Every sample of pure water, whatever its source, contains 11.19% hydrogen and 88.81% oxygen by mass. Which law does this illustrate?
Law of definite proportions
✓
Law of multiple proportions
Law of conservation of matter
Periodic law
Correct Answer
Law of definite proportions
The law of definite proportions states that a pure compound always contains the same elements in the same proportions by mass. The law of multiple proportions compares two or more different compounds made of the same elements. The law of conservation of matter concerns total mass before and after a chemical change. The periodic law concerns the repeating properties of elements, not the makeup of one compound.
Category: Early Atomic Theory
Question 2
Carbon forms both CO and CO2. For a fixed mass of carbon, what is the ratio of the mass of oxygen in CO to the mass of oxygen in CO2?
1:1
1:2
✓
2:3
1:3
Correct Answer
1:2
CO has one oxygen atom per carbon atom and CO2 has two, so for the same carbon the oxygen masses are in a 1:2 ratio, a small whole-number ratio predicted by the law of multiple proportions. A 1:1 ratio would mean the two compounds had the same composition. A 2:3 ratio does not match either formula. A 1:3 ratio would require a compound with three oxygen atoms per carbon atom.
Category: Early Atomic Theory
Question 3
Whose cathode ray experiments showed that atoms contain small, negatively charged particles?
Ernest Rutherford
Robert Millikan
J. J. Thomson
✓
John Dalton
Correct Answer
J. J. Thomson
J. J. Thomson showed that cathode rays are streams of negatively charged particles, later called electrons, far lighter than any atom. Ernest Rutherford discovered the nucleus with his gold foil experiment. Robert Millikan measured the charge of the electron. John Dalton proposed the early atomic theory, in which atoms were indivisible.
Category: Discovery of Subatomic Particles
Question 4
In the gold foil experiment, most alpha particles passed straight through the foil but a few bounced sharply back. What did this show?
Electrons are spread evenly through a sphere of positive charge
Atoms are solid, indivisible spheres of matter
Electrons orbit the center at fixed energy levels
An atom's mass and positive charge are concentrated in a tiny nucleus
✓
Correct Answer
An atom's mass and positive charge are concentrated in a tiny nucleus
Because most alpha particles passed through, atoms are mostly empty space, and the rare large deflections meant the positive charge and nearly all the mass sit in a very small, dense center. Electrons spread evenly through a sphere of positive charge is the plum pudding model, which the results disproved. Atoms as solid, indivisible spheres is Dalton's picture, which cannot explain particles passing through. Electrons orbiting at fixed energy levels is a later model and was not tested by this experiment.
Category: Discovery of Subatomic Particles
Question 5
What did Millikan's oil drop experiment determine?
The charge of an electron
✓
The mass of a proton
The existence of the nucleus
The charge-to-mass ratio of the electron
Correct Answer
The charge of an electron
Millikan measured the charges on tiny oil droplets and found they were always whole-number multiples of one value, the charge of an electron. The mass of a proton was not measured in this experiment. The existence of the nucleus was shown by Rutherford's gold foil experiment. The charge-to-mass ratio of the electron was measured earlier by Thomson.
Category: Discovery of Subatomic Particles
Question 6
Which subatomic particle has no electric charge?
Proton
Neutron
✓
Electron
Alpha particle
Correct Answer
Neutron
The neutron is electrically neutral and is found in the nucleus with the protons. The proton carries a charge of 1+. The electron carries a charge of 1-. An alpha particle carries a charge of 2+ because it contains two protons.
Category: Atomic Structure
Question 7
What determines the identity of an element?
The number of neutrons in its nucleus
The number of electrons it has gained
The number of protons in its nucleus
✓
Its total count of protons and neutrons
Correct Answer
The number of protons in its nucleus
The number of protons, the atomic number, defines the element; every atom with 6 protons is carbon. The number of neutrons can vary among isotopes of the same element. The number of electrons gained only changes the charge, forming an ion of the same element. The total count of protons and neutrons is the mass number, which differs between isotopes.
Category: Atomic Structure
Question 8
How many neutrons are in an atom of iron-56? (The atomic number of iron is 26.)
26
56
82
30
✓
Correct Answer
30
The number of neutrons is the mass number minus the atomic number: 56 - 26 = 30. The answer 26 is the number of protons. The answer 56 is the mass number, the total of protons and neutrons. The answer 82 comes from adding the two numbers instead of subtracting.
Category: Atomic Structure
Question 9
How many electrons does the ion S 2- have? (The atomic number of sulfur is 16.)
18
✓
16
14
32
Correct Answer
18
A neutral sulfur atom has 16 electrons, and a 2- charge means it has gained 2 more, giving 18. The answer 16 is the electron count of the neutral atom. The answer 14 comes from subtracting 2, as if the ion had lost electrons. The answer 32 is the mass number of the most common sulfur isotope, not an electron count.
Category: Atomic Structure
Question 10
A nitrogen atom that gains three electrons forms N 3-. What is this ion called?
Nitrate
Nitride
✓
Nitrite
Nitrogenide
Correct Answer
Nitride
A monatomic anion is named by adding the suffix -ide to the root of the element name, so N 3- is nitride. Nitrate is the polyatomic ion NO3-. Nitrite is the polyatomic ion NO2-. Nitrogenide is not a standard name because the -ide ending attaches to the root, not the full element name.
