CHEM 1120 Term 2 Chapter 3 & 4 Review – Complete Study Guide (ECU
Chemistry 2026)
A hypothetical element consists of two isotopes of masses 79.95 amu and 81.95 amu with
abundances of 31.3% and 68.7% respectively. What is the average atomic mass of this element?
- ANS ✔✔81.3 amu
naturally occurring copper exists in two isotopic forms: 63Cu and 65Cu. The atomic mass of
copper is 63.55 amu. What is the approximate natural abundance of 63Cu? - ANS ✔✔70%
A sample of ammonia has a mass of 89.9 g. How many molecules are in this sample? - ANS
✔✔3.8*10^24 molecules
how many moles of hydrogen sulfide are contained in 44.4 g sample of this gas? - ANS ✔✔1.30
mol
Calculate the molar mass of barium nitrite - ANS ✔✔229.35 g/mol
what is the molar mass of Ca3(PO4)2 - ANS ✔✔310.18 g/mol
what is the mass of a 4.110 mol sample of sodium hydroxide - ANS ✔✔164.4 g
the molar mass of the compound formed by sodium and selenium is - ANS ✔✔124.9 g/mol
one molecule of a compound weighs 2.50*10^22g. the molar mass of this compound is: - ANS
✔✔151 g/mol
,which of the following compounds has the same percent composition by mass as styrene, C8H8
- ANS ✔✔all of these:
Acetylene C2H2
Benzene C6H6
Cyclobutadiene C4H4
A-ethyl naphthalene C12H12
suppose you are given the percent by mass of the elements in a compound and you wish to
determine the empirical formula. which of the following is true? - ANS ✔✔you must convert
percent by mass to relative numbers of atoms
A hydrocarbon (a compound consisting solely of carbon and hydrogen) is found to be 85.6 %
carbon by mass. What is the empirical formula for this compound? - ANS ✔✔Ch2
Vitamin C contains the elects C, H, and O. it is known to contain 40.9% C and 4.58& H by mass.
The molar mass of vitamin C has been found to be about 180 g/mol. The molecular formula for
vitamin C is: - ANS ✔✔C6H8O6
A compound of sulfur and fluorine is found to contain 25.24% by mass of sulfur. If the molar
mass of the compound is 254.11 g/mol, what is its molecular formula? - ANS ✔✔S2F10
The molecular formula always represents the total number of atoms of each element present in
a compound - ANS ✔✔True
in balancing an equation, we change the ________ to make the number of atoms on each side
of the equation balance. - ANS ✔✔Coefficients of compounds
what is the coefficient for water when the following equation is balanced?
, As(OH)3 + H2O --> As2(SO4)3 + H2O - ANS ✔✔6
What is the coefficient for oxygen when the following equation is balanced? - ANS ✔✔7
Determine the coefficient for O2 when the following equation is balanced in standard
form(smallest whole numbers.)
C6H14 + O2 --> CO2 + H2O - ANS ✔✔19
What is the sum of the coefficient of the following equation when it is balanced using smallest
whole numbers? - ANS ✔✔9
wPCl5 + xH2O --> yPOCL3+ zHCl - ANS ✔✔the equation is balanced when:
w=1
x=1
y=1
z=2
Potassium forms an oxide containing 1 oxygen atoms of potassium. what is the coefficient of
oxygen in the balanced equation for the reaction of potassium with oxygen to form this oxide? -
ANS ✔✔1
if x equals 2, the coefficient z equals:
xP4O6 + yH2O --> zH3PO3 - ANS ✔✔none of these
Chemistry 2026)
A hypothetical element consists of two isotopes of masses 79.95 amu and 81.95 amu with
abundances of 31.3% and 68.7% respectively. What is the average atomic mass of this element?
- ANS ✔✔81.3 amu
naturally occurring copper exists in two isotopic forms: 63Cu and 65Cu. The atomic mass of
copper is 63.55 amu. What is the approximate natural abundance of 63Cu? - ANS ✔✔70%
A sample of ammonia has a mass of 89.9 g. How many molecules are in this sample? - ANS
✔✔3.8*10^24 molecules
how many moles of hydrogen sulfide are contained in 44.4 g sample of this gas? - ANS ✔✔1.30
mol
Calculate the molar mass of barium nitrite - ANS ✔✔229.35 g/mol
what is the molar mass of Ca3(PO4)2 - ANS ✔✔310.18 g/mol
what is the mass of a 4.110 mol sample of sodium hydroxide - ANS ✔✔164.4 g
the molar mass of the compound formed by sodium and selenium is - ANS ✔✔124.9 g/mol
one molecule of a compound weighs 2.50*10^22g. the molar mass of this compound is: - ANS
✔✔151 g/mol
,which of the following compounds has the same percent composition by mass as styrene, C8H8
- ANS ✔✔all of these:
Acetylene C2H2
Benzene C6H6
Cyclobutadiene C4H4
A-ethyl naphthalene C12H12
suppose you are given the percent by mass of the elements in a compound and you wish to
determine the empirical formula. which of the following is true? - ANS ✔✔you must convert
percent by mass to relative numbers of atoms
A hydrocarbon (a compound consisting solely of carbon and hydrogen) is found to be 85.6 %
carbon by mass. What is the empirical formula for this compound? - ANS ✔✔Ch2
Vitamin C contains the elects C, H, and O. it is known to contain 40.9% C and 4.58& H by mass.
The molar mass of vitamin C has been found to be about 180 g/mol. The molecular formula for
vitamin C is: - ANS ✔✔C6H8O6
A compound of sulfur and fluorine is found to contain 25.24% by mass of sulfur. If the molar
mass of the compound is 254.11 g/mol, what is its molecular formula? - ANS ✔✔S2F10
The molecular formula always represents the total number of atoms of each element present in
a compound - ANS ✔✔True
in balancing an equation, we change the ________ to make the number of atoms on each side
of the equation balance. - ANS ✔✔Coefficients of compounds
what is the coefficient for water when the following equation is balanced?
, As(OH)3 + H2O --> As2(SO4)3 + H2O - ANS ✔✔6
What is the coefficient for oxygen when the following equation is balanced? - ANS ✔✔7
Determine the coefficient for O2 when the following equation is balanced in standard
form(smallest whole numbers.)
C6H14 + O2 --> CO2 + H2O - ANS ✔✔19
What is the sum of the coefficient of the following equation when it is balanced using smallest
whole numbers? - ANS ✔✔9
wPCl5 + xH2O --> yPOCL3+ zHCl - ANS ✔✔the equation is balanced when:
w=1
x=1
y=1
z=2
Potassium forms an oxide containing 1 oxygen atoms of potassium. what is the coefficient of
oxygen in the balanced equation for the reaction of potassium with oxygen to form this oxide? -
ANS ✔✔1
if x equals 2, the coefficient z equals:
xP4O6 + yH2O --> zH3PO3 - ANS ✔✔none of these
