A student claims that one atom of carbon weighs 12.01 grams because carbon's atomic mass is 12.01. What is wrong with this reasoning?
AThe student is correct — atomic mass directly gives the mass of one atom in grams
BOne atom of carbon weighs 12.01 atomic mass units (u), not 12.01 grams — 12.01 g/mol applies to an entire mole (6.022 × 10²³ atoms) of carbon
CThe error is the number 12.01; carbon-12 weighs exactly 12.00 u
DAtomic mass cannot be used to determine mass at all — only density can
Atomic mass (u) and molar mass (g/mol) are numerically equal but entirely different quantities. A single carbon atom weighs ~12.01 u — an inconceivably tiny amount. Molar mass tells you the mass of one mole (Avogadro's number) of those atoms in grams. The numerical equality is by design: the mole was defined so that atomic mass in u maps onto molar mass in g/mol, bridging the microscopic and macroscopic scales.
Question 2 Multiple Choice
You have 36.04 grams of water (molar mass = 18.02 g/mol). What fraction of that mass comes from hydrogen?
A50% — water contains two hydrogen atoms and one oxygen atom, so hydrogen is 2/3 of the atoms
B11.2% — hydrogen contributes 2 × 1.008 = 2.016 g/mol out of 18.02 g/mol total
C88.8% — oxygen is the heavier element so hydrogen must be the minority
D33.3% — each of the three atoms contributes equally to mass
Percent composition by mass uses molar masses, not atom counts. Water has two H atoms (2 × 1.008 = 2.016 g/mol) and one O atom (16.00 g/mol), totaling 18.02 g/mol. Hydrogen's fraction is 2.016/18.02 = 11.2%. Option A confuses atom count ratio with mass ratio — a classic mistake. Oxygen, despite being only one atom out of three, contributes 88.8% of water's mass because it is sixteen times heavier than hydrogen.
Question 3 True / False
The numerical value of an element's atomic mass in atomic mass units (u) equals its molar mass in grams per mole. This is a convenient coincidence.
TTrue
FFalse
Answer: False
It is not a coincidence — it is by definition. The mole was specifically defined as the number of atoms in exactly 12 grams of carbon-12, which is Avogadro's number (6.022 × 10²³). Because carbon-12 is defined as exactly 12 u per atom, 12 grams of it contains exactly one mole of atoms. This definition ensures that atomic mass in u and molar mass in g/mol are always numerically identical, creating the bridge between microscopic and macroscopic chemistry.
Question 4 True / False
To find the number of molecules in a 50-gram sample of a compound, you primarily need the sample mass — no other information is required.
TTrue
FFalse
Answer: False
You also need the molar mass of the compound. The conversion chain is: mass → moles (dividing by molar mass) → number of molecules (multiplying by Avogadro's number). Without the molar mass, you cannot convert grams to moles. A 50-gram sample of water (18.02 g/mol) contains a very different number of molecules than a 50-gram sample of glucose (180.2 g/mol).
Question 5 Short Answer
Why must chemists convert between grams and moles rather than working directly in grams when calculating how substances react?
Think about your answer, then reveal below.
Model answer: Chemical reactions follow fixed ratios of atoms and molecules, not fixed mass ratios. A balanced equation like 2H₂ + O₂ → 2H₂O says two molecules of H₂ react with one molecule of O₂ — a 2:1 particle ratio. To use this stoichiometric ratio, you must work in moles (counts of particles scaled by Avogadro's number). Grams alone do not give you particle ratios because different elements have different masses. The mole is the translator between the measurable world of grams and the reactive world of atoms and molecules.
This is the fundamental reason the mole concept exists. Atoms react in whole-number ratios determined by the balanced equation, not in fixed-mass ratios. Once you convert mass to moles using molar mass, the stoichiometric coefficients directly give you the reaction ratios. This conversion chain (mass → moles → particles → moles of product → mass of product) underlies every quantitative calculation in chemistry.