Calculate the OH- of each aqueous solution
aspirin, 5.9×10−4 M
milk of magnesia, 1.5×10−9 M
To calculate the OH- concentration of each aqueous solution, we need to consider the compounds and their dissociation in water.
1. Aspirin (C9H8O4):
The chemical formula of aspirin is C9H8O4. It does not readily dissociate in water to release OH- ions. Therefore, the OH- concentration of an aspirin solution is negligible. We can assume it to be 0 M.
2. Milk of magnesia (Mg(OH)2):
The chemical formula of milk of magnesia is Mg(OH)2. It does dissociate in water to release OH- ions. Each formula unit of Mg(OH)2 generates two OH- ions. Therefore, the concentration of OH- ions in a milk of magnesia solution is twice the concentration of Mg(OH)2 in the solution.
Given:
- Concentration of milk of magnesia: 1.5×10^-9 M
Applying the dissociation equation:
Mg(OH)2 ⇌ Mg2+ + 2OH-
We can assume that the dissociation of Mg(OH)2 is complete, meaning that all the Mg(OH)2 molecules dissociate to form Mg2+ and OH- ions.
Therefore, OH- concentration = 2 × 1.5×10^-9 M = 3.0×10^-9 M.
So, for the given solutions:
- The OH- concentration for the aspirin solution is negligible.
- The OH- concentration for the milk of magnesia solution is 3.0×10^-9 M.
Call aspirin, HA.
HA ==> H^+ + A^-
Ka = (H^+)(A^-)/(HA)
Set up an ICE chart, substitute into the Ka expression and solve for (H^+), then convert to OH^-.
Mg(OH)2 ==> Mg^+2 + 2OH^-
1.5 x 10^-9 M is less than the solubility of Mg(OH)2; therefore, this should be the (Mg^+2). Twice that will be the concn of OH^-.