One mole of any substance contains 6.022 x 10^23 of that substance's entities, whether atoms, molecules, or formula units.
Correct Answer
True
True. A mole is a counting unit, like a dozen, defined as 6.02214076 x 10^23 entities. The kind of entity depends on the substance: atoms for an element such as Fe, molecules for H2O, formula units for NaCl.
Category: The Mole
Question 2
A 10 g sample of helium and a 10 g sample of argon contain the same number of atoms.
Correct Answer
False
False. Equal masses contain equal numbers of atoms only when the atoms have the same mass. Helium (about 4.00 g/mol) is roughly ten times lighter per atom than argon (about 39.95 g/mol), so 10 g of helium contains about ten times as many atoms.
Category: The Mole
Question 3
One mole of O2 molecules has twice the mass of one mole of oxygen atoms.
Correct Answer
True
True. Each O2 molecule contains two oxygen atoms, so a mole of O2 (32.00 g) has twice the mass of a mole of O atoms (16.00 g). The number of particles is the same in each, but each O2 particle is twice as heavy.
Category: The Mole
Question 4
The formula mass of a compound is found by adding the atomic numbers of all the atoms in its formula.
Correct Answer
False
False. Formula mass is the sum of the average atomic masses of all the atoms in the formula, each multiplied by its subscript. Atomic number counts protons and does not account for neutrons or isotope abundance.
Category: Formula Mass
Question 5
Using H = 1.008 amu and O = 16.00 amu, the molecular mass of water, H2O, is 18.02 amu.
Correct Answer
True
True. Two hydrogen atoms contribute 2(1.008) = 2.016 amu and one oxygen atom contributes 16.00 amu, for a total of 18.016, which rounds to 18.02 amu. The molar mass of water is therefore 18.02 g/mol.
Category: Formula Mass
Question 6
An empirical formula gives the simplest whole-number ratio of the atoms of each element in a compound.
Correct Answer
True
True. An empirical formula reduces the atom ratio to lowest whole numbers. It does not necessarily show the actual number of atoms in a molecule, which is the job of the molecular formula.
Category: Empirical Formulas
Question 7
Two different compounds that share the same empirical formula must also have the same molecular formula.
Correct Answer
False
False. Different compounds can share one empirical formula while having different molecular formulas. Acetylene, C2H2, and benzene, C6H6, both have the empirical formula CH.
Category: Empirical Formulas
Question 8
The percent composition by mass of a pure compound is the same no matter how large or small the sample is.
Correct Answer
True
True. A pure compound always contains its elements in a fixed mass ratio, so a 1 g sample and a 100 g sample of water both contain about 11.2% hydrogen by mass. This is why percent composition can be used to identify a compound.
Category: Percent Composition
Question 9
A compound's molecular formula is a whole-number multiple of its empirical formula.
Correct Answer
True
True. The molecular formula equals the empirical formula multiplied by a whole number n, found by dividing the molar mass by the empirical formula mass. When n is 1, the two formulas are identical, as in water.
Category: Empirical Formulas
Question 10
Adding water to a solution increases the number of moles of solute it contains.
Correct Answer
False
False. Adding solvent increases the solution volume but leaves the moles of solute unchanged. Because the same amount of solute is spread through a larger volume, the concentration decreases.
Category: Dilution
Question 11
Equal volumes of two solutions that have the same molarity contain the same number of moles of solute.
Correct Answer
True
True. Moles of solute equal molarity times volume, so if both factors are the same, the moles are the same. For example, 0.500 L of any 0.20 M solution holds 0.10 mol of solute.
Category: Molarity
Question 12
A 200 mL portion taken from a 0.50 M stock solution has twice the molarity of a 100 mL portion taken from the same stock.
Correct Answer
False
False. Concentration does not depend on how much of a solution is taken. Both portions are 0.50 M; the larger portion simply contains twice as many moles of solute.
Category: Molarity
Question 13
A solution that is 5.0% NaCl by mass contains 5.0 g of NaCl in every 100.0 g of solution.
Correct Answer
True
True. Mass percentage is the mass of solute divided by the mass of the whole solution, times 100%. So 5.0% by mass means 5.0 g of NaCl per 100.0 g of solution, which includes the 95.0 g of water.
Category: Concentration Units
Question 14
Volume percentage is used mainly to express the concentration of solid solutes dissolved in liquid solvents.
Correct Answer
False
False. Volume percentage is used mainly for solutions in which a liquid solute is dissolved in a liquid solvent, such as alcohol in water. Solid solutes are usually described by mass percentage or by mass-volume percentage.
Category: Concentration Units
Question 15
Rubbing alcohol labeled as 70% isopropanol by volume contains 70 mL of isopropanol in every 100 mL of solution.
Correct Answer
True
True. Volume percentage is solute volume divided by solution volume times 100%. A 70% (v/v) solution therefore has 70 mL of isopropanol per 100 mL of the final solution.
Category: Concentration Units
Question 16
Using N = 14.01 and H = 1.008, the molar mass of ammonia, NH3, is 15.02 g/mol.
Correct Answer
False
False. Ammonia has three hydrogen atoms, so its molar mass is 14.01 + 3(1.008) = 17.03 g/mol. The value 15.02 g/mol counts only one hydrogen atom.
Category: Formula Mass
Question 17
Using H = 1.008 and O = 16.00, hydrogen makes up about 11% of the mass of water.
Correct Answer
True
True. Water's molar mass is 18.02 g/mol, of which hydrogen contributes 2.016 g. The mass percentage of hydrogen is 2.016 / 18.02 x 100% = 11.2%, and oxygen accounts for the remaining 88.8%.
Category: Percent Composition
Question 18
One mole of CO2 contains 6.022 x 10^23 oxygen atoms.
Correct Answer
False
False. Each CO2 molecule contains two oxygen atoms, so one mole of CO2 contains 2 mol of oxygen atoms, or 1.204 x 10^24 oxygen atoms. It contains 6.022 x 10^23 carbon atoms.
Category: The Mole
Question 19
The empirical formula of ethane, C2H6, is CH2.
Correct Answer
False
False. Dividing both subscripts of C2H6 by their greatest common factor, 2, gives CH3. CH2 is the empirical formula of compounds with a 1:2 carbon-to-hydrogen ratio, such as ethylene, C2H4.
Category: Empirical Formulas
Question 20
Molarity is calculated by dividing the moles of solute by the volume of solvent used, not the total volume of the solution.
Correct Answer
False
False. Molarity is moles of solute per liter of solution, meaning the final total volume. A solution is prepared by dissolving the solute and then adding solvent until the desired total volume is reached.
Category: Molarity