AP Chemistry Unit 1-5 Progress Check: MCQ
Exam Questions with verified Answers 2026
Which of the following numerical expressions gives the number of moles in 5.0g of CaO?
5.0g ÷ 56 g/mol
WHY?
Dividing the mass of the sample by the molar mass of CaO gives the number of moles of CaO in the
sample.
In a lab a student is given a 21g sample of pure Cu metal. Which of the following pieces of information
is most useful for determining the number of Cu atoms in the sample? Assume that the pressure and
temperature in the lab are 1.0atm and 25°C.
The molar mass of Cu
WHY?
By dividing the mass of the sample by the molar mass of Cu, the number of moles of Cu in the sample
can be calculated. A simple multiplication of that number with Avogadro's number will provide the
desired information.
A 1.0mol sample of which of the following compounds has the greatest mass?
N2O5
WHY?
The mass of a sample of any compound can be calculated by n×M, where n is the number of moles of
molecules in the sample and M is the molar mass (mass per mole) of the compound. The compound
with molecules containing the greatest number of atoms of N and O has the greatest molar mass. Since
each sample contains the same number of molecules (1.0mol of molecules), the sample of N2O5 has the
largest mass.
The mass spectrum for an unknown element is shown above. According to the information in the
spectrum, the atomic mass of the unknown element is closest to
91 amu
WHY?
The relative abundances of the isotopes with atomic masses 90 and 92 have a weighted average that
would be less than 91. That result with the influence of the mass and abundance of the isotope with
atomic mass 94 would result in a weighted average closer to 91 than to 93.
The mass spectrum represented above is most consistent with which of the following elements?
,Gd
WHY?
The average atomic mass of Gd is close to 157amu. The relative abundance and masses of the isotopes
shown in the mass spectrum predict an average atomic mass close to 157amu.
Based on the mass spectrum of a pure element represented above, the average atomic mass of the
element is closest to which of the following?
186.3 amu
WHY?
Based on the mass spectrum, the 185amu isotope has a relative abundance close to 40% and the
187amu isotope has a relative abundance close to 60%. This predicts an average atomic mass slightly
higher than 186amu.
Ba2+(aq)+SO42−(aq)→BaSO4(s)
A student obtains a 10.0g sample of a white powder labeled as BaCl2. After completely dissolving the
powder in 50.0mL of distilled water, the student adds excess Na2SO4(s), which causes a precipitate of
BaSO4(s) to form, as represented by the equation above. The student filters the BaSO4(s), rinses it,
and dries it until its mass is constant. Which of the following scientific questions could best be
answered based on the results of the experiment?
Is the BaCl2(s)BaCl2(s) used in the experiment pure?
WHY?
The calculated number of moles of Ba in the precipitate can be compared with the expected number of
moles of Ba in 10.0g of BaCl2. The presence of fewer moles of in the precipitate would imply that the
BaCl2 was not pure.
A student measures the mass of a sample of a metallic element, M. Then the student heats the
sample in air, where it completely reacts to form the compound MO. The student measures the mass
of the compound that was formed. Which of the following questions can be answered from the
results of the experiment?
What is the molar mass of M?
WHY?
The number of moles of O in the compound is equal to the number of moles of M and can be calculated
using the difference in the masses of the sample and the compound. The molar mass of M is calculated
by dividing the mass of the sample of M by the number of moles of M.
A 42.0g sample of compound containing only C and H was analyzed. The results showed that the
sample contained 36.0g of C and 6.0g of H. Which of the following questions about the compound can
be answered using the results of the analysis?
What is the empirical formula of the compound?
, WHY?
The number of moles of C and H is calculated by dividing the masses of each element in the compound
and their respective molar masses. The ratio of the number of moles of C to H can be used to determine
the empirical formula of the compound.
A jar labeled NaCl contains a powder. The table above contains information determined by analyzing
a sample of the powder in the laboratory. What information in the table is the most helpful in
determining whether the powder is pure NaCl?
Mass percent of Na
WHY?
Percent by mass is an intensive property that is determined by the composition of a substance and
therefore is helpful for determining the purity of the powder.
A student obtains a mixture of the chlorides of two unknown metals, X and Z. The percent by mass of
X and the percent by mass of Z in the mixture is known. Which of the following additional information
is most helpful in calculating the mole percent of XCl(s) and of ZCl(s) in the mixture?
The molar masses of X and Z
WHY?
The molar mass (mass÷moles) is the quantity that establishes the ratio of the mass of a pure substance
in a mixture to its number of moles. This ratio can be used to calculate the mole percent of either XCl or
ZCl in the mixture.
A vessel contains a mixture of gases. The mass of each gas used to make the mixture is known. Which
of the following information is needed to determine the mole fraction of each gas in the mixture?
The molar mass of each gas
WHY?
The mole fraction of each gas is equal to the number of moles of the gas divided by the total number of
moles of gases in the mixture. Given the mass of each gas, the molar mass of each gas is needed to
determine the number of moles for each gas and the total number of moles in the mixture.
1s22s22p63s23p6
How many unpaired electrons are in the atom represented by the electron configuration above?
0
WHY?
The 1s,2s,2p,3s, and 3p orbitals are all full, and all the electrons are paired.
Which of the following represents the electron configuration of an oxygen atom in the ground state?
