AP Chemistry : AP Chemistry

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Limiting Reagent

Consider the reaction of potassium carbonate with calcium nitrate to form potassium nitrate and calcium carbonate:

Suppose 50ml of a 0.250M potassium carbonate solution was mixed with 100ml of a 0.175M calcium nitrate solution. What is the maximum amount of calcium carbonate that could be obtained?

Possible Answers:

Correct answer:

Explanation:

First, we must determine how many moles of each reactant begin the reaction by multiplying the molarity by the volume. Don't forget to convert volume to liters!

Next, use the reaction coefficients (i.e. the stoichiometry) to determine how many moles of calcium carbonate could be formed from each of the reactants. In this case, there is a 1:1 molar ratio between both reactants and calcium carbonate.

Thus, 0.0125 moles of potassium carbonate could form 0.0125 moles of calcium carbonate, while 0.0175 moles of calcium nitrate could form 0.0175 moles of calcium carbonate. The maximum amount of product is going to be determined by the limiting reactant, i.e. the reactant that provides the least amount of product. In this case, the limiting reactant is potassium carbonate, and the maximum yield of calcium carbonate is 0.0125mol.

For the final step convert this value to grams:

Example Question #21 : Stoichiometry

How is a limiting reagent problem recognized?

Possible Answers:

One of the reactants is always present in excess

The amount of product is always given

The initial quantities of at least two reactants are given

The initial quantities of one reactant and one product are given

The initial quantities of at least two reactants are given

Correct answer:

The initial quantities of at least two reactants are given

Explanation:

When two or more reactants of a chemical equation's quantities are given, the first step is to determine the limiting reagent of the reaction. The reaction will only proceed and produce as much product as the limiting reagent allows. For example if there are 5 moles of oxygen gas and excess hydrogen gas, only 10 moles of water can be produced regardless of how much hydrogen gas is present. 

Example Question #442 : Ap Chemistry

The following reaction is used to obtain small amounts of chlorine gas in the laboratory:

If  of  are allowed to react with  of , the limiting reactant will be:

 

Possible Answers:

There is no limiting reactant

Correct answer:

Explanation:

First of all, a product cannot be the limiting reactant. This problem can be solved in many different ways. Keep in mind that to solve a stoichiometry problem a good practice is to convert mass to mol. Let's find if the amount of  we have is enough to react with  of :

We have only  of   and  are needed. Then  is the limiting reactant.

Example Question #13 : Limiting Reagent

When grilling out, many people utilize the combustion of propane to provide the heat energy needed to cook their food.  The chemical equation for this reaction is shown below:

 

You notice that your gas grill is producing a large amount of soot, which is negatively impacting the taste of your food. Please select the best explanation of this phenomenon, and it's corresponding solution.

Possible Answers:

The limiting reactant is oxygen; therefore the best way to reduce the soot is to add an oxygen source such as , which releases oxygen by thermal decomposition.

Oxygen is in excess; therefore the best way to reduce the soot is to close the grill hood and close any vent holes preventing atmospheric oxygen from entering the system.

The soot production is due to a dirty grill and therefore your grill must be thoroughly cleaned.

The limiting reactant is propane; therefore the best way to reduce the soot is to increase the input of propane.

The limiting reactant is oxygen; therefore the best way to reduce the soot is to decrease the input of propane, which is in excess.

Correct answer:

The limiting reactant is oxygen; therefore the best way to reduce the soot is to decrease the input of propane, which is in excess.

Explanation:

Soot is a mixture incompletely combusted hydrocarbons. Therefore the production of soot occurs most when the hydrocarbon is in excess and the oxidizer (in this case oxygen) is the limiting reactant.

Incorrect answers and explanations:

The limiting reactant is propane, therefore the best way to reduce the soot is to increase the input of propane.  

This is incorrect because propane is in excess when soot production is maximized. Therefore adding more propane will only result in more soot.

The limiting reactant is oxygen; therefore the best way to reduce the soot is to add an oxygen source such as , which releases oxygen by thermal decomposition.

The limiting reactant is indeed oxygen and  does undergo thermal decomposition to produce oxygen. However it is also one of the primary ingredients for gunpowder (the other two ingredients have a high probability of being present in a device that processes tissue from living organisms), therefore the safety factor of this answer leads it to be incorrect. Not to mention that this method would not be the simplest solution.

