Question

Phosphoric acid is neutralized by potassium hydroxide according to the following reaction:

KOH (aq) + H3PO4 (aq) → K3PO4 (aq) + H2O (1)
What is the concentration of a phosphoric acid solution if 25.0 mL are exactly neutralized by 20.0 mL of 2.000 M KOH solution?

Answer 1

The concentration of the phosphoric acid solution is calculated by first finding the moles of KOH used, then using the molar ratio to find the moles of H3PO4, and finally dividing by the volume of H3PO4 to get the concentration, which is 0.533 M.

Step-by-step explanation:

The student is performing a titration to find the concentration of phosphoric acid (H3PO4) using a potassium hydroxide (KOH) solution. The reaction for the neutralization is as follows:

KOH (aq) + H3PO4 (aq) → K3PO4 (aq) + 3H2O (l)

We use the information that 25.0 mL of H3PO4 is neutralized by 20.0 mL of 2.000 M KOH to calculate the concentration of H3PO4. Given the balanced chemical equation, we can determine the molar ratios of the reactants. For each mole of H3PO4, three moles of KOH are required to reach the equivalence point.

Calculate the moles of KOH used: moles of KOH

= volume × concentration = (20.0 mL / 1000 mL/L) × 2.000 M = 0.0400 moles KOH.

Use the molar ratio from the balanced equation to find the moles of H3PO4: moles H3PO4 = moles KOH / 3 = 0.0400 moles / 3 = 0.0133 moles H3PO4.

Calculate the concentration of H3PO4: concentration = moles/volume = 0.0133 moles / (25.0 mL / 1000 mL/L) = 0.533 M.

Therefore, the concentration of the phosphoric acid solution is 0.533 M.

Answer 2

Balance the reaction first:

3KOH + H3PO4 —> K3PO4 + 3H2O

So for every mol of H3PO4, you need 3 mol of OH- to fully neutralize the acid, since H3PO4 is polyprotic.

0.0200 L KOH • (2.000 mol KOH / L KOH) • (1 mol H3PO4 / 3 mol KOH) = 0.0133 mol H3PO4

Divide this by the volume of H3PO4 to get the concentration.

0.0133 mol H3PO4 / 0.0250 L = 0.532 M H3PO4