1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
Which of the numbered diagrams best represents the particles in a small region of NaCl(aq)? Only a few of the water molecules are shown.
Answer and reasoning
ADiagram 1Correct In NaCl(aq) the Na⁺ and Cl⁻ ions are separated and each is surrounded by water molecules. The partially negative O atoms of water face the Na⁺ ion, and the partially positive H atoms face the Cl⁻ ion: these are the ion-dipole attractions that hold the water molecules around each ion.
BDiagram 2 A student who thinks water's H atoms point toward a cation picks this. O carries the partial negative charge in water, so O faces the Na⁺ ion and the H atoms face the Cl⁻ ion.
CDiagram 3 A student who thinks a dissolved ionic compound stays as intact formula units picks this. When NaCl dissolves, the Na⁺ and Cl⁻ ions separate and each is surrounded by its own water molecules.
DDiagram 4 A student who thinks dissolving is a chemical reaction picks this picture of NaOH and HCl units. Dissolving NaCl separates the ions; no bonds form between the ions and the atoms of water.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
3.8.A.1 Particulate representation of a solution Fix
Particulate representation of a solution
A drawing of a small region of a solution that shows its components as particles (molecules or ions), communicating the structure and properties of the solution through the relative numbers of each kind of particle and through the interactions drawn between them.
Relative concentration in a particulate diagram
In a diagram, the concentration of a solute is shown by the number of solute particles per unit of volume represented. Samples of equal volume are compared by counting solute particles; samples of different volumes are compared by the number of particles per unit volume.
Hydrated ions (ion-dipole interactions)
In an aqueous solution each ion is surrounded by water molecules oriented by the ion's charge: the partially negative O atoms point toward a cation, and the partially positive H atoms point toward an anion.
Representing a dissolved ionic compound
A dissolved ionic compound is drawn as separate, hydrated ions in the ratio given by its formula, for example two Na⁺ ions for every SO₄²⁻ ion in Na₂SO₄(aq). A polyatomic ion such as SO₄²⁻ stays intact, because its atoms are held together by covalent bonds.
Representing a dissolved molecular compound
A molecular solute such as glucose or methanol is drawn as intact molecules dispersed among the solvent molecules: dissolving separates molecules from one another but does not break covalent bonds within them, and such solutes produce essentially no ions.
Solute-solvent interactions in a diagram
The attractions drawn between solute and solvent particles must match their structures: ion-dipole attractions for ions in water, and hydrogen bonds where an H atom bonded to N, O or F in one molecule is attracted to an N, O or F atom of another, as between CH₃OH and H₂O.
Students often think Water molecules surround a cation with their H atoms pointing toward it, and an anion with their O atom pointing toward it. In fact The O atom, which carries the partial negative charge, points toward a cation; the partially positive H atoms point toward an anion.
Students often think A dissolved ionic compound is present as intact formula units, like molecules, rather than as separate ions. In fact No. A soluble ionic compound dissolves as separate ions, each surrounded by water molecules, in the ratio given by its formula.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
Each numbered box in the diagram represents a sample of a solution of the same solute, drawn to the volume labeled below it; solvent molecules are not shown. Which sample has the highest concentration of the solute?
Answer and reasoning
ASolution 1 A student who takes the sample with the most solute particles to be the most concentrated picks this. Its 8 particles are spread through 1.5 mL, 5.3 per mL, which is less than 6.0 per mL.
BSolution 2Correct Concentration is particles per unit volume: 6 particles in 1.0 mL is 6.0 per mL, higher than 8 in 1.5 mL (5.3 per mL), 2 in 0.5 mL (4.0 per mL) or 7 in 2.5 mL (2.8 per mL).
CSolution 3 A student who thinks the largest sample is the most concentrated picks this. Its 7 particles in 2.5 mL give only 2.8 per mL, the lowest concentration of the four.
DSolution 4 A student who thinks the smallest sample must be the most concentrated picks this. Its 2 particles in 0.5 mL give 4.0 per mL, less than 6.0 per mL.
Working Concentration = solute particles ÷ volume. 6 particles ÷ 1.0 mL = 6.0 per mL; 8 ÷ 1.5 mL = 5.3 per mL; 7 ÷ 2.5 mL = 2.8 per mL; 2 ÷ 0.5 mL = 4.0 per mL. The highest is 6.0 particles per mL, the 1.0 mL sample.
