Study Guide

# Stoichiometry - Test Your Knowledge

## Test Your Knowledge

### Atomic Mass

1. The mass value shown on the periodic table for each element is equal to ____________.

A. the atomic mass of the most abundant isotope
B. the atomic mass of the least abundant isotope
C. the average atomic mass of the element
D. the number of protons in the atom
E. none of the above

2. Carbon-12 has how many neutrons?

A. 12
B. 13
C. 6
D. 7
E. None of the above

3. On the periodic table the average atomic mass of an element is given in units of what?

A. Grams
B. Moles
C. Average mass units
D. Atomic mass units
E. None of the above
4. The atomic number is defined as:

A. The number of protons in an atom
B. The number of neutrons in an atom
C. The total number of protons and neutron in an atom
D. The number of electrons in an atom
E. None of the above

5. The mass number is defined as:

A. The number of protons in an atom
B. The number of neutrons in an atom
C. The total number of protons and neutron in an atom
D. The number of electrons in an atom
E. None of the above

### Moles

1. A mole of something is equal to the quantity:

A. 10.022 × 1023
B. 8.022 × 1023
C. 6.023 × 1023
D. 10.022 × 10-23
E. 6.023 × 10-23

2. Avogadro's number is named in honor of:

E. None of the above

3. The molar mass of something is:

A. The mass of exactly 12 grams of something
B. The mass of 6.023x10-23 somethings
C. The mass of a mole of something
D. The weight of 5 furry moles
E. None of the above

4. The scientific term for objects we can see and measure is:

A. Microscopic
B. Macroscopic
C. Tiny
D. Huge
E. None of the above

5. Why is it useful to keep track of and cross units out as we work our way through problem?

A. It's a quick and easy way to find silly mistakes.
B. It helps us achieve the units we ultimately need.
C. It makes complicated problems seem easy.
D. It helps us from dividing when we should be multiplying and multiplying when we should be dividing.
E. All of the above

### Molecular Mass

1. Molecular weight is defined as:

A. The sum of the atomic weights of all the atoms in a molecule
B. The sum of the atomic weights of all of the atoms in a mole of a particular molecule
C. The weight of one molecule of carbon-12 atoms
D. The weight of one mole of carbon-12 molecules
E. None of the above

2. Molar mass is defined as:

A. The sum of the atomic weights of all the atoms in a molecule
B. The sum of the atomic weights of all of the atoms in a mole of a particular molecule
C. The mass of one mole of a particular substance
D. The mass of one mole of carbon-12 atoms
E. None of the above

3. Another term for molecular mass is:

A. Mole weight
B. Mole
C. Molar mass
D. Molecular weight
E. None of the above

4. The molecular mass of water and the molar mass of water are:

A. Numerically the same
B. Not related
C. The molecular mass is the molar mass divided by the number of moles of water
D. The molar mass is the molecular mass divided by the number of moles of water
E. None of the above

5. The molar mass of water is:

A. 1.08 g/mol
B. 12.04 g/mol
C. 16.02 g/mol
D. 18.02 g/mol
E. None of the above

### Percent Composition

1. To convert from a fractional composition to a percent composition you have to:

A. Multiply by 100
B. Divide by 100
C. Divide by 10
D. Nothing, the values are the same
E. None of the above

2. Percent composition is defined as:

A. The total weight of the compound divided by the number of moles of the compound
B. The mass of each element divided by 100 in a compound
C. The percent by mole quantity of an element in a compound
D. The percent by mass of each element in a compound
E. None of the above

3. There are how many moles of hydrogen in 1 mole of H2O2?

A. One
B. Two
C. Three
D. Four
E. None of the above

4. How many moles of carbon are in 4 moles of CH4?

A. One
B. Two
C. Four
D. Sixteen
E. None of the above

5. The total percent composition of all of the elements in a compound should be:

A. 0%
B. 10%
C. 25%
D. 100%
E. None of the above

### What is Stoichiometry?

1.Which term describes a process in which a substance (or substances) is changed into one or more new substances?

