Answer to Question #119978 in General Chemistry for Cass

Question

7. You are washing the walls in your kitchen with a product to remove oils and grease before you paint. (oils and grease make it more difficult for paint to adhere) The chemical you are using is called TSP and the active ingredient is Na3PO4 (Kb = 5.9 *10 -3) The instructions have you dissolve1/4 cup (40 grams) into 1 gallon (3.785Liters). What is the pH of the wall cleaning solution

Answer 1

Molar Mass of Na₃PO₄ = 3 × 23 + 31 + 64 = 164 gm/mol .

Concentration of Na₃PO₄ = (40)/ (164 × 3.785)

= 0.0644 M.

So, Concentration of PO₄^3- = 0.0644 M

PO₄^3- gives HPO₄^2- and OH^- after hydrolysis.

, If y is degree of hydrolysis of PO₄^3- then Concentration of OH^- is 0.0644 y M.

Concentration of H₂PO₄^- is 0.0644 y M.

Concentration of PO₄^3- is 0.0644 (1-y ) approx is equal to 0.0644 M.

So, K_b = (0.0644 y ) × (0.0644 y) / 0.0644 .

So, (0.0644 y)² = K_b × 0.06444.

So,( 0.0644 y)² = 0.0644 × 5.9 × 10^-3 .

0.0644 y is the Concentration of OH^- .

So, Concentration of OH^- = 1.94 × 10^-2 M..

So, P^OH = -log^{concentration of OH-}

So, P^OH = 1.712 (approx.)

So, P^H = 14 - 1.712 = 12.287 (approx ).

At 25⁰ c .