AP Chemistry 1.3 Elemental Composition of Pure Substances- MCQs - Exam Style Questions
▶️ Answer/Explanation
1. Calculate % Cl in Compounds:
– NaCl: \( \frac{35.45}{58.44} \times 100\% = 61\% \) Cl
– NaI: \( 0\% \) Cl
– KCl: \( \frac{35.45}{74.55} \times 100\% = 48\% \) Cl
– LiCl: \( \frac{35.45}{42.39} \times 100\% = 84\% \) Cl
2. Analyze Mixture:
Observed: \( 75\% \) Cl > pure NaCl (\( 61\% \))
Need impurity with higher % Cl than NaCl.
LiCl has \( 84\% \) Cl, which would increase overall % Cl.
✅ Answer: (D)
Question
\(Ba^{2+}(aq)+SO_{4}^{2-}(aq)\rightarrow BaSO_{4}(s)\)
A student obtains a 10.0g sample of a white powder labeled as \(BaCl_{2}\). After completely dissolving the powder in 50.0mL of distilled water, the student adds excess \(Na_{2}SO_{4}(s)\), which causes a precipitate of \(BaSO_{4}(s)\) to form, as represented by the equation above. The student filters the \(BaSO_{4}(s)\), rinses it, and dries it until its mass is constant. Which of the following scientific questions could best be answered based on the results of the experiment?
A Is the \(Na_{2}SO_{4}(s)\) used in the experiment pure?
B Is the \(BaCl_{2}(s)\) used in the experiment pure?
C What is the molar solubility of \(BaCl_{2}\) in water?
D What is the molar solubility of \(BaSO_{4}\) in water?
▶️Answer/Explanation
Ans:B
The calculated number of moles of Ba in the precipitate can be compared with the expected number of moles of Ba in 10.0g of \(BaCl_{2}\). The presence of fewer moles of in the precipitate would imply that the \(BaCl_{2}\) was not pure.
Question
A student measures the mass of a sample of a metallic element, M. Then the student heats the sample in air, where it completely reacts to form the compound MO. The student measures the mass of the compound that was formed. Which of the following questions can be answered from the results of the experiment?
A What is the density of M?
B What is the molar mass of M?
C What is the melting point of M?
D What is the melting point of MO?
▶️Answer/Explanation
Ans:B
Correct. The number of moles of O in the compound is equal to the number of moles of M and can be calculated using the difference in the masses of the sample and the compound. The molar mass of M is calculated by dividing the mass of the sample of M by the number of moles of M.