a) From the Chapter 1 quiz: The densities of the coinage metals (copper, silver
ID: 525291 • Letter: A
Question
a) From the Chapter 1 quiz: The densities of the coinage metals (copper, silver and gold) are as follows: copper = 8.95 g/cm3, silver = 12.59 g/cm3, gold = 19.32 g/cm3 A sample of material is found to weigh 10.8 grams, and have a volume of 1.20 cm. The sample could be which of the coinage metals? Correct answer: Copper. The most commonly selected incorrect answer on this question was “none of the above”. Prove that the answer is, indeed, copper and provide an explanation why a student might have chosen “none of the above.”
b) From the Chapter 5 quiz: “Which of the following will occur if the temperature of a gas is increased from 20°C to 40°C at constant pressure?” More than ¼ of respondents selected - The volume will double. Explain why this answer is incorrect.
c) From the Chapter 6 quiz: This was the HARDEST question asked all term: “Which of the following solutions has the highest osmolarity?” The options were: 0.20 M NaCl, 0.15 M BaCl2, 0.10 M AlCl3 and 0.35 M sucrose. The correct answer is 0.15 M BaCl2. The most often selected incorrect response was 0.35 M sucrose. I can now tell you that the BaCl2 has the highest osmolarity and the sucrose is the lowest. Your challenge now is to explain which has the higher osmolarity between the NaCl and the AlCl3. Explain your answer.
d) From the first Chapter 8 quiz: “When solid sodium hydroxide is dissolved in water which of the following species is not present in the solution? The correct answer is NaOH(aq). Why is this answer correct?
Explanation / Answer
Answer to Qa)
Density = mass / volume
= 10.8 g / 1.2 cm3
Note volume is always expressed in cm3 and never in cm. If it is mentioned in cm then it is definitely a typographical error.
Density = 9 g cm-3
This is equal to that of copper. Hence the coinage metal is copper.
Answer to Qb)
For a gas at constant pressure volume is directly proportional to temperature.
Let us assume that 1 L gas was present at 20 deg C = 20 + 273 K = 293 K
The volume of gas at 40 deg C = 40 + 273 K = 313 K can be calculated as follows
V proportional to T
V1 / V2 = T1 /T2
V2 = V1 T2/ T1
= 1 L X 313 K / 293 K
= 1.07 L
Thus the volume will only increase slightly by 7%.
Why many students assumed that the volume goes double is that they did not convert deg C to K.
Answer to Qc)
Osmolarity = Molarity X number of osmoles
One mole NaCl will give 2 osmoles (1 Na+ and 1 Cl-)
Osmolarity of NaCl = Molarity X number of osmoles
= 0.2 X 2 = 0.4 osm
One mole BaCl2 will give 3 osmoles (1 Ba2+ and 2 Cl-)
Osmolarity of BaCl2 = Molarity X number of osmoles
= 0.15 X 3 = 0.45 osm
One mole AlCl3 will give 4 osmoles (1 Al3+ and 3 Cl-)
Osmolarity of AlCl3 = Molarity X number of osmoles
= 0.1 X 4 = 0.4 osm
Sucrose does not dissociate. One mole sucrose will give 1 osmole
Osmolarity of sucrose = Molarity X number of osmoles
= 0.35 X 1 = 0.35 osm
Thus we can show that BaCl2 has highest osmolarity while sucrose has the least.
From the above calculations we can say that NaCl and AlCl3 has equal osmolarity.
Answer to Qd)
The bond that binds Na-O in NaOH is ionic while the O-H bond is covalent. Hence when NaOH is dissolved in water it dissociates at Na+(aq) and OH-(aq) and therefore NaOH(aq) species is not present in the solution.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.