Question 1Level 2 · ExamSingle correct
Consider the following statements about Dalton's atomic theory (as Dalton originally stated it) and later observations.
I. The existence of the isotopes 35Cl and 37Cl contradicts Dalton's postulate that all atoms of a given element are identical in mass.
II. The solid iron(II) oxide whose composition varies with the method of preparation (for example Fe0.95O or Fe0.93O) is a departure from the law of definite proportions.
III. Dalton's theory, as originally stated, explains why 1 volume of H2(g) and 1 volume of Cl2(g) give 2 volumes of HCl(g) at the same temperature and pressure.
IV. Dalton's theory, as originally stated, distinguishes between an atom and a molecule of an element (for example between O and O2).
Which of the statements are correct? Exactly one option is correct.
AI, II and IV only
BII and III only
CI and II only
DI, III and IV only
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Answer: C
I: correct. Dalton held that all atoms of an element have the same mass; 35Cl and 37Cl are atoms of the same element with different masses.
II: correct. The law of definite proportions requires a fixed mass ratio of the elements in a compound; Fe1−xO has a composition that depends on preparation (a non-stoichiometric solid), so it departs from that law.
III: incorrect. For Dalton the 'compound atom' HCl came from one H atom and one Cl atom, so 1 volume + 1 volume should give 1 volume. Two volumes of HCl can be explained only with Avogadro's idea that hydrogen and chlorine are diatomic molecules, H2 + Cl2 → 2HCl.
IV: incorrect. The atom-molecule distinction came from Avogadro, not Dalton.
Correct statements: I and II only, option (C).
Difficulty: HardQuestion 2Level 2 · ExamNumerical answer
A gaseous mixture contains only CH4 and NH3. Its mass is 6.60 g and it contains 4.00 mol of electrons in total. Using H = 1, C = 12, N = 14 (g mol−1) and NA=6.022×1023 mol−1, find the total number of hydrogen atoms in the mixture, expressed in units of 1023. Give the answer correct to two decimal places.
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Answer: 8.43
Both CH4 and NH3 have 10 electrons per molecule (6 + 4 and 7 + 3). Let n1 and n2 be the amounts of CH4 and NH3.
Electrons: 10(n1+n2)=4.00⇒n1+n2=0.400 mol.
Mass: 16n1+17n2=6.60. Subtracting 16×0.400=6.40: n2=0.200 mol, so n1=0.200 mol.
H atoms: 4(0.200)+3(0.200)=1.40 mol =1.40×6.022×1023=8.4308×1023.
Answer: 8.43.
Difficulty: HardQuestion 3Level 2 · ExamOne or more correct
Use H = 1, O = 16, Al = 27, S = 32, K = 39 (g mol−1) and NA=6.022×1023 mol−1; STP means 273 K and 1 atm. Which of the following statements are correct? One or more options may be correct.
A9.48 g of potash alum, KAl(SO4)2⋅12H2O, contains 0.400 mol of oxygen atoms.
B1.80 g of H2O contains 6.022×1023 electrons.
C3.20 g of O2 and 3.20 g of O3 contain the same number of molecules.
D22.4 L of H2O at STP contains 6.022×1023 molecules.
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Answer: A, B
(A) Potash alum KAl(SO4)2⋅12H2O: M=39+27+2(96)+12(18)=474 g mol−1. 9.48 g = 0.0200 mol. O atoms per formula unit = 8 + 12 = 20, so O = 0.400 mol. Correct.
(B) 1.80 g H2O = 0.100 mol; each molecule has 10 electrons, so 1.00 mol electrons = 6.022×1023 electrons. Correct.
(C) Equal masses of O2 and O3 contain the same number of O atoms (0.200 mol each), but different numbers of molecules: 0.100 mol O2 against 0.0667 mol O3. Incorrect.
(D) Water is a liquid (ice/water) at 273 K and 1 atm; 22.4 L of it is about 22.4 kg, i.e. about 1244 mol, not 1 mol. The molar volume 22.4 L applies only to gases. Incorrect.
Answer: A, B.
Difficulty: Hard