(C)
WHY?
Exam Questions with verified Answers 2026
Which of the following numerical expressions gives the number of moles in 5.0g of CaO?
5.0g ÷ 56 g/mol
WHY?
Dividing the mass of the sample by the molar mass of CaO gives the number of moles of CaO in the
sample.
In a lab a student is given a 21g sample of pure Cu metal. Which of the following pieces of information
is most useful for determining the number of Cu atoms in the sample? Assume that the pressure and
temperature in the lab are 1.0atm and 25°C.
The molar mass of Cu
WHY?
By dividing the mass of the sample by the molar mass of Cu, the number of moles of Cu in the sample
can be calculated. A simple multiplication of that number with Avogadro's number will provide the
desired information.
A 1.0mol sample of which of the following compounds has the greatest mass?
N2O5
WHY?
The mass of a sample of any compound can be calculated by n×M, where n is the number of moles of
molecules in the sample and M is the molar mass (mass per mole) of the compound. The compound
with molecules containing the greatest number of atoms of N and O has the greatest molar mass. Since
each sample contains the same number of molecules (1.0mol of molecules), the sample of N2O5 has the
largest mass.
The mass spectrum for an unknown element is shown above. According to the information in the
spectrum, the atomic mass of the unknown element is closest to
91 amu
WHY?
The relative abundances of the isotopes with atomic masses 90 and 92 have a weighted average that
would be less than 91. That result with the influence of the mass and abundance of the isotope with
atomic mass 94 would result in a weighted average closer to 91 than to 93.
The mass spectrum represented above is most consistent with which of the following elements?
,Gd
WHY?
The average atomic mass of Gd is close to 157amu. The relative abundance and masses of the isotopes
shown in the mass spectrum predict an average atomic mass close to 157amu.
Based on the mass spectrum of a pure element represented above, the average atomic mass of the
element is closest to which of the following?
186.3 amu
WHY?
Based on the mass spectrum, the 185amu isotope has a relative abundance close to 40% and the
187amu isotope has a relative abundance close to 60%. This predicts an average atomic mass slightly
higher than 186amu.
Ba2+(aq)+SO42−(aq)→BaSO4(s)
A student obtains a 10.0g sample of a white powder labeled as BaCl2. After completely dissolving the
powder in 50.0mL of distilled water, the student adds excess Na2SO4(s), which causes a precipitate of
BaSO4(s) to form, as represented by the equation above. The student filters the BaSO4(s), rinses it,
and dries it until its mass is constant. Which of the following scientific questions could best be
answered based on the results of the experiment?
Is the BaCl2(s)BaCl2(s) used in the experiment pure?
WHY?
The calculated number of moles of Ba in the precipitate can be compared with the expected number of
moles of Ba in 10.0g of BaCl2. The presence of fewer moles of in the precipitate would imply that the
BaCl2 was not pure.
A student measures the mass of a sample of a metallic element, M. Then the student heats the
sample in air, where it completely reacts to form the compound MO. The student measures the mass
of the compound that was formed. Which of the following questions can be answered from the
results of the experiment?
What is the molar mass of M?
WHY?
The number of moles of O in the compound is equal to the number of moles of M and can be calculated
using the difference in the masses of the sample and the compound. The molar mass of M is calculated
by dividing the mass of the sample of M by the number of moles of M.
A 42.0g sample of compound containing only C and H was analyzed. The results showed that the
sample contained 36.0g of C and 6.0g of H. Which of the following questions about the compound can
be answered using the results of the analysis?
What is the empirical formula of the compound?
, WHY?
The number of moles of C and H is calculated by dividing the masses of each element in the compound
and their respective molar masses. The ratio of the number of moles of C to H can be used to determine
the empirical formula of the compound.
A jar labeled NaCl contains a powder. The table above contains information determined by analyzing
a sample of the powder in the laboratory. What information in the table is the most helpful in
determining whether the powder is pure NaCl?
Mass percent of Na
WHY?
Percent by mass is an intensive property that is determined by the composition of a substance and
therefore is helpful for determining the purity of the powder.
A student obtains a mixture of the chlorides of two unknown metals, X and Z. The percent by mass of
X and the percent by mass of Z in the mixture is known. Which of the following additional information
is most helpful in calculating the mole percent of XCl(s) and of ZCl(s) in the mixture?
The molar masses of X and Z
WHY?
The molar mass (mass÷moles) is the quantity that establishes the ratio of the mass of a pure substance
in a mixture to its number of moles. This ratio can be used to calculate the mole percent of either XCl or
ZCl in the mixture.
A vessel contains a mixture of gases. The mass of each gas used to make the mixture is known. Which
of the following information is needed to determine the mole fraction of each gas in the mixture?
The molar mass of each gas
WHY?
The mole fraction of each gas is equal to the number of moles of the gas divided by the total number of
moles of gases in the mixture. Given the mass of each gas, the molar mass of each gas is needed to
determine the number of moles for each gas and the total number of moles in the mixture.
1s22s22p63s23p6
How many unpaired electrons are in the atom represented by the electron configuration above?
0
WHY?
The 1s,2s,2p,3s, and 3p orbitals are all full, and all the electrons are paired.
Which of the following represents the electron configuration of an oxygen atom in the ground state?
(C)
WHY?