A. Chemical reaction
B. Chemical equation
C. Stoichiometry
D. Metamorphosis
E. None of the above

2. According to the law of conservation of mass:

A. Mass can be created but not destroyed in any chemical reaction.
B. Mass can only be destroyed but not created in any chemical reaction.
C. Mass is neither created nor destroyed in any chemical reaction.
D. Mass must be weighed before and after all reactions.
E. None of the above

3. When balancing a chemical equation it is not okay to:

A. Change the subscript of a chemical formula
B. Change the coefficient and a product
C. Change the coefficient of a product
D. Change the coefficient of a reactant
E. All of the above

4. Which of the following is FALSE regarding chemical equations?

A. Chemical equations are read left to right.
B. Reactants are on the right hand side of the arrow.
C. The arrow means, "To yield."
D. A plus sign means, "When reacted with."
E. None of the above

5. Which of the following statements is true?

A .One mole of hydrogen gas and one mole of oxygen gas has the same number of molecules.
B. One mole of hydrogen gas and one mole of oxygen gas weigh the same amount.
C. One mole of hydrogen gas has more molecules present than one mole of oxygen gas.
D. One mole of hydrogen gas has fewer molecules present than one mole of oxygen gas.
E. None of the above

### Reactants and Products

1. What is stoichiometry?

A. The qualitative study of reactants and products in a chemical reaction
B. The quantitativestudy of reactants and products in a chemical reaction
C. The mass of products consumed in a reaction
D. The mass of reactants formed in a reaction
E. None of the above

2. What is the mole method?

A. The method that says reactants only react in a 1:1 mole ratio
B. The method that says the coefficients in a chemical equation can be interpreted as the number of grams of each substance
C. The method that says the subscripts in a chemical equation can be interpreted as the number of moles of each substance
D. The method that says the coefficients in a chemical equation can be interpreted as the number of moles of each substance
E. None of the above

3. The first step in any stoichiometry problem is to:

A. Convert the given quantity into moles
B. Convert the given quantity into grams
C. Convert the given quantity into liters
D. Bang your head on the wall
E. None of the above

4. How does one convert between grams and moles?

A. By using the molar mass of the substance
B. By using the ratio of products and reactants
C. By using the density of the substance
D. By weighing the substance on a scale
E. None of the above

5. If 3A +4B → 12C which of the following is not true:

A. 9 moles of A produces 36 moles of C
B. 2 moles of B produces 6 moles of C
C. 6 moles of A reacts with 8 moles of B
D. 2 moles of B produces 8 moles of C
E. None of the above

### Limiting Reagents

1. What is the limiting reagent?

A. The reactant in a chemical reaction that determines the amount of product that can be produced
B. The reactant in a chemical reaction that is present in the least amount of grams
C. The reactant in a chemical reaction that is present in the least amount of moles
D. The reactant in a chemical reaction that is present in a greater amount than necessary to react to completion
E. None of the above

2. What is the excess reagent?

A. The reactant in a chemical reaction that determines the amount of product that can be produced
B. The reactant in a chemical reaction that is present in the least amount of grams
C. The reactant in a chemical reaction that is present in the least amount of moles
D. The reactant in a chemical reaction that is present in a greater amount than necessary to react to completion
E. None of the above

3. Pretend you have a job building tricycles. If you had 100 handlebars, 150 wheels, 250 pedals, and 75 seats, how many bikes could you build?

A. 50
B. 75
C. 120
D. 125
E. None of the above

4. In the previous question, how many pedals are left over after you have built all of the tricycles you can build?

A. 0
B. 100
C. 150
D. 200
E. None of the above

5. Consider N2 + 3H2 → 2NH3. If 18 moles of N2 is reacted with 36 moles of H2, which of the following is false?

A. N2 is in excess
B. 24 moles of NH3 will be produced
C. H2 is the limiting reagent
D. N2 is the limiting reagent
E. None of the above

### Reaction Yields

1. The actual yield is:

A. The amount of product that would result if the entire limiting reagent reacted
B. The amount of product that is actually obtained from a reaction
C. The amount of product that would result if the entire excess reagent reacted
D. The amount of product from that would result from one mole of reactant
E. None of the above

2. The theoretical yield is:

A. The amount of product that would result if the entire limiting reagent reacted
B. The amount of product that is actually obtained from a reaction
C. The amount of product that would result if the entire excess reagent reacted
D. The amount of product from that would result from one mole of reactant
E. None of the above

3. Percent yield is calculated by:

A. Dividing the theoretical yield by the actual yield and dividing by 100%
B. Dividing the theoretical yield by the actual yield and multiplying by 100%
C. Dividing the actual yield by the theoretical yield and dividing by 100%
D. Dividing the actual yield by the theoretical yield and multiplying by 100%
E. None of the above

4. The theoretical yield of a reaction is 8.3 g. The actual yield is 6.4 g. What is the percent yield?

A. 0.012%
B. 75.8%
C. 77.1%
D. 129%
E. None of the above

5. The theoretical yield of a reaction is 9.2 g. The percent yield is 63%. What is the actual yield?

A. 4.5 g
B. 5.4 g
C. 5.8 g
D. 6.8 g
E. None of the above

### Atomic Mass

1. The mass value shown on the periodic table for each element is equal to ____________.

C. the average atomic mass of the element

Elements have several naturally occurring isotopes. The average atomic mass of all of the naturally occurring isotopes for each element appears on the periodic table.

