  H₂   

• Mg
• H2
• MgCl2
• HCl