Chemistry 102 – Chapter 4 Summary and Review – Answers 1. What is the concentration of the chloride ion when 10.0 g of iron(III) chloride is combined with enough water to make 225 mL of solution? Answer: Molar concentration = moles (solute) / volume (liters) of solution [ FeCl3 ] =
⎛ mol FeCl3 ⎞
⎟
⎝ 162.2 g ⎠ = 0.274 M 0.225 L
(10.0 g FeCl3 ) ⎜
This is the concentration of iron(III) chloride. The concentration of the chloride ion, however is three times this, as there are three moles of the chloride ion for every mole of iron(III) chloride. −
⎛ 0.274 mol FeCl3 ⎞ ⎛ 3 mol Cl ⎞
−
⎜
⎟ = 0.822 M Cl ⎜
⎟
L
⎝
⎠ ⎝ 1 mol FeCl3 ⎠
2. What is the oxidation number of chlorine in elemental chlorine, sodium chloride, sodium chlorite and sodium perchlorate? Answer: Remember, the oxidation number of an atom is determined by assuming the substance is 100% ionic. We will be able to elaborate more on this as we discuss electronic structure, chemical bonding, etc. Name Formula
Ox Num Cl2 0
sodium chloride NaCl
–1
sodium chlorite NaClO2
+3
sodium perchlorate NaClO4
+7
elemental chlorine Reasoning
all free elements have an oxidation number of zero – this includes diatomic molecular elements when halogens are combined with a metal (and thus called halides), the oxidation number is –1 when halogens combine with oxygen to form a polyatomic ion – oxygen has an oxidation number of –2 and the halogen has a positive oxidation number* when halogens combine with oxygen to form a polyatomic ion – oxygen has an oxidation number of –2 and the halogen has a positive oxidation number* *
Fluorine will always be –1 and the oxidation number of an element can never be greater than the group number of the element (chlorine can have an oxidation number of +7 but not +8) [Do a double check to make sure these are correct and the compound is neutral.] 3. What would you add to a solution of Fe3+, Pb2+, and Ba2+ positively identify each ion? Give one net ionic equation for each precipitation. Answer: To identify lead(II) ions, you can add sodium chloride and lead(II) chloride will precipitate while the remaining ions will not. Filter the lead(II) chloride. Pb2+(aq) + 2Cl–(aq) → PbCl2(s) To identify barium ions, add sodium sulfate and barium sulfate will precipitate while iron(III) sulfate will not. Filter the barium sulfate. Ba2+(aq) + SO42–(aq) → BaSO4(s) To identify iron(III) ions, add sodium hydroxide. Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s) 4. Explain on a macroscopic and a particle level how diluting a solution will not change the number of moles of solute but will change the concentration. Answer: Macroscopic: a solution is identified as a homogeneous mixture – this is observed by looking consistent (properties appear the same) and only by making other observations (performing other tests) can it be determined that the solution is a homogeneous mixture rather than a pure substance. If a solution is colored because the solute has color and the solvent is colorless, diluting this solution will result in the color remaining the same but the intensity decreasing. This is shown in the figures below. If the solution is colorless, then diluting on the macroscopic will not appear to change the solution (observed only by looking). Of course, testing the two solutions will reveal that the diluted solution has a lower concentration. By considering the colored solutions, the solute is unchanged (the color remains the same) but the intensity has decreased. This is due to the addition of the colorless solvent. Volume of the solvent has increased, the concentration has decreased (the color intensity has decreased). Particle: now it does not matter if the solution/solute/solvent particle have color macroscopically (there is color on the particles to explicitly identify the difference between solute and solvent particles). Adding this to the figure, the number of solute particles remains unchanged as the number of solvent particles increases. If the number solvent particles increases (adding more solvent) with a constant number of solute particles, the concentration of the solution will decrease or be more dilute. 5. Show the difference between a strong acid (use HNO3(aq)) and a weak acid (use HNO2(aq)). Show the neutralization reactions for both with sodium hydroxide. What is the molar concentration of sodium hydroxide when 25.00 mL of sodium hydroxide is neutralized with 18.75 mL of a 0.175 M solution of nitric acid? Answer: A strong acid will ionize 100% in water or the species present in water for nitric acid are H+ and NO3–. A weak acid will not ionize 100% in water (and much less) or the species present in water for nitrous acid are H+, NO2– and HNO2. Nitric acid: HNO3(aq) → H+(aq) + NO3–(aq) Nitrous acid: HNO2(aq) U H+(aq) + NO2–(aq) To calculate the molar concentration of sodium hydroxide, use the concentration and volume of the nitric acid to calculate the number of moles of nitric acid; then use the mole ratios of nitric acid and sodium hydroxide; finally use the volume to determine the molar concentration. ⎛ mol NaOH ⎞ ⎛
1
⎛ 0.175 mol HNO3 ⎞
⎞
⎜
⎟ ( 0.01875 L ) ⎜ mol HNO ⎟ ⎜ 0.02500 L ⎟ = 0.131 M NaOH L
⎠
⎝
⎠
3 ⎠⎝
⎝
6. Show the balanced equation for the reaction of zinc with hydrochloric acid. What is oxidized and what is reduced? What is the oxidizing agent and what is the reducing agent? How many electrons are transferred in the process (for your equation as written)? How is this reaction observed macroscopically and represented on the particle level? Answer: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) Zinc is oxidized (from 0 to +2) and hydrogen is reduced (from + to 0); zinc is the reducing agent and hydrochloric acid is the oxidizing agent; as written, two electrons are transferred. This reaction is observed macroscopically by a piece of solid zinc is placed in the acid to react by dissolving the zinc and hydrogen gas bubbles forming. This reaction is represented on the particle level by solid zinc with all electrons, reacting with the hydrogen ion (and more accurately hydrated hydrogen ion, for instance, hydronium ion) to transfer electrons from the zinc to the hydrogen ion. This causes the zinc ions to go into solution and the reduced hydrogen molecules to collect and form gas bubbles.