2. Carbon-12 has how many neutrons?

C. 6

Carbon-12 has an mass number of 12, which means it has 12 total protons and neutrons. The periodic table indicates carbon has an atomic number of 6, which means it has 6 protons. Some fancy math should tell you that carbon-12 has 6 neutrons.
3. On the periodic table the average atomic mass of an element is given in units of what?

D. Atomic mass units

The average atomic mass for each element is given in atomic mass units (amu) which is defined as a mass equal to one-twelfth the mass of a single carbon-12 atom.

4. The atomic number is defined as:

A. The number of protons in an atom

The atomic number is by definition the number of protons in an atom. The atomic number of carbon, for example, is 6, which means it has 6 protons.

5. The mass number is defined as:

C. The total number of protons and neutron in an atom

The mass number is, by definition, the total number of protons and neutrons in an atom. The mass number of carbon, for example, is 12, which means it has 12 protons and neutrons total.

### Moles

1. A mole of something is equal to the quantity:

C. 6.023 × 1023

A mole is just a quantity. A mole of something is 6.023 x 1023 items of that something.

2. Avogadro's number is named in honor of:

Avogadro's number is named in honor of the Italian scientist Amedeo Avogadro.

3. The molar mass of something is:

C. The mass of a mole of something

By definition, molar mass is the mass of one mole of something.

4. The scientific term for objects we can see and measure is:

B. Macroscopic

The term macroscopic describes things we can see and measure with our unaided eyes.

5. Why is it useful to keep track of and cross units out as we work our way through problem?

A. It's a quick and easy way to find silly mistakes.
B. It helps us achieve the units we ultimately need.
C. It makes complicated problems seem easy.
D. It helps us from dividing when we should be multiplying and multiplying when we should be dividing.
E. All of the above

Following units on chemistry homework, quiz, and exam problems is the best way to make difficult questions seem easy. If you know the units you need in the end, arrange the information you have in a way that all of the other units cancel out. Easy Peasy.

### Molecular Mass

1. Molecular weight is defined as:

A. The sum of the atomic weights of all the atoms in a molecule

Molecular weight is, by definition, the sum of the atomic weights of all the atoms in a molecule.

2. Molar mass is defined as:

C. The mass of one mole of a particular substance

Molar mass is, by definition, the mass of one mole of a particular substance.

3. Another term for molecular mass is:

D. Molecular weight

Molecular mass and molecular weight mean the same thing. The terms can (and are) used interchangeably.

4. The molecular mass of water and the molar mass of water are:

A. Numerically the same

For all molecules, the molecular weight and molar mass are numerically the same.

5. The molar mass of water is:

D. 18.02 g/mol

molar mass H2O = 2(atomic mass of H) + atomic mass of O
molar mass H2O = 2(1.008 g) + 16.00 g = 18.02 g

### Percent Composition

1. To convert from a fractional composition to a percent composition you have to:

A. Multiply by 100

To covert from a fractional composition to a percent composition you have to multiply the value by 100.

2. Percent composition is defined as:

D. The percent by mass of each element in a compound

By definition the percent composition is the percent by mass of each element in a compound.

3. There are how many moles of hydrogen in 1 mole of H2O2?

B. Two

There are two moles of hydrogen in one mole of H2O2. We can also say for every two moles of hydrogen there are two moles of oxygen.

4. How many moles of carbon are in 4 moles of CH4?

C. Four

There is one mole of carbon in every mole of CH4. Therefore, four moles of CH4 contains four moles of carbon.

5. The total percent composition of all of the elements in a compound should be:

D. 100%

This is a quick and easy way to check your work. The total percent composition of all the elements in a compound should be 100%.

### What is Stoichiometry?

1.Which term describes a process in which a substance (or substances) is changed into one or more new substances?

A. Chemical reaction

Be definition a chemical reaction is a process in which a substance (or substances) is changed into one or more new substances.

