General Chemistry Review: Moles, Stoichiometry, and Reactions

Comprehensive practice questions covering the stoichiometric principles, chemical reaction types, yields, and density based on the lecture notes.

What is the definition of a mole in chemistry?

A counting unit equal to Avogadro's number: $6.022 \times 10^{23}$ particles

What numerical value is known as Avogadro's number?

$6.022 \times 10^{23}$ particles

How many atoms are contained in $1$ mole of a monatomic substance?

$6.022 \times 10^{23}$ atoms

How many molecules are contained in $1$ mole of a molecular compound?

$6.022 \times 10^{23}$ molecules

How many formula units are present in $1$ mole of an ionic compound?

$6.022 \times 10^{23}$ formula units

According to the DAT application, how many molecules are in $1$ mole of $CO_2$?

$6.022 \times 10^{23}$ molecules

Does $1$ mole of $H_2O$ contain the same number of molecules as $1$ mole of $CO_2$?

Yes, because all moles contain Avogadro's number of particles.

What is the definition of molar mass?

The mass of one mole of a substance.

What are the standard units for molar mass?

$$g\,mol^{-1}$$

Where can you find the values needed to determine an element's molar mass?

On the periodic table.

What is the molar mass of oxygen gas ($O_2$)?

$$32\,g\,mol^{-1}$$

How is the molar mass of $O_2$ calculated ($16 \times 2$)?

$$32\,g\,mol^{-1}$$

What is the molar mass of $CO_2$?

$$44\,g\,mol^{-1}$$

What sum of atomic masses ($12 + 16 + 16$) gives the molar mass of carbon dioxide?

$$44\,g\,mol^{-1}$$

What is the equation relating moles ($n$), mass ($m$), and molar mass ($M$)?

$$n = \frac{m}{M}$$

In the equation $n = \frac{m}{M}$, what does $n$ represent?

moles

In the equation $n = \frac{m}{M}$, what does $m$ represent?

mass

In the equation $n = \frac{m}{M}$, what does $M$ represent?

molar mass

How many moles are present in $48\,g$ of $O_2$?

$$48\,g \div 32\,g\,mol^{-1} = 1.5\,mol$$

What is the molar outcome of dividing $48\,g$ of $O_2$ by its molar mass?

$$1.5\,mol$$

What is the procedure to convert moles to particles?

Multiply by Avogadro's number.

How many molecules are in $2$ moles of $H_2O$?

$2 \times (6.022 \times 10^{23}) = 1.204 \times 10^{24}$ molecules

What is the definition of percent composition?

The percentage of a compound's mass contributed by a specific element.

What is the equation for percent composition?

$$(\text{Mass of element} \div \text{Molar mass of compound}) \times 100$$

Why is knowledge of percent composition useful in chemistry?

It is used for determining empirical and molecular formulas.

What is the percent composition of hydrogen in water ($H_2O$)?

$$11.1\%$$

Do the percent compositions of all elements in a single compound always add up to $100\%$?

Yes.

Between $NaCl$ and $NaOH$, which compound has a greater percent sodium?

$$NaOH$$

Why does $NaOH$ have a greater percent sodium than $NaCl$?

Because the overall molar mass of $NaOH$ is smaller.

What is a molecular formula?

The actual number of each atom present in a molecule.

What is the molecular formula of ethane?

$$C_2H_6$$

What is an empirical formula?

The simplest whole-number ratio of atoms.

What is the empirical formula of ethane ($C_2H_6$)?

$$CH_3$$

What is the empirical formula of a compound with the molecular formula $C_6H_{18}O_3$?

$$C_2H_6O$$

According to the DAT Trap, can an empirical formula and molecular formula be identical?

Yes. Example: $CO_2$

What is the first step in finding an empirical formula from percent composition?

Assume a $100\,g$ sample.

What is the second step in finding an empirical formula from percent composition?

Convert grams to moles.

What is the third step in finding an empirical formula from percent composition?

Divide by the smallest mole value.

What is the final step in finding an empirical formula after dividing by the smallest mole value?

Convert to a whole-number ratio.

If a compound is $96\%$ carbon and $4\%$ hydrogen, what is its empirical formula?

$$C_2H$$

How do you find a molecular formula from an empirical formula?

Divide the actual molar mass by the empirical molar mass and multiply all subscripts by that factor.

What is the molecular formula if the empirical formula is $HCO_2$ and the actual molar mass is $90\,g\,mol^{-1}$?

$$H_2C_2O_4$$

According to the DAT SUPER TRAP, can subscripts be changed when balancing equations?

NO. Never.

What is the only component that can be changed when balancing chemical equations?

Coeqicients only.