A student draws a particulate diagram of Na₂SO₄(aq). The diagram shows four Na⁺ ions and four SO₄²⁻ ions, each surrounded by water molecules. Which statement best evaluates the diagram?
Answer and reasoning
AIt is consistent: a solution of an ionic compound has equal numbers of cations and anions. A student who thinks electrical neutrality means equal numbers of cations and anions picks this. Neutrality requires equal amounts of positive and negative charge; each SO₄²⁻ ion carries −2, so two Na⁺ ions are needed for each SO₄²⁻ ion.
BIt is inconsistent: each SO₄²⁻ should be drawn as a separate S ion and four O ions. A student who thinks polyatomic ions break apart on dissolving picks this. The S and O atoms of a sulfate ion are held together by covalent bonds, so SO₄²⁻ stays intact in solution.
CIt is inconsistent: Na₂SO₄ should be drawn as whole Na₂SO₄ units, not as separate ions. A student who thinks a dissolved ionic compound stays as intact formula units picks this. Soluble ionic compounds dissolve as separate, hydrated ions.
DIt is inconsistent: there should be twice as many Na⁺ ions as SO₄²⁻ ions.Correct Na₂SO₄ contains two Na⁺ ions for each SO₄²⁻ ion, and both kinds of ion are released when it dissolves, so a correct diagram shows Na⁺ and SO₄²⁻ in a 2:1 ratio (for example eight Na⁺ with four SO₄²⁻). This also balances the charge: 2(+1) = +2 for each −2 ion.
A student is drawing a particulate diagram of a solution of methanol, CH₃OH, in water. Which interaction between CH₃OH molecules and H₂O molecules should the diagram show?
Answer and reasoning
AHydrogen bonds between the H atoms of the CH₃ group and the O atoms of the water molecules A student who thinks any H atom can form a hydrogen bond picks this. The H atoms of the CH₃ group are bonded to C, which is not electronegative enough to give them the large partial positive charge needed; only the H on O takes part.
BCovalent bonds that join each CH₃OH molecule to an H₂O molecule to form one new compound A student who thinks dissolving is a chemical reaction picks this. Methanol and water molecules stay intact in the solution; they are held near one another by intermolecular attractions, not joined by covalent bonds.
CHydrogen bonds between an H atom bonded to O in one molecule and an O atom of the otherCorrect Both molecules have an O–H group. An H atom bonded to O in one molecule (CH₃OH or H₂O) is attracted to a lone pair on the O atom of the other: a hydrogen bond. These strong solute-solvent attractions are why methanol and water mix in all proportions.
DIon-dipole attractions between H₂O and the CH₃O⁻ and H⁺ ions that methanol forms A student who thinks molecular solutes separate into ions picks this. Methanol dissolves as intact, neutral CH₃OH molecules; essentially none of them ionize in water.
A student draws particulate diagrams of equal volumes of 0.10 M glucose, C₆H₁₂O₆(aq), and 0.10 M NaCl(aq), with the water molecules omitted. Which claim about the diagrams, with its reasoning, is correct?
Answer and reasoning
AThe NaCl diagram should show twice as many solute particles, because each NaCl unit separates into two ions.Correct Equal volumes of 0.10 M solutions contain equal amounts of solute. Glucose dissolves as intact molecules, one particle per molecule, while each NaCl formula unit separates into a Na⁺ ion and a Cl⁻ ion, so the NaCl diagram shows twice as many solute particles.
BThe two diagrams should show equal numbers of solute particles, because NaCl stays as whole NaCl units. A student who thinks a dissolved ionic compound stays as intact formula units picks this. NaCl dissolves as separate Na⁺ and Cl⁻ ions, two particles per formula unit.
CThe glucose diagram should show more solute particles, because each glucose molecule breaks into its atoms. A student who thinks dissolving breaks molecules into atoms picks this. Glucose dissolves as whole molecules; the covalent bonds inside each molecule stay intact.
DThe two diagrams should show equal numbers of solute particles, because glucose also forms two ions. A student who thinks molecular solutes separate into ions picks this. Glucose dissolves as neutral molecules and forms essentially no ions, so it gives one particle per molecule.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
