# Homework Help: Easy chem

1. Feb 6, 2006

### hola

What is the pH of a solution containing 50.0g of (Na_3)(PO_4) in a liter of solution? We are given that (Na_3)(PO_4) is very basic.

2. Feb 9, 2006

### PhiJ

pH = 14 + log[OH]
as it is a strong base, the concentration of OH- ions should be the same as the concentration of (Na_3)(PO_4). Can you work it out from there?

3. Feb 9, 2006

### GCT

You'll need to use the equilibrium constants, that's probably what your teacher/professor wants. I'll give you a general outline, although in most cases it's not the best way to learn the problem....

convert from grams of the compound to moles compound, and using the molar ratios, convert to moles of the anion.

The anion has three Kbs, in most cases however, you can ascribe the pH to the first Kb (or is it the "third" Kb). The rest is pretty simple, use the Kb equation
$$K_{b} = \frac{[HPO_{4}^{2-}][OH^{-}]}{[PO_{4}^{3-}]}$$

solve for the hydroxide concentration and then use the equation phiJ recommended to solve for pH.

After solving for the OH- concentration try to determine whether the usage of the second Kb was necessary for a magnitude of error.