Answer the questions a) - c) on composition determination of fumarolic gas. Before sampling fumaroli — Physical Chemistry — Thermodynamics Chemistry Question
Quantitative composition analysis of volcanic gas
Answer the questions a) - c) on composition determination of fumarolic gas.
Before sampling fumarolic gas, 20 cm3 of 5 mol dm-3 NaOH solution was loaded in a syringe and air was removed from the syringe. A titanium tube was inserted into a fumarole and the tube was sufficiently heated by the hot fumarolic gas to prevent condensation of fumarolic gas in the tube. By connecting the titanium tube with the syringe using a rubber tube, the fumarolic gas was introduced into the syringe by slow drawing. During the sampling procedure, the syringe was cooled with a wet towel. After completing the gas collection, the syringe was equilibrated with the surrounding mountain atmosphere condition (0 °C, 1013 hPa) and the volumes of gas a nd liquid components recorded from the scale on the syringe were 50.0 cm3 and 38.0 cm3, respectively.
A 10 cm3 aliquot was collected from the 38.0 cm3 solution in the syringe, and all the sulfur species were oxidized to sulfate ion using an oxidant. After adding diluted HCl into the solution and making the solution slightly acidic, the addition of 20 cm3 of 10% BaCl2 solution to the solution resulted in a white precipitate. The precipitate was dried and weighed. Its mass was 0.30 g.
The volume of 1 mol of a gas is assumed to be 22.4 dm3 in the standard condition (0 °C and 1013 hPa).
Calculate the volume component of water vapor (in %) in the collected fumarolic gas assuming that the volume of NaOH solution did not change by absorption of gases other than water vapor. The density of NaOH solution was during the experiment constant (1.0 g cm–3).
Model Answer
This corresponds to 1 mole of water.
The volume is 22.4 dm3 at the standard condition.
22.4 × 1000 /（22.4 × 1000 + 50.0 + 109.8 + 28.0）= 0.992
0.992 × 100 = 99 %
Calculate the quantity of sulfur (in mol) contained in the fumarolic gas collected into the syringe. Moreover, calculate the total volume (in cm3) of H2S and SO2 gas components at the standard conditions (0 °C, 1013 h Pa). Give the answer to two significant figures.
Model Answer
The precipitate is BaSO4.
The formula mass is:
Mr(BaSO4) = 137.3 + 32.1 + 16.0 × 4 = 233.2
0.30 g of BaSO4 corresponds to 0.30 g / 233.2 g mol–1 = 1.29×10–3 mol of BaSO4
Total sulfur amount in the initial solution is
n(S)total = 1.29×10–3 × 38.0 / 10.0 = 4.90×10–3 mol
The volume of the gas is
22.4 dm3 × 4.90×10–3 mol = 109.8 cm3 = 1.1×102 cm3
Mole ratios of H2S to SO2 are estimated utilizing the reactivity difference between H2S and SO2 gases. Fumarolic gas is bubbled into a testing tube which contains 30 cm3 of KIO3 – KI solution with a concentration of 0.03 mol dm-3 and 3 cm3 of HCl solution (c = 4 mol dm-3). Describe by chemical equations the reactions occurring for H2S and SO2 gases, respectively.
Model Answer
SO2 + I2 + 2 H2O → SO42– + 2 I– + 4 H+
H2S + I2 → S + 2 I– + 2 H+