AP CHEMISTRY PRACTICE MIDTERM
REVIEW SHEET 2026 FULL
QUESTIONS AND ACCURATE
ANSWERS
◉ Mass Spectroscopy. Answer: Technique used to identify the isotopes
of an element and their relative abundance in nature
◉ Empirical Formula. Answer: Lowest whole number ratio of atoms of
the elements in a compound
◉ A student obtains a sample of a pure solid compound. In addition to
Avogadro's number, which of the following must the student know in
order to determine how many molecules are in the sample?
A) Mass of the sample, volume of the sample
B) Mass of the sample, density of the sample
C) Molar mass of the compound, mass of the sample
D) Molar mass of the compound, density of the sample. Answer: C)
Molar mass of the compound, mass of the sample
,-Dividing the mass of the sample by the molar mass of the compound
yields the number of moles of the compound, which, when multiplied by
Avogadro's number, yields the number of molecules of the compound in
the sample.
◉ A student has a 1g sample of each of the following compounds: NaCl,
KBr, and KCl. Which of the following lists the samples in order of
increasing number of moles in the sample?
A) NaCl < KCl < KBr
B) NaCl < KBr < KCl
C) KCl < NaCl <KBr
D) KBr < KCl < NaCl. Answer: D) KBr < KCl < NaCl
-The number of moles is represented by the ratio mass/molar mass.
Since all samples have the same mass, the greater the molar mass, the
smaller the number of moles.
◉ A student has two samples of NaCl, each one from a different source.
Assume that the only potential contaminant in each sample is KCl. The
student runs an experiment to determine the percent by mass of chlorine
in each sample. From the results of this experiment alone, which of the
following questions is most likely to be answered?
A) Which sample has the higher purity?
, B) Which sample has the higher density?
C) What is the source of the contaminants present in each of the
samples?
D) Which sample came from a salt mine, and which sample came from
the ocean?. Answer: A) Which sample has the higher purity?
-For each sample, the percent of chlorine by mass can be compared to
the percent of chlorine by mass of pure NaCl. The sample with the
greater percent of chlorine by mass has the higher purity.
◉ Complete combustion of a sample of a hydrocarbon in excess oxygen
produces equimolar quantities of carbon dioxide and water. Which of the
following could be the molecular formula of the compound?
A) C2H2
B) C2H6
C) C4H8
D) C6H6. Answer: C) C4H8
-The balanced chemical equation is C4H8 + 6 O2 ➝ 4 CO2 + 4 H2O, so
combustion produces CO2 and H2O with a mole ratio of 1 to 1, in
agreement with the observed equimolar ratio.
REVIEW SHEET 2026 FULL
QUESTIONS AND ACCURATE
ANSWERS
◉ Mass Spectroscopy. Answer: Technique used to identify the isotopes
of an element and their relative abundance in nature
◉ Empirical Formula. Answer: Lowest whole number ratio of atoms of
the elements in a compound
◉ A student obtains a sample of a pure solid compound. In addition to
Avogadro's number, which of the following must the student know in
order to determine how many molecules are in the sample?
A) Mass of the sample, volume of the sample
B) Mass of the sample, density of the sample
C) Molar mass of the compound, mass of the sample
D) Molar mass of the compound, density of the sample. Answer: C)
Molar mass of the compound, mass of the sample
,-Dividing the mass of the sample by the molar mass of the compound
yields the number of moles of the compound, which, when multiplied by
Avogadro's number, yields the number of molecules of the compound in
the sample.
◉ A student has a 1g sample of each of the following compounds: NaCl,
KBr, and KCl. Which of the following lists the samples in order of
increasing number of moles in the sample?
A) NaCl < KCl < KBr
B) NaCl < KBr < KCl
C) KCl < NaCl <KBr
D) KBr < KCl < NaCl. Answer: D) KBr < KCl < NaCl
-The number of moles is represented by the ratio mass/molar mass.
Since all samples have the same mass, the greater the molar mass, the
smaller the number of moles.
◉ A student has two samples of NaCl, each one from a different source.
Assume that the only potential contaminant in each sample is KCl. The
student runs an experiment to determine the percent by mass of chlorine
in each sample. From the results of this experiment alone, which of the
following questions is most likely to be answered?
A) Which sample has the higher purity?
, B) Which sample has the higher density?
C) What is the source of the contaminants present in each of the
samples?
D) Which sample came from a salt mine, and which sample came from
the ocean?. Answer: A) Which sample has the higher purity?
-For each sample, the percent of chlorine by mass can be compared to
the percent of chlorine by mass of pure NaCl. The sample with the
greater percent of chlorine by mass has the higher purity.
◉ Complete combustion of a sample of a hydrocarbon in excess oxygen
produces equimolar quantities of carbon dioxide and water. Which of the
following could be the molecular formula of the compound?
A) C2H2
B) C2H6
C) C4H8
D) C6H6. Answer: C) C4H8
-The balanced chemical equation is C4H8 + 6 O2 ➝ 4 CO2 + 4 H2O, so
combustion produces CO2 and H2O with a mole ratio of 1 to 1, in
agreement with the observed equimolar ratio.
