State Henry's law and write its mathematical form. How does temperature affect the solubility of a gas? Give two applications of Henry's law.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the law in one line. At constant temperature, the partial pressure of a gas above a solution is proportional to its mole fraction in the solution.
2 · Write the formula with symbols.p = K_H x, where p is the partial pressure of the gas, x its mole fraction in solution and K_H the Henry's law constant (bar or kbar).
3 · Sketch the graph. Partial pressure p (y-axis) against mole fraction x (x-axis): a straight line through the origin with slope K_H.
4 · Explain the effect of temperature. K_H increases as temperature rises, so gases are less soluble in warm water; dissolving a gas is exothermic.
5 · Give two applications.
Soda bottles are sealed under high CO₂ pressure
Divers' air tanks are diluted with helium to avoid the bends
6 · Close in one line. So a gas dissolves best at high pressure and low temperature.
Scoring tip: Name the symbols right after the formula. A one-line sketch of p against x, a straight line through the origin, earns a quick extra mark.
