UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper II
Q1(i). Pure-component vapour pressures
Question
At 50° C, an ideal solution containing 1 mol A and 2 mol B has total vapour pressure 300 torr. Adding 1 mol A increases it to 400 torr. Find the pure-liquid vapour pressures.
Answer in brief
pA°=700 torr and pB°=100 torr.
Key points
- Raoult' s law gives p=xAp_A°+xBp_B°. Initially, pA°/3+2pB°/3=300, so pA°+2p_B°=900.
- After addition there are two moles of each component, so (pA°+p_B°)/2=400, giving pA°+p_B°=800.
- Subtraction yields pB°=100 torr, and then pA°=700 torr. Adding the more volatile A raises the total vapour pressure, consistent with the data.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversPure-component vapour pressuresIdeal solutions
All parts of Question 1
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- 1(b) Units of attraction parameter5 marks
- 1(c) Surface energy of cube and sphere5 marks
- 1(d) Atoms in a BCC sample5 marks
- 1(e) Volume independence of ideal-gas heat capacity5 marks
- 1(f) Entropy on doubling volume5 marks
- 1(g) Single-ion activity coefficients5 marks
- 1(h) Ammonia buffer5 marks
- 1(i) Pure-component vapour pressures5 marks
- 1(j) Zero-order half-life5 marks
- 1(k) Physical and chemical adsorption5 marks
- 1(l) Iron redox-electrode potential5 marks
- 1(m) Energy of one einstein5 marks
- 1(n) Bohr quantisation from matter waves5 marks
- 1(o) Spectral wavelength shifts5 marks
- 1(p) Bromobutane with one NMR signal5 marks