What is the pH of a 1 * 10^-3 M solution of Na3PO4? You can ignore the formation of H2PO4- and H3PO4^2-.

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Identify the relevant chemical species in the solution: Na3PO4 dissociates completely in water to form 3 Na+ ions and 1 PO4^3- ion.

Recognize that PO4^3- is a base and can accept protons from water, forming OH- ions and HPO4^2- ions. The reaction is: \( \text{PO}_4^{3-} + \text{H}_2\text{O} \rightleftharpoons \text{HPO}_4^{2-} + \text{OH}^- \).

Use the base dissociation constant \( K_b \) for PO4^3- to find the concentration of OH- ions. The \( K_b \) can be calculated using the relation \( K_w = K_a \times K_b \), where \( K_w \) is the ion-product constant of water and \( K_a \) is the acid dissociation constant for the conjugate acid HPO4^2-.

Set up an expression for \( K_b \) using the concentrations: \( K_b = \frac{[\text{HPO}_4^{2-}][\text{OH}^-]}{[\text{PO}_4^{3-}]} \). Assume \([\text{HPO}_4^{2-}] = [\text{OH}^-] = x\) and \([\text{PO}_4^{3-}] = 1 \times 10^{-3} - x\), then solve for \( x \).

Calculate the pOH from the concentration of OH- ions using \( \text{pOH} = -\log[\text{OH}^-] \), and then find the pH using \( \text{pH} = 14 - \text{pOH} \).

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pH and pOH

pH is a measure of the hydrogen ion concentration in a solution, defined as the negative logarithm of the hydrogen ion concentration (pH = -log[H+]). A lower pH indicates a more acidic solution, while a higher pH indicates a more basic solution. The pOH is similarly defined for hydroxide ions, and the relationship between pH and pOH is given by the equation pH + pOH = 14 at 25°C.

Strong vs. Weak Bases

In chemistry, bases can be classified as strong or weak based on their ability to dissociate in water. Strong bases, like NaOH, completely dissociate into their ions, while weak bases only partially dissociate. Sodium phosphate (Na3PO4) is a strong base because it dissociates completely in solution, contributing to the increase in hydroxide ion concentration and thus raising the pH.

Dissociation of Sodium Phosphate

Sodium phosphate (Na3PO4) dissociates in water to produce sodium ions (Na+) and phosphate ions (PO4^3-). The phosphate ions can react with water to produce hydroxide ions (OH-), which increases the pH of the solution. Understanding this dissociation process is crucial for calculating the resulting pH of the solution, as it directly influences the concentration of hydroxide ions present.

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