Category: Chemical Nomenclature
Question 11
One atomic mass unit is defined as exactly 1/12 of the mass of one atom of which isotope?
Hydrogen-1
Oxygen-16
Carbon-12
✓
Carbon-14
Correct Answer
Carbon-12
The atomic mass unit is defined so that one carbon-12 atom has a mass of exactly 12 amu. Hydrogen-1 has a mass of about 1.008 amu but is not the defining standard. Oxygen-16 is not the modern standard, although oxygen was the basis of older mass scales. Carbon-14 is a radioactive isotope and has a mass of about 14 amu.
Category: Atomic Structure
Question 12
Boron has two natural isotopes: boron-10 (10.013 amu, 19.9% abundance) and boron-11 (11.009 amu, 80.1% abundance). What is boron's average atomic mass, to three significant figures?
10.5 amu
10.2 amu
11.0 amu
10.8 amu
✓
Correct Answer
10.8 amu
The average atomic mass is the abundance-weighted sum: 0.199 x 10.013 + 0.801 x 11.009 = 1.993 + 8.818 = 10.811, which rounds to 10.8 amu. The value 10.5 amu is a simple average that ignores abundance. The value 10.2 amu comes from swapping the two abundances. The value 11.0 amu uses only the more abundant isotope.
Category: Atomic Structure
Question 13
The molecular formula of glucose is C6H12O6. What is its empirical formula?
CH2O
✓
C6H12O6
C2H4O2
CHO
Correct Answer
CH2O
An empirical formula shows the simplest whole-number ratio of atoms; dividing each subscript in C6H12O6 by 6 gives CH2O. C6H12O6 is the molecular formula itself, not reduced. C2H4O2 is reduced only by 3, so it is not the simplest ratio. CHO has the wrong ratio of hydrogen to carbon.
Category: Chemical Formulas
Question 14
Which type of formula shows how the atoms in a molecule are connected to one another by bonds?
Molecular formula
Structural formula
✓
Empirical formula
Formula unit
Correct Answer
Structural formula
A structural formula draws the bonds between atoms, showing which atoms are attached to which. A molecular formula gives only the number of each kind of atom. An empirical formula gives only the simplest ratio of atoms. A formula unit is the simplest ratio of ions in an ionic compound and shows no bonds.
Category: Chemical Formulas
Question 15
In which group of the periodic table are the halogens found?
Group 1
Group 2
Group 17
✓
Group 18
Correct Answer
Group 17
The halogens, including fluorine, chlorine, bromine and iodine, make up group 17. Group 1 holds the alkali metals. Group 2 holds the alkaline earth metals. Group 18 holds the noble gases.
Category: The Periodic Table
Question 16
Which element is an alkali metal?
Calcium
Magnesium
Aluminum
Potassium
✓
Correct Answer
Potassium
Potassium is in group 1, the alkali metals, along with lithium and sodium. Calcium is an alkaline earth metal in group 2. Magnesium is also an alkaline earth metal in group 2. Aluminum is a metal in group 13.
Category: The Periodic Table
Question 17
Which of these compounds is ionic?
KBr
✓
CO2
NH3
SO2
Correct Answer
KBr
KBr forms from potassium, a metal, and bromine, a nonmetal, so it is made of K+ and Br- ions. CO2 is a molecular compound of two nonmetals. NH3 is a molecular compound of two nonmetals. SO2 is also a molecular compound of two nonmetals.
Category: Ionic and Molecular Compounds
Question 18
What is the charge of the ion that calcium forms in its compounds?
1+
2+
✓
2-
1-
Correct Answer
2+
Calcium is a group 2 metal, and it loses two electrons to form Ca 2+. A 1+ charge is typical of group 1 metals such as sodium. A 2- charge is typical of group 16 nonmetals such as oxygen. A 1- charge is typical of group 17 nonmetals such as chlorine.
Category: Ionic and Molecular Compounds
Question 19
Which name correctly identifies the ionic compound Fe2O3?
Iron(II) oxide
Iron(VI) oxide
Iron(III) oxide
✓
Iron(III) trioxide
Correct Answer
Iron(III) oxide
Each oxide ion is O 2-, so three of them total 6-, and two iron ions must total 6+, making each Fe 3+; the Roman numeral shows that charge, giving iron(III) oxide. Iron(II) oxide is FeO. Iron(VI) oxide wrongly uses the total iron charge instead of the charge on each iron ion. Iron(III) trioxide wrongly combines a Roman numeral with a Greek prefix.
Category: Chemical Nomenclature
Question 20
Using Greek prefixes, how is the molecular compound N2O5 named?
Nitrogen pentoxide
Dinitrogen tetroxide
Pentanitrogen dioxide
Dinitrogen pentoxide
✓
Correct Answer
Dinitrogen pentoxide
Binary molecular compounds use Greek prefixes for the number of each atom: di- for 2 nitrogen atoms and penta- for 5 oxygen atoms, giving dinitrogen pentoxide. Nitrogen pentoxide leaves out the di- prefix for the two nitrogen atoms. Dinitrogen tetroxide is N2O4. Pentanitrogen dioxide reverses the counts.
Category: Chemical Nomenclature