Oxygen is in excess; therefore the best way to reduce the soot is to close the grill hood and close any vent holes preventing atmospheric oxygen from entering the system.

Oxygen is the limiting reactant; therefore cutting off the supply of oxygen would only create more soot or depending upon the degree to which the oxygen supply is cut off, this could stop the reaction altogether.

The soot production is due to a dirty grill and therefore your grill must be thoughly cleaned.  

A dirty grill may have soot in it, but unless the dirtiness is blocking off the supply of oxygen, the soot currently being produced is not caused by the dirt. In any case the root cause of any soot production is that oxygen is limiting and this answer does not address that fact.

Correct Answer and explanation:

The limiting reactant is oxygen; therefore the best way to reduce the soot is to decrease the input of propane, which is in excess.

The limiting reactant when soot is produced is oxygen. In most propane grills, the oxygen supply is maxed out at the level of atmospheric oxygen. Therefore the only reactant under your direct control is the propane. By reducing the quantity of propane, you can make it no longer in excess.

Example Question #1 : Other Stoichiometric Calculations

You have an unidentified colorless, odorless liquid in a thin cubic container. The container is 3.2 inches by 3.4 inches by 3.3 inches and the liquid fills the entire container. The mass of the liquid minus the mass of the container is 0.370 pounds. What is the liquid's density in grams per milliliter?

(1 inch = 2.54 cm and 1 kg = 2.2046 pounds)

Possible Answers:

Correct answer:

Explanation:

First we must determine the volume of the container:

The density is .

Now we will convert this into grams per milliliter.

Example Question #441 : Ap Chemistry

How much heat is released from the reaction of 18 moles of methane if each mole of methane liberates 5kJ of heat, but the reaction is only 60% efficient?

Possible Answers:

Correct answer:

Explanation:

Each mole of methane to react releases 5kJ of heat. Since we have 18 moles of methane, we expect a potential of 90kJ of heat to be released.

We also have to take into account that the process is only 60% efficient; therefore, we effectively only liberate 60% of the total expected amount.

Example Question #3 : Other Stoichiometric Calculations

When  of  are dissolved in   of a   solution of  

Possible Answers:

after filtration  of a solid are obtained

we obtain a clear solution

the approximate concentration of  in the resulting solution will be  and after filtration   of a solid are obtained

a precipitate of sodium nitrate will be obtained

Correct answer:

the approximate concentration of  in the resulting solution will be  and after filtration   of a solid are obtained

Explanation:

The net ionic equation that occurs is:

 and  are spectator ions. Hence,   concentration is not affected. The molecular mass of  is , then  of NaCl yields  of . Since we have  of a  solution of , we have  of . There is no limiting reactant. Hence  of  are formed. Being the molecular mass of ,  are formed. Result should be given with tree significant figures.

Example Question #442 : Ap Chemistry

What is the mass of ?

 

Possible Answers:

Correct answer:

Explanation:

Remember: the given measurement has 4 significant figures, so the answer must also have 4 significant figures.

Example Question #443 : Ap Chemistry

How many atoms are there in 17.0g of sulfur?

Possible Answers:

Correct answer:

Explanation:

Remember, there really isn't a way to go straight from grams to atoms, but, it's possible to change grams into moles (using the molar mass), and it's possible to change moles into atoms (using Avogadro's number).

Example Question #31 : Stoichiometry

For the molecular formula, , how many moles of bromine are in 20.0 grams of this molecule?

Possible Answers:

Correct answer:

Explanation:

The first step is to determine the molecular weight of the molecule:

The molecular weight serves as a conversion factor to convert grams to moles as implied its units . So, given that we are dealing with 20.0 grams, the moles of 20.0 grams can be calculated using the calculated molecular weight as seen below in which the grams cancel out leaving the units in moles:

Based on the molecular formula, for every mole of the compound , there is 1 mole of bromine. Based on our calculations 20.0 grams of  is 0.1835 moles of the compound. Because there is a 1:1 molar ratio of bromine to every molecule, we have 0.1835 moles of bromine in 20 grams of our compound, .

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