What volume of a 0.25 m phosphoric acid solution is required to react completely with 1.0 l of 0.35 m sodium hydroxide?

The moles have to be equal. Always start with a balanced equation. This reaction is a double replacement.

The basic equation is

NaOH + H3PO4 = = = > Na3PO4 + HOH

The balanced equation is.

3NaOH + H3PO4 = = = > Na3PO4 + 3HOH

moles of NaOH = moles of H3PO4

mols of NaOH = molarity * Volume

molarity = 0.35 mol/L

Volume = 1.0 L

moles NaOH = 0.35 * 1 = 0.35 mols

Find the moles of H3PO4

For every mol of H3PO4 used, you require 3 mols of NaOH

3/1 = 0.35/x

3x = 0.35

x = 0.35/3

x = 0.1167 moles of H3PO4 needed for this reaction.

Now the volume needs to be calculated.

n = 0.1167

M = 0.25 mol/L

V = ?

Formula

M = n/V

V = n/M

V = 0.1157/0.25

V = 0.467 L

Note: I merely copied the reaction given myself to the next line. I'm not copying from an outside source.