Consider a solution of CO₂( g) dissolved in water in a closed container.
Which one of the following plots correctly represents variation of log (partial pressure of CO₂ in vapour phase above water) [y-axis ] with log (mole fraction of CO₂ in water) [x-axis ] at 25°C ?

Solution

Chemistry ma’am · 4 steps

Take the log of Henry's law and it becomes a straight line. Now just read its slope and intercept.

1. Henry's law. At a fixed temperature, the partial pressure of a gas above a solution is proportional to its mole fraction in the solution: p = K_H x.

   log p = log x + log K_H

2. Read the line. This has the form y = mx + c. The slope is 1, so the line rises. The intercept is log K_H, which is fixed at 25°C.
3. Sign of intercept. CO₂ is only slightly soluble in water, so K_H is large: about 1.67 kbar, or 1670 bar. So log K_H ≈ 3.2, a positive intercept, not zero.
4. So. The plot is a straight line with a positive slope that cuts the y-axis above the origin. That is option (c).

The idea to keep. Whenever a law says one quantity is proportional to another, a log–log plot is a straight line. The slope is the power, and the intercept is the log of the constant.

Answer from the NTA final answer key. Solution by PrepWiser.