Balance the following equation: $H_2 + O_2 \rightarrow H_2O$

$$2H_2 + O_2 \rightarrow 2H_2O$$

Balance the following equation: $N_2 + H_2 \rightarrow NH_3$

$$N_2 + 3H_2 \rightarrow 2NH_3$$

According to the DAT Trap, what should you do if you obtain a coeqicient of $1/2$ while balancing?

Multiply the entire equation until all coeqicients are whole numbers.

What does stoichiometry use to relate reactants and products?

Mole ratios from balanced equations.

In the reaction $2H_2 + O_2 \rightarrow 2H_2O$, how many moles of $H_2O$ are produced for every $1$ mole of $O_2$ consumed?

$$2\,moles$$

Using the equation $N_2 + 3H_2 \rightarrow 2NH_3$, how many moles of $NH_3$ are produced from $250\,mol$ of $N_2$?

$$500\,mol\,NH_3$$

Using the equation $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, how many moles of $O_2$ are needed for $2\,mol$ of $CH_4$?

$$4\,mol\,O_2$$

According to the DAT TRAP, can grams be converted directly to grams using coeqicients?

No. Always: grams → moles → moles → grams

What is the definition of the limiting reagent?

The reactant that gets used up first.

What specific reactant determines how much product forms in a reaction?

The limiting reagent.

What is the definition of the excess reagent?

The reactant left over after the reaction.

If a reaction stops because reactant A is completely consumed, what is reactant A called?

The limiting reagent.

According to the DAT Trap, is the reactant with the smaller mass always the limiting reagent?

No. You must compare moles and stoichiometric ratios.

What is the mandatory first step in every limiting reagent problem?

Convert all reactants into moles.

Which reagent must be used to calculate the theoretical yield?

The limiting reagent.

What is the definition of density?

Mass per unit volume.

What equation defines density?

$$\rho = \frac{m}{V}$$

What is the density of a substance with a mass of $20\,g$ and a volume of $10\,mL$?

$$2\,g\,mL^{-1}$$

If an object has a density of $0.8\,g\,mL^{-1}$, will it float in water ($1.0\,g\,mL^{-1}$)?

Yes.

Will an object with a density of $3.0\,g\,mL^{-1}$ float or sink in water?

Sink.

Does the density change if you double both the mass and the volume of a sample?

No. Density remains constant.

What is the volume of a $50\,g$ sample with a density of $2\,g\,mL^{-1}$?

$$25\,mL$$

What is the definition of theoretical yield?

The maximum possible product predicted by stoichiometry.

What is the definition of actual yield?

The amount of product actually obtained experimentally.

What does percent yield represent?

The Eqiciency of a reaction.

What equation is used for calculating percent yield?

$$(\text{actual} \div \text{theoretical}) \times 100$$

What is the percent yield if the actual yield is $15\,g$ and the theoretical yield is $50\,g$?

$$30\%$$

In a properly performed experiment, can the percent yield exceed $100\%$?

Typically no; values above $100\%$ indicate contamination or measurement error.

Which yield is calculated through stoichiometry?

Theoretical yield.

Which yield results from laboratory work?

Actual yield.

What is the definition of an element?

A substance made of one type of atom.

Is $Cl_2$ classified as an element or a compound?

Element.

Is $CO_2$ classified as an element or a compound?

Compound.

What is the definition of a compound?

Two or more diqerent elements chemically bonded.

What is a physical change?

A change where no new substance forms.

Is melting ice a physical or chemical change?

Physical change.

Is evaporating water a physical or chemical change?

Physical change.

Is tearing paper a physical or chemical change?

Physical change.

What is a chemical change?

A change where a new substance forms.

Is burning paper a physical or chemical change?

Chemical change.

Is combining sodium and chlorine to form $NaCl$ a physical or chemical change?

Chemical change.

What is a synthesis reaction?

A reaction where two or more substances combine into one product.

What is the reaction type for $A + B \rightarrow AB$?

Synthesis.

What is a decomposition reaction?

A reaction where one compound breaks into smaller substances.

What is the reaction type for $AB \rightarrow A + B$?

Decomposition.

What is a single displacement reaction?

A reaction where one element replaces another.

What is the reaction type for $A + BC \rightarrow AC + B$?

Single displacement.

What is a double displacement reaction?

A reaction where two compounds exchange ions.

What is the reaction type for $AB + CD \rightarrow AD + CB$?

Double displacement.

To which category do neutralization reactions belong?

Double displacement.

To which category do precipitation reactions belong?

Double displacement.

What is a combustion reaction?

A reaction with oxygen that releases heat and light.

What are ALWAYS the products of complete hydrocarbon combustion?

$CO_2$ and $H_2O$.

What is the reaction type for $CH_4 + O_2 \rightarrow CO_2 + H_2O$?

Combustion.

In rapid fire, what does $1$ mole equal?

$6.022 \times 10^{23}$ particles

In rapid fire, how is mass converted to moles?

$$m \div M$$