Question

(a) Calculate the Coulomb electrostatic force and electrostatic energy between a Na+ ion and a Cl- ion at a distance of 0.22 nm. Is this attractive or repulsive? The magnitude of the ionic charge of Na+ and Cl- ion is 1.6 x 10-19 C. (b) Calculate the Coulomb electrostatic energy between one casein submicelle and one Ca++ ion, assuming that the submicelle has negative charge - 1 x 10-17 C, and the separation between the centers of the submicelle and the Ca++ ion is 12 nm. The charge of an Ca++ ion is 3.2 x 10-19 C. Is the interaction between the submicelle and the Ca++ ion attractive or repulsive? (c) In Lab #1 you are making cheese by adding vinegar to milk. Which of the following 4 statements is TRUE 1. Adding vinegar to milk produces CO2 which makes milk ferment. 2. Adding vinegar to milk neutralizes the electrostatic repulsion between casein micelles causing them to aggregate. 3. Adding vinegar to milk increases the electrostatic repulsion between casein micelles causing them to aggregate. 4. Milk will curdle if you use baking soda instead of vinegar.

Answer 1

(a) We know that the coloumb force is given by
F = kq₁q₂/r²
where k is constant = 9*10⁹
q₁ and q₂ are the charges
r is distance between them
Charge on Na⁺ ion (q₁) = + 1.6*10^-19 C
Charge on Cl^- ion (q₂) = -1.6*10^-19 C
Distance between them (r) = 0.22 nm = 0.22*10^-9 m
Putting all the values in the above equation
F = (9*10⁹)*(1.6*10^-19)² /(0.22*10^-9)² = 4.76*10^-9 N
Now since both are opposite charge therefore force will be attractive.
(b) Charge on Ca⁺⁺ ion (q₁) = (2*1.6*10^-19) C
Charge on Submicelle (q₂) = -1*10^-17 C
distance between them (r) = 12 nm =12*10^-9 m
Force =[ (9*10⁹)*(2*1.6*10^-19)*(1*10^-17)]/(12*10^-9)²
Force (F) = 2*10^-10 N
Since both the charge are opposite therefore force will be attractive.