[Campbell Biology P.853] A gardener suspects that soil near a walkway is contaminated with s... | Practice Question

A gardener suspects that soil near a walkway is contaminated with salt NaCl . If the soil contains an additional 50 mM NaCl and the temperature is 20°C, calculate the solute potential Ψs contribution from this added salt, and predict its effect on water movement. Use R = 0.00831 L·MPa/ mol·K and assume NaCl completely ionizes i=2 .

  • A: -0.244 MPa; This decreases the soil's water potential, causing water to move out of the roots.
  • B: -0.122 MPa; This increases the soil's water potential, causing water to move into the roots.
  • C: +0.244 MPa; This increases the soil's water potential, causing water to move into the roots.
  • D: -0.244 MPa; This increases the soil's water potential, causing water to move into the roots.

Explanation

The solute potential Ψs is calculated using the formula Ψs = -iCRT. Given i=2, C=0.050 mol/L 50 mM , R=0.00831 L·MPa/ mol·K , and T=293 K 20°C + 273 , Ψs = - 2 0.050 0.00831 293 ≈ -0.244 MPa. A more negative solute potential lowers the overall water potential of the soil. Water moves from an area of higher water potential to an area of lower water potential. Therefore, a lower soil water potential will cause water to move out of the plant roots and into the soil, leading to plant wilting.