Answer: 40.5 mol
Explanation:
Since this is a double replacement reaction, the unbalanced equation is something like follows:
[tex]\text{K}_{3}\text{PO}_{4}+\text{NaOH} \longrightarrow \text{KOH}+\text{Na}_{3}\text{PO}_{4}[/tex]
Balancing this equation,
[tex]\text{K}_{3}\text{PO}_{4}+3\text{NaOH} \longrightarrow 3\text{KOH}+\text{Na}_{3}\text{PO}_{4}[/tex]
From this equation, we can conclude that for every 3 moles of sodium hydroxide consumed, 1 mole of potassium phosphate is consumed.
This means the answer is 13.5(3)=40.5 mol