2. According to the law of conservation of mass:

C. Mass is neither created nor destroyed in any chemical reaction.

Be definition, the law of conservation of mass states that mass is neither created nor destroyed in any chemical reaction.

3. When balancing a chemical equation it is not okay to:

A. Change the subscript of a chemical formula

Changing the subscript in a chemical formula alters the chemical identity of the reactant or product. These changes are not allowed when balancing a chemical equation.

4. Which of the following is FALSE regarding chemical equations?

B. Reactants are on the right hand side of the arrow.

A chemical equation is read left to right with reactants being on the left hand side of the arrow and products being on the right hand side of the arrow. The arrow symbol means "to yield" and a plus sign means "when reacted with."

5. Which of the following statements is true?

A. One mole of hydrogen gas and one mole of oxygen gas has the same number of molecules.

A mole is a quantity (6.023 × 1023) therefore a mole of hydrogen atoms and a mole of oxygen atoms as the same number of molecules present.

### Reactants and Products

1. What is stoichiometry?

B. The quantitativestudy of reactants and products in a chemical reaction

Stoichiometry is the quantitative study of reactants and products in a chemical reaction. In other words, stoichiometry is learning exactly how much of a reactant is consumed and exactly how much of a product is formed. It's all about the numbers.

2. What is the mole method?

D. The method that says the coefficients in a chemical equation can be interpreted as the number of moles of each substance

In the mole method the coefficients in a chemical equation can be interpreted as the number of moles of each substance. This method makes hard stoichiometry problems become very manageable.

3. The first step in any stoichiometry problem is to:

A. Convert the given quantity into moles

The first and essential step to solving any stoichiometry problem is to first convert the given quantity into moles.

4. How does one convert between grams and moles?

A. By using the molar mass of the substance

To convert between grams and moles the molar mass of the substance must be calculated using the periodic table. Remember: the definition of molar mass is the mass of 1 mole of any given substance.

5. If 3A + 4B → 12C which of the following is not true:

D. 2 moles of B produces 8 moles of C

Using the coefficients we can determine the exact molar ratios of products and reactant. 2 moles of B would produce 6 moles of C, not 8.

### Limiting Reagents

1. What is the limiting reagent?

A. The reactant in a chemical reaction that determines the amount of product that can be produced

By definition the limiting reagent is the reactant in a chemical reaction that determines the amount of product that can be produced. Once the limiting reagent is used up, no more product can be formed.

2. What is the excess reagent?

D. The reactant in a chemical reaction that is present in a greater amount than necessary to react to completion

By definition the excess reagent is the reactant in a chemical reaction that is present in a greater amount that necessary for the reaction to go to completion. Upon the completion of the reaction some of the excess reagent will still be present.

3. Pretend you have a job building tricycles. If you had 100 handlebars, 150 wheels, 250 pedals, and 75 seats, how many bikes could you build?

A. 50

Each tricycle needs three wheels. Therefore we can only make 50 tricycles with 150 wheels. The number of wheels is the limiting reagent.

4. In the previous question, how many pedals are left over after you have built all of the tricycles you can build?

C. 150

The pedals are in excess. Once we build 50 tricycles we have used 100 pedals (2 per tricycle). That means we have 150 pedals left over.

5. Consider N2 + 3H2 → 2NH3. If 18 moles of N2 is reacted with 36 moles of H2, which of the following is false?

D. N2 is the limiting reagent

H2 is the limiting reagent because it can only produce 24 moles of NH3while the given amount of N2 can produce 36 moles of NH3.

### Reaction Yields

1. The actual yield is:

B. The amount of product that is actually obtained from a reaction

The actual yield is the amount of product that is actually obtained from a reaction. A lot of things can go wrong in a reaction, and some reactions don't even go to completion.

2. The theoretical yield is:

A. The amount of product that would result if the entire limiting reagent reacted

The theoretical yield is the amount of product that would result if the entire limiting reagent reacts to completion. This is the maximum amount of product that can be achieved in a reaction.

3. Percent yield is calculated by:

D. Dividing the actual yield by the theoretical yield and multiplying by 100%

The percent yield is calculated by dividing the actual yield by the theoretical yield and multiplying by 100%: 4. The theoretical yield of a reaction is 8.3 g. The actual yield is 6.4 g. What is the percent yield?

C. 77.1%  5. The theoretical yield of a reaction is 9.2 g. The percent yield is 63%. What is the actual yield?

C. 5.8 g  Solve for actual yield to get 5.8 g.

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